direct synthesis of organic silicates
TRANSCRIPT
• • . • • • • • • S D 0 1 0 0 0 2 1
'University of Khartoum
Faculty of Education
Department of Chemistry
Direct Synthesis of Organic
Silicates
A Thesis Submilled lor llie Degree oI 'MSc. in Chemistry
By
liana Hassan Cisinalla
Supervisor
Dr. Onicr Yousif Onier
June 2000
3.2/. 2 7
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ContentsQuran :.-..,.. , .. .... , I
Dedication.. ..... ... IIContents...... IllList of Tables: ....... VI
List of Figures VII]
Acknowledgments.. , IXabstract [Arabic] .,. :,... ..... XAbstract [English] , :....; XI
CHAPTER ONE: LITERATURE REVIEWI. Literature Review
[A. Introduction^............ ...........,.;.. ......... \.:....'.' 1
1.1.1. Objectives of the Study v. , ........... 3
.2. Some Aspects of Silicon Chemistry 4
1.2.1. Elemental Silicon , .:.. <.. 4
1.2.2. The Nature of Bonding in Silicon-Oxygen compouiuls 7
1.2.3. Structure of the Silicates , 10
.3. Organic Silicates..... , ''. 13
•>1.3.1. lThysical Prbpertiesof Organic Silicates
1.3.1.1 .Volatility, Molecular Complexities & Thermodynamic
Data 15
1.3.1.2. Structural Aspects..... W
: •1.3.1.3. Dipole Moments 20
1.3.1.4. Densities, Viscosities and Surface Tensions 20
1.3.2. Characterization Methods 21
1.3.2.1. Infra-red Spectra of Organic Silicates ' 21
1.3.2:2. Nuclear Magnetic Resonance spectra of organic 24
Silicates.,' ; 24
1.3.2.3. Mass Spectra of Organic Silicates 26
1.3.2.4. Gas-Iiquiil Chromatography of Organic Silicates 27
in
1.3.3. Chemical Properties of Organic Silicates... 30
1.3.3.1. Hydrolysis of Organic Silicates. ;• ,. 31
1.3.3.2. Hydrolysis and Condensation of Tetraethqxysilane 33
1.3.3.3. Formation of Double Metal Alkoxides 34
1.3.4. Uses'of Organic Silicates 35
1.4. Preparative Methods 36
1.4.E The Halosilane Route to Tetraalkoxysilanes.......: 361.4.2. The Exchange Route to Tetraalkoxysilanes 411.4.3. The direct Synthesis. 42
1.4.3.1. The Elemental Route Using Metalic & Metal Salt; Catalysts 43
1.4.3. 2. The Elemental Route Using Metal Alkoxide catalysts 44.
CHAPTER Two: EXPERIMENTAL2. Experimental
2.1. Materials :... 46
2.1.1. Ethanol ; ..' :...: 46
2.1.2. Magnesium , 46
2.1.3 Mercury (I) Chloride , 46
2.1.4. Silicon Powder , .: , ............:.. 46
2.1.5. Magnesium Ethoxide 46
2.1.6. Anhydrous Tin Tetrachloride... ; 46
2.1.7. Tin(ll) Oxide..... , 47
2.2. Equipment & Apparatus. 47
2.2.1. General 47
2.2.2. Infra-red Spectrometer 47
2.2.3. Cias-liquid Chromatographic System 47
2.3. Experimental Procedure....' • 492.3.1. General 942.3.2. Preparation of Magnesium Ethoxide catalyst 492.3.3. Direct Synthesis of Tetraetlvoxysilane :..-..- ;. 50
IV
2.3.3.1. Catalysed by Magnesium Ethoxide 5.0
, ' \ 2.3.3.1 Catalysed by Tin Tetrachloride.. .,..*.. - 51
2.3.3.3. Catalysed by Ti.n(II) Oxide..... .........;....... 52
CHAPTER THREE: RESULTS
3. Results
3.1. Theoretical & Experimental Yields «. 54
3.2. Magnesium Ethoxide Catalyst 54
3.2.1. IR Spectrum of Solid Product... .. 54
3.2.2. IR Spectrum of Liquid Product 55
3.3. Si I icon Ethoxide , ... 5 8
3.3.1. Catalysed by Mg(GC21I5)2 • • • • 58
3.3.l.l.IR Spectrum......... 58
3.3.1.2. GL Chromatograph 60
3.3.2. Catalysed by SnCf,. : . . . . . . 62
3.3.2.1. IR Spectrum....- ' 62
3.3.2.2. GI. Chiomatogiaph 64
3.3.3. Catalysed by SnO...; 66
3.3.3.4. IR Spectrum... ., 66
3.3.3.2. GL Chromatograph. '. 68
CHAPTER FOUR: DISCUSSION & CONCLUSION4. Discussion & Conclusion
4.1. Discussion ;-.'. 71
4.1.1. Preparation of Magnesium Ethoxide by the Reaction of
Magnesium & Ethanol 71
4.1.2. Preparation of Telraethoxysilane by the Reaction of Silicon &
1'thanol 72
4.2. Conclusion 75
Re f e re n-c es 76
List of Tables
TABLE
No.
1.1.
1.2.
1.5.
1.6.
1.7.
.8.
.9.
1.10.
2.1.
2:3.
2.4.
3.1.
TITLE
Some Selected Physical Properties of Silicon.
Selected Values of Electronegativities on Pauling Scale.
I'AGK
6
-.-•• f
i Selected Values for the (Si-O) Bond Length in Some
Organosilicon Compounds. 9
Bond Energies of Some Silicon & Carbon Bonds. -
Physical Properties of Some Metal Ethoxide.
Boiling Points and Complexities of Titanium & Zirconium.
Physical Constants of Tetraethoxysilane.
Density & Surface Tension of Some Alkoxysiiane Derivatives.
Vibrational Spectrum of Ethoxypolysiloxanes.
H'NMR Spectra of the Product of the Reaction Between Ethanof
& Silicon.
GLC Analysis of the Product of the Reaction Between Ethanol
& Silicon Metal Usi'ng Magnesium Ethoxide Catalyst.
Summary of the Reaction Between Ethanol & Magnesium
Catalysed by Mercury(l) Chloride.
Summary of the Reaction Between Elhanol and Silicon
Catalysed by Magnesium Ethoxide,
Summary of the Reaction Between Ethanol & Silicon Catalysed
by fin Telrachloride.
Summary of the Reaction Between Ethanol & Silicon Catalysed
by Tin Oxide.
Yields of Tetraethoxysilane Obtained Using Different Catalysts
Infra-red Spectra of the Solid Product of the Reaction Between
Ethanol & Magnesium-Catalysed by Mercury(l) Chloride.
7;T
22
24
28
50
53
"54"
54
1 3 :
3.4.
3.5.
3.6.
3.7.
3.9.
~4~2.
43 .
Infra-red Spectra of the Liquid Product of the Reaction Between• • • ' ' • • • v • ' ' • ; . ' - / • • • • • - • ' : - : '
& Magnesium Catalysed by Mercury(I) Chloride.
Infra-red Spectra of the Product of the Reaction Between4
Ethanol & Silicon Catalysed by Magnesium Ethoxide.
GLC Analysis of the Product of the Reaction Between Ethanol
& Silicon Catalysed by Magnesium Ethoxide.
Infra-red Spectra of the Product of the Reaction Between
Ethanol & Silicpn Catalysed by Tin Tetrachloride.
GLC Analysis of the Product of the Reaction Between Ethanol
& Silicon Catalysed by Tin Tetrachloride.
Infra-red Spectra of the Product-of the Read ion Between
Ethanol & Silicon Catalysed by Tin Oxide. . \
GLC Analysis of the Product of the Reaction Between Ethanol
& Silicon Catalysed by Tin Oxide.
Reported and Obtained IR Characteristic Bands for.Mg(OC;>l I5)2
Reported and Obtained IR Characteristic Bands for Si(OC2l I5)i
Prepared Using MgCOCiHs): Catalyst.
Reported and. Obtained IR Characteristic Bands for SU
Prepared Using SnCi.4"Catalyst.
Reported and Obtained IR Characteristic Bands for Si(O(\I lO
Prepared I Ising SnO Catalyst.
60
62
64
66
68
7 C
72
73
v 11
List of Figures
Fig.
No.
1.1.
1.3.
7A
2.1
3.2.
3.3.
3.4.
3.5.
3.6.
3.7.
3.8.
TITLE
(pn - dn) Bonding in the (Si - O) Bond.
Assigned Infra-red Spectrumof Tetraethoxysi lane. '
H'NMR Spectrum of Tetraethoxysi lne.
G1C of the Product of the Reaction Between Ethanol & Silicon
Metal.
Reaction System for Experimental Apparatus.
Infra-red Spectra of the Solid Product of the Reaction Between
Ethanol & Magnesium. '
Infra-red Spectra the Liquid Product, of the Reaction Between
Ethanol & Magnesium.
Infra-red Spectra of the Product of the Reaction Between
Ethanol & Silicon Using Mg(OC2M5)2 Catalyst.
Gas-liquid Chromatograph of the Product of the Reaction
Between Ethanol & Silicon Using Mg(()El)2 Catalyst.
Infra-red Spectra of the Product of the Reaction'Between
Ethanol & Silicon Using SnCl.t Catalyst.
Gas-liquid Chiomatograph of the Product of the Reaction
Between Eihanol & Silicon Using SnCl.| Catalyst.
Infra-red Spectra of the Product of the Reaction Between
Ethanol & Silicon Using SnO Catalyst.
Gas-liquid Chrbmatbgraph of the Product of the Reaction
Between Ethhnol & Silicon.Using SnO Catalyst.
•I»AC;IC
8
1 3
"25
29
•56
•57
6
65
67
69
v I 11 .•
Abstract
Tetraethoxysilane was prepared using. the direct synthetic
procedure in; .presence of magnesium ethoxide, tin telrachloride and tin
oxide as catalysts.
Magnesium ethoxide was prepared firstly, identified by spectral
analysis and then used in the preparation of tetraethoxysilane.
The method adopted is reliable and significant as far as synthetic
routes are concerned. ,
The product obtained was analysed using infra-red speetroscopy
and gas-liquid chromatography, these indicated that the final reaction
product can be obtained in high yield and. purity. Spectral analysis
obtained are in good agreement with reported data for tetraethoxysilane.
XI
Chapter One
Literature Review
1. Literature Review:
J.I. Introduction: *• . . ' • . ' . ; ' • . •
• - • • " • • • - • ' • • • . • . . • ' • . . » • • • '
Actually, though, silicon chemistry has deep roots in* human/ '
history, dating from the dawn of the race and extending through all of
geology, mineralogy, and the ancient ceramic arts, .
The development of silicone materials is, in perspective, as part of
the fascinating involvement of the element silicon in our daily life, from
the: stuff the earth and the moon aremade of to the modern use of ultra-
pure silicon in transistors and computers, and the use of ordinary
elementary silicon to make silicone rubber, silicone oil, silicon resins,
silicon-containing polishes, drugs and fragrances.
Of course these are not our.only connections with silicon. The
natural compounds of silicon and oxygen (the silicates) are the starting
materials for making bricks, tile, cement, glass, and a host of modern
ceramic products. •
The widespread usefulness of silicon and its compounds conies'"
about for two reasons: First, there is so much of it, and second, it is so
versatile. Its chemical and physical properties are so unusual and so
varied that they just cry out for research into creative and ingenious uses.
Moreover, silicon is a rather friendly element, devoid of a specific
elemental toxicity like thai'of arsenic or lead or plutonium, and so
accustomed to long association with the insides and out sides of living
systems (including the human body)'that silicone polymers are even used .
in cosmetics, medicines, and prosthetic pails for the body. Therefore
silicon and. its compounds are not strangers to us, nor need silicones be.
The interest in organic silicates.(me(al alkoxicles) --subject of this
work - arose due to their tendency to form highly polymeric compounds.
Although metal alkoxides have been known for many years and used in a
number of organic reactions, it is surprising how little systematic work
lias been carried out on these compounds. It is only since a round 1950. :.;•
that a rapid overall development can be discerned in the field of alkoxide
chemistry in general.
In the 1950 the alkoxides of only a dozen elements were known where as
the alkoxide chemistry of almost all the metalic and metalloidal elements
has been investigated'"' during the last two and a half decades.
Metal alkoxides have the general'formula M (OR)X, where M is a
metal of valency x and R is an alkyl group, and can be considered1'1"'1 ^ to .'
be derivatives of alcohols (ROM) in which the hydroxylic hydrogen has
been replaced by a metal (M). Metal alkoxides involve (M^'~()'v-(') bonds,
• which are" •< polarized in the direction shown due to the highly
electronegative character of oxygen.
The degree of polarization in an alkoxide molecule depends upon
the eleclronegativity of the central element (M) and the nature of these
compounds,; varies from essentially covalent volatile monomers as in
cases of electronegative elements like silicon, germanium, phosphorous
and sulphur to more electrovalent polymeric solids in the cases of
electropositive elements such as (he alkali and alkaline earlh metals as
well as the lanthanons. For derivatives of the same element, the covalent
character of the (M-O) bond increases with greater!! inductive effect of
the alky 1 group. 1 he decreasing order of molar conductivities of sodium
methoxide, ethoxide, isopropoxide and tert-butoxide in their parent
alcohol (i.e. 92.0, 45.0, 2.5 and 0.01 mhos respectively) appears to arise
at least in part from the increasing 11 inductive effect of the alkyl group.
The polarity of the. M l ) bond may also be partially offset in cases of
electrophilic metals, which undergo covalency expantion, by
intermolecular coordination through the oxygen atom of the alkoxy
tp\. This type of molecular association appears to be sensitive to stericV » ; ' ' ' •' * • •
)r&vsuch as the ramification of the alky 1 group.
|; In view of the hydroxy derivatives/of elements behaving as bases,
ifoxyacids according to the electronegativity values of the central
llment, there has been some confusion in the literature regarding the
'riienclature of these alkoxy derivatives. The alkoxy derivatives of
iements with electronegativity of 2.0 or less appears to have been
generally termed as alkoxides whilst the others are termed orthoesters. It
ias been sometimes fouild more convenient for comparison to name theL r ' ' • " . > • ' • '
palkoxy derivatives of all the elements as alkoxides.. Further in keeping
with the nomenclature, generally adopted for metal alkoxides by most of
the authors, the common names like meth'oxides, cthoxides, propoxides
(n-and iso-), butoxides (n-, iso-, sec- and tert-). It is only in the cases of
higher alkoxides, e.g: Th (OCMe Et IV) that the nomenclature is derived
strictly from IUPAC conventions.
There has been a noticeable increase in the industrial applications of
metal alkoxides, and these developments have emphasized the
inadequacy of our present fundamental knowledge of these compounds.
l.l.hObjectives oHhe Study:
The objectives of this work are:
(i) To prepare tetraethoxysilane by the direct procedure
avoiding any silanol intermediate,
(ii) • To investigate this facile reaction ol elemental silicon with
alcohol with a view of improving this procedure and to
investigate lavtors affecting reaction process,
(iii) To characterize final reaction products using appropriate
analytical methods.
1.2. Some Aspects of Silicon Chemistry:
1.2.1. Eleni-ental Silicon: .
Silicon, with atomic number 14 (Is" 2s ' 2p6 3s" 3p" 3d") and atomic-
weight 28, is second only to oxygen in abundance (27.2 wt%). Because of
its pronounced tendency to combine with oxygen, about half the earth
crust consist of silica SiO2 and silicate ( l '6). ' ;
Silicon is an analogue of carbon as regards the number of valence
electrons, but its atom is larger, its ionisation energy lower, and its
electron affinity and Polarisability higher. Therefore as an element of the
third period, it differs substantially from carbon, an element of the second
period, in structure and properties. Silicon is active at high temperatures
but sluggish in its reactions at room temperature (has a high activation
energy), its covalent compounds are not stable towards air and water the
way those of carbon are (chemists would point out that silicon16'7'81 has d-
orbitals readily available in its atomic structure and so it can expand its
bonding capacity to six instead of being limited to lour, as carbon is).
Crystalline silicon -- the most stable form - has a diamond type
cubic lattice structure and is a hard brittle substance with high
temperatures of fusion and vaporisation16' :>. :
The element is a semiconductor at room temperature with a distinct
shiny dark-grey metalic luster. Selected values for the physical properties
of silicon are represented'71 in fable 1.1.
Silicon has not. been recognized1 ' as an element until 1823-, when
Ber/.elius reduced potassium fluorosilicate with potassium and obtained a
brown powder:
. I<2 Sil r, M K •-> Okf (soluble in water) I-' Si . /
Still it was another thirty fpiif years before Devi le succeed in melting the
powder and obtained' steel-grey pellets that could be recognized a l
elementary silicon. , • .^
Silicon is a metalloid (neither metal nor non-metal, it looks like
metals, but behave decidedly different both in the physical and chemical
sense).
Elementary silicon is now produced*L6'7> commercially by reducing
the oxide SiO2 with carbon in an electric furnace at 3000C:
Si()2 + 2C : — > S i + 2CO
Silicon in its diamond lattice crystalline form is relatively
unreactive except at high temperatures, this is due to (he formation of a
thin protective layer of silica (SiO2), so oxidation in air occurs at above
95O'C\
Also a catalyst is sometimes needed to. activate elementary'silicon.
A copper-silicon alloy has been found"1 to react with hydrogen chloride
faster••than does pure silicon. Similarly copper was found to be a much
more effective catalyst in accelerating the reaction between silicon and
alkyl or aryl halides to form alkyl or aryl halosilanes as follows:
RX-i-Si . •Cii/>250^ RnSiX.».n ,
(R -- Al.kyl or aryf groii]"), X r : I lalogen)
Silicon is tetrahedraljy coordinated in most of its compounds, like
silica, hydrides and-halkles, (with mainly sp hybridization), but other
cooi'dinalion geometries e.g. tii.agonal bipyi'amidal with sp cl, octahedral
with sp'd" hybridization) are also known.
Table 1.1. Some Selected Physical Properties of Siiieon(7):
Property
'Electronic configuration
Melting point/C
Boiling point
Density (20 C)g/cnr
AMflls/KJmol -i
AHV;||1/Kjntor
Crystal lattice
Lattice constant (25 C)pm
Covalent radius in crystal/pm
Ionic radius in SLO.j" 7pm
Pauling electronegativity
Specific heat/KJmol"
1 (l) /KJmol '
1 (2)"7KJ mof
I (3)/KJmor
ViAj/KimoY1'
Ivntiopy/KJinor1
Thermal conductix it\ KJiuM'sec"
Value
[Ne] 3 s" 3:p"
1420
3280
2.33'6,
506+1.7
383+10
Diamond
ao = 541.99
117.6
26 7 ' '" T
T.8
0.1 135-253 to 1-96
0.7428- lOlo.lOOC
786
1575
3220
4350
18.93 (solid)
168.02 (gas)
83.7
1.2.2. The Nature of Bonding in Silicon - Oxygen Compounds:
r' '•••'-From chemical analysis of vast numbers of rocks and minerals over
a period, of-'a hundred years, a good idea of what is the earth crust is made
of has been obtained01. It is found that tile earth crust is 77.5% silicon and
oxygen, and even the next 22.5 of it consists of those metals whose ions
fit into a framework of silicon and oxygen to make the myriad metal
silicate rocks and minerals. These materials are polymers, with highly.
stable Si-0 backbones'6'.
According to Pauling's original scale of elect rone-gat ivity of
elements'71, /Table 1.2., silicon has a rather low electronegativity
compared with the elements. As a result of this it is expected that silicon
would be the most electropositive in many bondings even for Si-M, Si-C,
Si-1 bonds.
Table: 1.2. Selected Values of Flectronegativities on Pauling Scale,
"; 0(3.5)H(2.1)— —
Si(7.S)--------
As(2.0) • Se(2.4)
(4.0)
Hr(2.3
1(2.5)
Silicon with its electronic configuration of |Ne| 3s~ 3p'~ 3d
resemble carbon in forming predominantly tetracovalent compounds with
main!)' sp ! hybridisation. However, the presence of 3d-orbitals affect the
chemical behaviour of silicon in increasing the coordination number
beyond four.
In all tetracovalent silicon compounds, the silicon - oxygen bond is
caused by the o bonding of tlie hybridised s and p electrons ofthe silicon
atom with the p-electrons of oxygen and the additional ^interaction of
the unshared p-electrons of oxygen with 3d-orbitals of silicon which has
been termed (Pn - d j conjugation, (see Fig. 1.1.).
Vacant Si 3ds
orbital
filled oxygen
Multiple bond
Fig. 1.1. (l\ --tln) bonding in the (Si ()) bond.
Ti-bonding of (Si-O) should also be expected1'1 lo inlluence the
chemical reactivity of Si-0 compounds. The basicity of the .oxygen in
siloxanes was reduced by such 7r-bond'mg.
The (Si-O) bond length of majority of organosilicon compounds
('fable 1.3.) equal to (I64±3pm), this value is much smaller than the
calculated value of the Si-O bond length (183pm), which was calculated
with addition ofthe atomic radii of silicon (117.7pm), and that of oxygen
(66pm), this indicates a partial double bond character which arises as a
result of (IV; d;t) interaction.
Table 1.3. Selected Values for the Si-O Bond'Length in sonic• • • • • . • • , • - • • • • • • • * • . ' • • • • • • • • • • • • • • ' • '
Grganosilicon Coinpounds(9):
Compound
(H3Si)2O
(Me3Si)2O
Si(OSiMe3).,
Et2Si(OH)2
Si(OMe).,
(F3Si)2O '
(Me2Si0j3
Me2Si(OII)2
PhOSiH3
Me3SiOM
SP = Spectroscc
e = Electron dif
X - X-ray diflVf
M = K, Rb, Cs
Bond length pm
163.4
163.0
163.0
163.0
164.0
185.0
166.0
163.0
164.0;
160.0
>pic.
Vaction.
iction.
Method of determination
SP
• . . e .
e
e
e
•• ' e
e
X
e
Structural evidence indicates that it-bonding takes place in silicon -
oxygen compounds, the-'-vSi-.O-Si x - bond angle is larger in disiloxanes
(140-160) than would be expected if only signia bonding of normal
(sp or sp"") h)'bridisai ion were taking place at oxygen.
The Si-0 bond is one oft he most stable bonds'''S) (see Table I A.).
The greater energy of formation of Si-O bond is due to existence of
(p;t (i.[) interaction.
Table 1.4. Bond Energies of some Silicon and Carbon Bonds(8):i
. ft
Bond
Si = Si
1 S i - S i
Si - H
S i - C
S i - 0
S i - F
Si - Cl
Bond Energy
Kcal/mol
78
54 ' ,
76
• ' 7 3
111
143
' 96
Bond
C = Cz^i f \L — L
c-c.0-11
c-o.C-F
C-Cl
Bond Energy
Kcal/mol
200
146
83
99
86
117
78
It is found that the strong 7i-bonding in organosilieon compounds is
a dative 7i-bonding.
The oxygen atom uses only one pair 'ofthe two unshared pair of
Tc-electroils to occupy only a single 3d-orbital of silicon.
Generally speaking, the silicon-carbon bond is reasonably stable11"1
(the order of energies B - O Si-C> Al-C is inversely related to the order
of polarities and susceptibility to attack); because of this certain
organosilanes and their derivatives have found commercial usage.
Probably the- most important class of organosilanes consists o f the
organosiloxanes, commonly known as silicones. These materials are
polymers with highly stable (Si-O) backbones.
1.2.3. Structure of Hie Silicates:
Organosilicon compounds with Si-O-Si bonds may show1'" arrange
of structural features. Silicon, unlike carbon, almost never forms double
bonds -; chemists o n l y recently succeeded in making a double-bonded
compounds of silicon by special and complex methods -.
Id
. The basic chemical .unit' of silicates is ( l l ) the.(SiO.))'tetrahedron
shaped an ionic group with a negative four charge (-4). The central silicon
ion has a charge of positive four while each oxygen has a charge of
negative two (-2) and thus each silicon - oxygen bond is equal to one
halfC/2) the total bond energy of oxygen. This condition leaves the
oxygens with the option of bonding to another silicon ion and therefore
linking one (SiO.4) tetrahedron to another and another etc..
The silicates tetrahedra form complicated structures. They can
form as single units, double units, chains, sheets, rings and framework
structures"""'.So we must picture each silicon atom forming ' single bonds
to four separate oxygen atoms, with each oxygen atom linked to two
separate silicon atoms, thus:
; ' • • ' • • ( ) • . . ' 0 " :
V
Si
" 0 ; ( ) • • •
Actually, in three dimensions the silicon sits at the '. center of a
tetrahedron, 'with'oxygen" at each of the four corners. This is true of
almost all metal silicates. Obviously the silicon oxygen teterhedra can
bond to each other, and indeed must do so in the soikl silicates, for the
oxygen atoms are bivalent and 'nuis t attach to two silicon atoms. This
leads to chains and' rings of linked (Si-O) telrahedra, and even to
continuous sheets of such linked tetrahedra.
As for the mineral silicates [there are also non mineral silicates
Which are volatile liquids; such as ethylsilicate, Si (OC2Il;0i, b.p. 1 68.5 C
I I
fthese are esters of orthosificic acid, Si(OH)4], they are of several distinct
| t y p e s : ' - " - - ' • * • •I ? . • • • • ' • . ' •
' l . Silicates witlr.discrete negatively-charged silicate ions, as in:
a. Those in which there are discrete orthosilicate ions, SiO.i'1" (no
oxygen atoms are shared by other silicate tetrahedra; all four
; .charges are balanced by positively-charged ions of metal such
as Na . K . Ca~\ Mg" etc.). The gem mineral, zircon Zr StOi,
is an example.
b. Those in which there are discrete disilicate ions, SiiO7(>", in
which two silicate tetrahedra share one coiner.
c. Those containing cyclic polysilicate ions in which three or
more tetrahedra share two corners, as in:
• O / O \ O
Si "Si
Q
o 02. Silicate with infinite chains of tetrahedra, each sharing two corners
with the outside oxygen atoms bearing negative charges:
O" O" O' (J
: Si . •• . Si ;
/ \ / \ • •
( ) • ( ) ' O O •'(.) ' 0\
( )"
Si
()"
' ' \,/ N
0 0" (Y
Si/ \/ \
0
12
Such ions and the. ones in 1 .£., have the average composition (SiC).-}),,2""
are all tailed metasilicates.
3. Silicates in which^the tetrahedra share three corners, leading to flat
sheets of alternate silicon and oxygen atoms. This conformation is
typical of the layered minerals. Such as the clays and micas.
The different ways that the silicate tetrahedra combine is what
makes the silicate class the largest, the most interesting and complicated
class of minerals, and also give them their characteristic properties which
allow them to be used widely.
1.3. Organic Silicates:
1.3.1. Physical Properties of Organic Silicates:
Organic silicates exhibit"''great differences in physical properties
(see 'fable 1.5.) depending primarily on the. position of the metal in the
Periodic Table, and secondarily on the alky 1 group.
Table 1.5. P
Alkoxiile
LiOC2II5
NaOC2H5
KOC2H5
Mg(QC2ll5)2Ca(OC2H5)2
Ba(OC2H5)2
U(OC2U5)5
Ce(OC2H5)4 .
Sn (OC2I-I5)4
u(oc2n5)6
hysieal Properties !
Colour & PhysicalForm
White solid
White solid
White solid
White solid
White solid :
White solid
Dark-brown liquid
Yellow solid
White solid
Dark-red liquid
Some Metal VA
ni .p .C
20-24260250270270270—200Un meltable
hoxidcs(l
b.P:c•/pa1'
n.d.
n.cl.
n.d.
n.cl
n.cl.
n.cl.
1 60_.n.d72/0.13
3):
Solubility inOrganic Solvents-t-
+
+—1-
-1-
b = to convert Pa to mml Ig divide by 1 33 .3 .
n.cl. ;^ ncit disti lable.
d = less soluble.
The alkoxyderivatives of metals have at least one (M-.O-C) system.
Due to*the strongly electronegative character of oxygen (electronegativity
Value, 3.5 on the Pauling scale), alkoxides of metalic elements exhibit
strongly polar character'2'. ThulM-0 bond in these derivatives could be
expected to have about 65% ionic character for metals with
eleclronegativity values of 1.5 — 1.3 (e.g. Al) to about 80% for more
electropositive metals with electronegativity values of the order of
1.2 -0.9 (on Pauling scale) (e.g., alkali metals and alkaline earths).
However, most of these alkoxides show a fair degree of volatility and
solubility in common organic solvents; properties which can be
considered as, characteristic of covalent compounds. The two factors
which have been postulated^' ' for explaining the attenuation in the
polarity of the (M-O) bond, are the inductive effect of the alky 1 or aryl
groups at the oxygen atom (this increases with the branching of the alkyl
chain) and the formation of oligomers through-dative association of the
type:
• < /
M ' • *
O
The latter tendency is expected to decrease with the ramification on
the alkyl group due to steric factors.
The .applications of more sophisticated spectroscopic and magnetic
techniques have thrown clearer light on the structures of organic silicates.
Physical characteristic of metal alkoxides are divided into the
following headings:
14
1.3,-1.1. Volatility, Molecular Complexities and Therinoclynamic
Data:
Volatility of m^tal aikoxides depends mainly upon three Factors.
(i) Molecular size and shape of the alkoxide group:
For homologous series of monomeric aikoxides M(OR)X, the
volatility decreases with increase in n-alkyl chain length
whilst in an isomeric series of monomeric aikoxides,
branching of the alkyl chain may lead to small increase in
volatility due to the effect of the shape of the molecule on its
intermolecular forces. In the case of oligomeric aikoxides
•[M(OR)x]n, the volatility decreases due to its greater size and
intermolecular forces.
(ii) Nature of the central metal atom:
•The size of the central metal atom influence volatility and
, •• molecular complexity of the aikoxides. (volatility increase
with the decrease in size). '
(iii) The nature of (M - 0 - C) bond:
The chain branching of the alkyl group affectsnot oim' the
degree of polymerization, but also the electron releasing
tendency which makes the (M-() - - • C) bond less polaj4 and
tend to increase the volatility of particular alk||xide
••derivatives. Table 1.6. show1 ' the influence of branching of
the alkyl group- on volatility and complexity using titanium
and zirconium amyloxides as examples.
Tabie 1.6. Boiling Points and Complexities of Titanium and of Zirconium(13):
- CH--
' -CHr-
-CH;—
i -CH :-
-CH (
RinM(OR)4
-CH-.-CH.-CH.-CH,x/CH3
CH, CH-
/ C H ; C H ,C H "
^ C H v/ CH,
C —CH3
" ^ CH?
^ CH:—CH?
v CH2—CH3
/ C H ; - C H : - C H 3
Titanium
: Bp.C /pad
I . - ' • •175/80
: ;.• 184 /10
154/50
105 •'?
112 '5
i 135/100
Alkoxide: - Molecular:"' complexity! 5.4; 1 . 2 ' . - • •i
1.1
i • • ; ' - . 1 . 3ii •
i 1-0
i . o - '
ZirconiumBp,c7pa
a
255/1247/10
238/10
188/20
178/5
175/5
i Alkoxide .M o l e c u l a r •'•'
- c o m p l e x i t y ••..3.2
• i - i -•• • -
•• J . J •
I
3,7 "
• 2 . 4 '• /
2 . 0 ' •
2.0
-C HCH-,
CH,-CH, - CH?
CH,
• - 9 8 , 1 0 1.0 95/10 .0
a: To convert Pa to mm Hs divide bv
•It .has been establishe(^(l4) that the structure and volatility of the
alkoxides of titanium and zirconium are governed by the configuration of
the alky 1 group.
(A) Alkoxides of the alkali metals:
They are ionic in character due to the strongly electropositive
nature of the alkali metals. Sodium ethoxide has been shown to
behave as a strong base in ethanol,
(B) Alkoxides of group (II) elements:
Their primary alkoxides derivatives are generally non-volatile
compounds. Where as their secondary and tertiary alkoxides tend
to be Comparatively more volatile and soluble in organic solvents.
(C) Alkoxides of group (III) elements: ,
Aluminum alkoxides are thermally stable and even in (he lowest
member of the series. . '
[AL(OMe)i] may be sublimed with difficulty at 240 C under high
vacuum. The higher alkbxides are all soluble, distillable prodiicl
and the "'melting points of the solids increase with increasing
ramification of the alkyl chain.
In view of .'the comparatively lower values of entropies of
vaporization of aluminum alkoxides, il appears (hat they might-be
associated in the vapour': phase also, it has been shown that Al-
isopropoxide is dimeric in the vapour phase while it is trimcric in
solution. This special behaviour of aluminum alkoxides and'the
stability of bridges even in the vapour phase was explained16'by
Mehrotra-on the basis of electron delicient nature of tricovalently
bonded aluminum atoms.
17
Y '• ' AL ' AL
The variation in solubility and melting points ofalkoxides is due to
the extent of hydrolysis which depends on the percentage
composition of the alkoxides.
(D) Alkoxides of group (IV) elements:
Silicon alkoxides are highly volatile and most of their derivatives
distilled unchanged at atmospheric pressure, e.g. tetraethoxide and
tetraphenoxide distilled at 168 C and 41 7 - 420 C respectively.
And the low boiling points of silicon alkoxides are due to(2) their
monomeric nature irrespective of the chain length and branching of
the alkoxy groups.
Metal alkoxides are1 'colored when the corresponding metal ions
are colored, otherwise not.
The alkoxysilanes generally have sweet, fruity odors that-become
less apparent as molecular weight increases.
Tetraethoxysilane is; a colourless mobile liquid, with a plaasant
ester like odor, flammable, irritant, also possesses highly toxic
properties. Some physical constants of tetraethoxysilane are listed
i n T a b l e 1.7.(7).
Table 1*7.,Physical Constants of Tetracthoxysilane(7):
Property
Density gm/cni
Boiling point
Refractive Index
Heat of Combustion
At 25C, p30 atmo.
Specific heat. Capacity at 60C1
Heat of formation
Autogenous Ignition temp
Molar Entropy of vaporisation
Silica Contents w/w%
Value
0.933
1-66C
1.3834
-1322.9±2.2
Kcal/mol
2.41KJKg"K\
-318.9+2.2 Kcal.mol
235 C
121.8KJmoF
28' 1
1.3.1.2. Structural Aspects:
Although direct structural evidence for metal alkoxides is not
available expect in a few isolated cases, important properties like tiegree
of association, volatility and reactivity with different reagents throw
considerable light on the possible structural features of these derivatives.
Bradley, in 1958 proposed'"' a simple structural theory according to
which alkoxides 'derivatives adopt the smallest possible structural unit
consistent with all atoms attaining a higher coordination number.
Furthermore, the coordination number of the oxygen atom should not
exceed four and therefore the sterochemist'ry of the alkoxides was
proposed.
In recent decades, much work has been done on the structure of the
metal alkoxides113'11 A?>. The simple alkali alkoxides have an ionic lattice
19
and a layer-like structure,' but alkaline earth alkoxides show-more .
tova-lent character.
The* aluminum alkoxicfes have been thoroughly studied and there is no • $
doubt as to their covalent nature, the lower alkoxides are cyclic, even in
solution and in the vapor phase.
The aging phenomenon observed*2' was explained by Bradley who
considered it to arise from the tendency of tetrahedral AL to change to
octahedral configuration
1.3.1/3. Dipole Moments:
The ease of hydrolysis of metal alkoxides leaves less scope for (he
measurement of their dipole moments.
The dipole moments of a number of Si tetraalkoxides -were measured1"' by
Bradley who concluded that the free rotation of alkoxy groups a round
M O bond was possible m the case of Si. Also the dipole moment of
variety of Si phenoxides have recently been measured by Bradley, who
concluded that the free rotation of the phenoxy groups a round Si O bond
depend on the steric factors.
1.3.1.4. Densities, Viscosities and Surface'1 ensioiis:
These constants are helpful in identifying the products. It has been
observed that the density, viscosity and surface tension are dependant on
temperature.
fable I.(S. Show'"1 the densih and surface tension of some alkoxy
lienvatives of silicon
20
Table 1.8. Density and Surface Tension of Some Alkoxysihme .
Derivatives(2):
Si(OMe).,
Si(OEt).,
Si(OPrn),<
Density g/ciii
.02804
0.9320
0.9158
Surface Tension dynes/em
21.67
23.58
1.3.2. Charactrizatioii Methods:
1.3.2.1. Infra-red Spectra of Organic Silicates:
Infra-red spectroscopy has been utilised'2"'1 to support the identity
of metal alkbxides by observing bands characteristic of the bonded
alkoxide groups (e.g. M O and C--0 stretching vibrations). ;
Thus the \' (C--O) band in metal alkoxides appears at 1070, 1025, 1375,
1365, 1170, 1 150, 980 and 950; for melhoxides, elhoxides and
isopropoxides. . • .
The vibralional spectrum of (CNI lsO)4Si is well known and the
assignments have been made" '''Sl (see Table 1.9. and l;ig. .12.),
\'as(Si O) and vs(vSi-;O) bands in Si tetraalkoxides appears in the range
720 - 880 and 640 • 780cm'1 respectively.
21
Table 1.9, Vibrational Spectrum of Ethoxypo!ysiloxanes(17):
Frequency cm"1 & Intensity
478 m
656 w
789 s ,
962 s
1080vs
1 102 vs
1163
1291 m
1391 m ,
1 4 4 1 w . •''•
1456 w.
in r:- moderate.
\v -- weak.
s r-r- strong.
vs ^ very strong
Assignment
OSiO, SiOC and CO deformation
SiO4 symmetric frequency
SiO_t asymmetric stretch frequency
C - C stretch frequency
C - O stretch frequeiicy
CH3andCH2 internal
and external deformation
Wave-number (em1)
4000 ,3600 -3200 .2800 2400 2000, . . ; i ;•• , / ' i ... . . . . . . i . • . ' . i i i—!_J
1800 1600 1400 1200 1000 S00 600
Si-O-Cstretch antisymmetric
stretch .
Fig. (1.2.): Assigned Infra-Red Spectrum of Tetraethoxysilane.
1.3.2.2. Nuclear Magnetic Resonance Spectra of Organic
,Silic'ates(NMR):
The NMR spectra of only a few. soluble tertiary alkoxide , '
derivatives of alkyl magnesium have been recently studied12'. .For a
tetrameric methyl magnesium tert-butoxide in benzene, the (wo broad'
signals at 8.06 and 5.94 are due to methyl magnesium and tert-butoxide
protons respectively. The NMR spectrum of molten Al-isopropoxide was
measured at 198 C showing'the presence of a pair of signals.
The PMR spectra of alkyl silicon alkoxides R.^SiCOR),, had been
studied'-3"6' in" detail to substanciate the presence of Vn-dn bonding
between (Si-O) bonds. It has been observed.that when methyl group of
tetramethoxysilane is replaced by an ethoxy group, the " Si resonance
shift down Field from -12 to-27-ppm and the silicon methyl protons from
Zero to 0.005 ppm, indicating that the electron withdrawal being greater
than (he it-bonding contribution. The replacement of two methyl groups
of tetramethoxysilane by two ethoxy groups however, shifted the 29Si
resonance towards high field from -2 to 16 ppm and the methyl protons
from Zero to '-0.022 ppm, indicating the increase in the election density
on silicon atom..
The I l'NMK spectrum of tetraelhoxysilane is well established'7'1^.
The 111 chemical shift of tetraethoxysilane was obtained"''1 (see Fig. 1.3.).
'fable 1.10. show the II'NMR spectra of the product of the reaction
between etliaiiol and silicon1 '.
Table 1.10. H'NINIR Spectra of the -Product of the Reaction Hehveen
Kthaiml and Silicoii(7):
The protons
<CH.,)t.
ici
C h e m i c a l shift p p i n1.19 ~3.75
I n t e g r a t i o n f a c t o r : - 2 : 3 . i : t r iplet , CJ= qua r t e t .
Fig. (1.3.) I OOMEz H NMR Spectrum of Tetraethoxysilane (7)
• .fj f - 1 , i .
0 H -
o(CH3) =
i. !-' ( " ' I • I
1.3.2.3. Mass Spectra of Organic Silicates: •
The "recorded*I7J mass spectra of triethoxy and tetraethoxysilanes
suggested that the common features of the decomposition were loss of
proton, alkyl or alkoxy group from the parent molecule.
In some cases, CH3 is lost' 3) from an ethoxy group of (Col IsO)^Si, then —
CH2 and finally O.
The mass spectra of tetraalkoxysilanes (RO).|Si where R is C| to C5
alkyl have indicated common features. The fragments obtained due to the
loss of a proton or alkyl and alkoxy group from the parent molecule give
the parent molecular ion and a free radical, according to the following
(1-6) decomposition pathways:
(1) (RO).,Si ->(RO).,Si'1 HKX
(2) (RO)3Si - [O(CM2)MMe] -> (RO)3Si O C! U' + Me(CIl2)"n-i
Olefmic and aldehyde elimiiiation occurs after the primary
decomposition:
(3) R, R,
(4)
•I
Si R,
Si — I I ' c = 0
v -.- o
1 2 1 1 . • I
Si Si
OClh
26
(5) Rearrangement of (SiOCH2) to (Sill)
... O
Si C ——>- Si" - H + IICO"
H II
(6) Rearrangement of(OSiOCH2) to (SiO.11)
O 1-1 II
Si : C > Si' 0-H-i-ilCO"
: • • ( ) ' ' .
The principal ion peaks and decomposition products required in
(5) and (6) path ways have been clearly identified.
1.3.2.4. Gas - Liquid Cliromatography of Organic Silicates:
Gas liquid chronatography (GI.C) has been used to separate
mixtures of.tetraalkoxyislanes.
, GI..C may be considered to have replaced the traditional fractional
distillation method in the initial separation and analysis of
tetraalkoxysilanes. Nevertheless, problems with respect to the full
separation of all individual alkoxysilane members still exist especially for
high molecular weight components. Generally the abundance of each
component detected in the mixture analysed progressively decreases as
the molecular weight of the component increases' ~(".
The most important alkoxysilanes separations can be made on
pohdimetliylsiloxauc stationary phase or an organic grease such as a
pie/on I . A standard ..! meter column or fluid (SF.--30 or ()V-~1()1 ) on
chiomosorb \V pro\ ides adequate separation of most common
alkox;si lanes'
11
However, using GLC analysis sometime presents encountering
problems and is not a perfect and reliable method of analysis. For more
accurate.data a comparative .GLG-mass spectra is required.
The GLC results of final reaction mixture, where the catalyst was a
solution of magnesium ethoxide have generally indicated'7'20 the presence
of ethanol, tetraethoxysilane and the lower siloxane oligomers, (see 'fable
1.11. and Fig. 1.4.).
Table 1.11. GLC Analysis of the Product of the Reaction Between
Ethanol and Silicon metal Using Magnesium Ethoxide
Catalyst(7):
Peak Nuiiibei
2
3
4
Assignment
Ethanol
Tetraethoxysilane
1 lexaelhoxydi siloxane
I lexaelhoxytrisiloxane
Hexaelhoxylelrasiloxane
Fig. (1.4.) GLC ol the Product of the Reaction Between Ethauol and
Silicon i\Ietal(7).
29
1.3.3* Chemical Properties of Organic Silicates:
Alkoxysilanes are characterised by the presence of (Si-'-O-C)'
functional groups, which show similar common reactions depending on
the nature of the alkoxy group and substituent.
Alkoxysilanes are very reactive species which may be due to the
presence of electronegative alkoxy groups making the metal atoms highly
prone to nucleophilic attack. The metal alkoxides are, therefore,
extremely susceptible to hydrolysis by atmospheric moisture and require
careful handling.
Metal al'koxides readily react with excess . of hydrogen halides or
acylhalides giving the metal haiides. However, by using stiochiometric
amounts of these haiides, the metal halide alkoxides may, be prepared.
Alkoxides readily react with the protons of a large number of organic
hydroxy compounds such as alcohols'"'"', glycols'""'" ', carboxylic acids,
hydroxyacids, (3-diketones, alkanolamines etc., containing reactive
hydroxy groups with the replacement of the alkoxy group by the new
organic ligand:
:. M(OR)X I xllOX -> M(OX)X + xROH
These reactions are quite versatile, and appear to be subject mainly to
kinetic factors, e.g., the reaction with highly ramified alcohol arc
generally slower and ma)' be even stcrically hindered in some cases.
A l . ( ( ) C 2 l l 5 h ( 2 C 4 I I . / O H ' • a t ' i i t > A L ( ( ) C 2 l l 5 ) ( ( K : , l | i ; ) > i 2 r , I I 5 ( ) I I ,
2 A L ( ( ) C 2 l l 5 ) ( ( ) C 4 i l . / ) 2 - M C \ , H l / ( ) l l i > i o v L > A L 2 ( ( ) C \ I 1 5 ) ( ( ) C ' . , 1 1 , / ) 3 I C 2 1 I 5 ( ) I I .
2 A I . ( ( ) C M I O ( O C , I f.) l),-i ( ' , ( f / O I I - l l ^ ^ ^ [ A I U O C i l I . , 1 ) , ] ' - I - 2 C M | , O I I.
Also the alkoxides are sometimes reactive towards other molecules
having reactive protons such as those having -Nil or-SI I. In these cases,
the reactions are controlled by thermodynamic factors and arc governed
30
by the comparative1 stability of (M-O), (M-N) and (M-S) bonds.
Metalalkoxides sometimes behave as weak Lewis acids forming
coordination compounds .with suitable ligands, although, in general, the
metal atoms in alkoxides prefer to attain the higher coordination state
through intermolecular alkoxy bridge formation rather than by
coordination with an external reagent.
Also the unsaturated substrates like A=B readily insert a cross the
(M-.O) bonds of certain metal alkoxides with molecular.re-arrangement
resulting in the formation of insertion products12'.
It was established*2'^ that Bu3SnH effectively catalyzed the silicon
hydride mediated reductive cyclization of enals and enones. Employing
Bu3SnH as a catalyst for this transformation requires the use of
stiochiometric a mount of a second metal hydride capable of regenerating
Bu3SnH from tributyltinalkoxide.
C a t . H113S11I I, 0 . 5 P h S i l l. •w
r ad ica l in i t i a to r 0 1
I ' t O H , Phi l o r t o l u e n e , A
S i l i c o n h y d r i d e s reac t w i th a l k o x i d e s to afford tin h y d r i d e s and silyl
ethers, provide (lie basis for a new catalytic process1"""*"26'.
1.3.3.1. Hydrolysis of Organic Silicates:
All metal alkoxides- so far investigated1" have been characterized
by the ease with which they are hydrolysed.
In many cases the alkoxides are so sensitive even to traces of
water, that very special precautions have to be adopted in order to study
their properties. When restricted amounts of water are added, these metal
alkoxides undergo partial -hydrolysis reactions yielding in some cases
products of • definite composition called metal oxide alkoxides
.11
MOn (OR)X. Arid when an excess of water is present, the ultimate product
is the metal hydroxide or more commonly the hyclraled metal' oxide, and
these compounds, which are polymeric, are extremely interesting, from-
the structural point of view.
Hydrolysis of metal alkoxides using pure reagent is slow, therefore,
and acid or base are usually used(2'2930) as catalysts.
M(OR)X + 112O H7OIT ^ M(OR)X., 011 + ROM
Catalyst |[H20
• • ' . • . , . . M(Ok)x.2--(Ol-I)2 i RO11
Hydrolysis is usually followed by condensation polymerization. So
higher temperatures, longer times, higher acid/base concentrations and
higher 1120/M(0R)x ratios, all tend to shift the molecular size to higher
values'311. Also on the basis of the water consumed during the hydrolysis
reaction of Ti(OHt).t, it was concluded'2' that the alkoxide derived from
aromatic alcohols are more resistant to hydrolysis than the corresponding
normal aliphatic alkoxides. The resistance to hydrolysis increases with
the increase in length of the alky! chain.
The proposed mechanism of hydrolysis of metal alkoxides involves
the coord ina t ion <.if water molecu le •through its oxygen to the metal in a
facile nucleophilic process: '
11 11
\ • • v \
( ) : - > M ( 0 R ) , • > . (.): - * M(()RK-i •-> M(OI I)(OR)X . | -i-ROI 1
11 11 :0-R
32
£.. Acid Catalysed Mechanising '.
= M - OR * iI3OB:.-4- = M - OR *-> = M ~ Oil + NOR I 1 IB
, V ; HOHHB
I HB
or: H,O + = M - OR + MB -> HoO: M-OR -*= M-OIH1IOR + 1 IB
. / \
B. Base Catalysed Mechanism*32':
H O - M =
R O " ' • »
One of the protons on the water molecules interacts1" with the oxygen of
an alkoxide group through hydrogen bonding and, Ibllowing an electronic
rearrangement a molecule of alcohol is expelled. The hydroxy metal
alkoxides formed may react further to form the oxide alkoxide by either
of reactions (2) or (3):
(2). M(OI D(RO)V| i Mf OR)X - > (Up),., MOM(OR)vl -I- RO11
(3). •2M(0il)(R0)v,->(RO)V, MOM(OR)X., .-i-N2O
1.3.3.2. Hydrolysis and Condensation ()t'rretraethoxysilane:
The stability of some tetraalkoxysilanes to atmospheric moisture
was studied'7'.. It is found that tetraethoxysilane (TEOS) undergoes
hydrolysis after two months. Hydrolysis of tetraethoxysilane occurs
readily in tire presence of acidic or basic catalysts, but due to the
immiscibility of water and IT-OS, a mutual solvent such as alcohol and
ketones is often used to obtain homogeneous mixture.
Base catalyst: r
I.!,('f:2H5n)38j7.OC2H5 < OH / N [(C2H5O)3-SiOH -OC2H5.P
V * \ p =^(C 2H 5O) 3 SiOM + C2H5O"
2. C2U5O- + H2O -> C2H5OM + 011"
Acid catalyst:
1: (C 2 H5O)3Si -OC 2 H5+H 3 O + ^ = ^(C2H5O)3 Si l l ' - OC2MS+ M 20
- = ^ ( C 2 I - I 5 O ) 3 S i — 0112 -i- C2115Oil
2: (C2I-I5O)3 Si-OH2 + H 2 0 -» (C2I-f50).i SiOH +11.,O'
Condensation of Hydrolysed Species:
l:(C2ll5O)3SiOIJ+C2H.5OSi(OC2H5)f==^(C2IIsO)3SiOSi((X;2Il5)rK12lI5
2: 2(C2 lf5O)3SiOri,—^ (C2I {5O).vSi O - Si(OC2l l5)., 1 -I I2O
When (Oil) is more than one in the hydrolysate molecule, gelation
occurs129'32' through the cross-linking bfthe groups at each silicon atom.
1.3.3.3. Formation of Double Metal Alkoxides:
A large number of double metal alkoxides involving more than one
metal atom within the molecular species were synthesised(2J'" during the .
late twenties by Mcerwein and Bersin.
However, the apparently covalent behaviour ofdouble alkoxides of
•strongly " .electropositive elements (e.g., alkali, alkaline earths and
lanthanide elements) has attracted special attention. Alkoxides-of strongly
electropositive elements like alkali metals has been reported to behave as
strong bases particularly in their parent alcohols, similar to the reactions
of alkalies like caustic soda with amphoteric hydroxides like;Zn(OH)2 in i
aqueous medium to give (hydroxo salts) of the type Na2/n(011).|.
Meerwein and Bersin reported that the titrations of strong bases like alkali
alkoxides with alkoxides of less electropositive elements, like Zn and AL, '
carried out in' parent alcohols, benzene or nitrobenzene gave end points
(using thymolphthaiein as indicator) corresponding to the formation of
derivatives of types Na2fZn(OR)4], or Na[AL(OR).]] which were termed,
"alkoxosalts" corresponding to "hydroxo slats" in aqueous systems.
The formation of double alkoxides may be considered to arise
partially from the mutual neutralisation of acidic and basic alkoxides and
partially from the tendency of the metal to form coordination complexes.
Thus if two metal halides or alkoxides having different electropositive
character, but capable of increasing their coordination number are
allowed to react under suitable conditions in the presence of the parent
alcohdl, the formation of double alkoxide derivatives may be achieved.
A novel yttrium-copper double alkoxide clusters - which act as
good candidates for sol-gel processes - have been synthesized'2-' recently.
1.3.4. Uses of. Organic Silicates:
The uses of metal alkoxides depends on their chemical reactivity in
common organic solvents'2'. The chemical'reactivity is manifest in the
variety of catalytic applications of the alkoxides ranging from redox
catalysts (Aluminium alkoxides), to accelerators for the drying of paints
and inks. Ultimately the alkoxicles are valuable precursors of high purity
metal oxides through hydrolysis, pyrolysisor combustion.
Aluminum alkoxides and other organic aluminum compounds are used as
dryers in paints.
Silicon alkoxides copolymers are used as protective film forming media
and in thermally stable inorganic polymers. Dihydrido carbonyltris
(triphenylphosphine) ruthenium (Ru) catalysed copolymerisation of
disiloxane compounds'1''. .
Alkoxides .of aluminum could be used for water-proofing of textiles'""0'.
Monomeric organometalic precursors may be converted through
hydrolysis reaction into gels and ultimately glasses or ceramic* "'(>' '.
Tetraalkoxysilanes are used for the preparation of ceramics by the sol-gel
process^ . . . . ' ' • . '
Tetraethoxysilane is an important industrial material, it can be used as
heat transfer fluid, as an electric, coolant, and can be hydrolysed under
controlled conditions to form hydrolysates which act as binders for
refractory grains. This latter applications is particularly important in the
production of precision cast material for ceramic and foundry
applications1"' '. ,
A variety of carbocyclic derivatives of silicon' which was
biologically active have found a medicinal interest'V)|.
Purified methyl chlorosilane are used to prepare the various methyl
silicplie resins, oils and elastomers.
Trichlororsilane HSiCL.i is the preferred source of hyperpure silicon for
the transistor and integrated circuits which go into every radio, television
and telephone appliance and which are the hearl of every computer
system"1. • '
1.4. Preparative Methods:
•Organic silicates can be prepared by a number of selected methods
These include the following: the halosilane route, the exchange route and
the elemental route.
1.4.1. The Ilalosi lane Route to Tetr . inlkoxysihines:
Tetraalkoxysilanes can be prepared by (he reaction of halosilane.
with the corresponding alcohol as follows:
' Sf X i i KOI 1 ->-.S.i(OK).i i -t NC'lt
Where R is an alkvl or aryl group, and X is a halogen.
When anhydrous ethaiml is used 'the product is telraethoxysilane, bu!
when1 industrial spin! or aqueous ethanol is used, the product is technical
36
ethylsilicate(7'13'l9'29). Technical ethylsilicate is a mixture of
.ethoxypolysilbxanes, comprising hexaethoxydisiloxane and. the higher
oligomers. . T-he reason that ethylpolysilicates are formed is that the
condensation-polymerization reactions occur in the presence of small•i
amounts of .water in the reaction mixture which are catalysed by (he
hydrogen chloride by-product.
Lower yields of alkoxysilanes, particularly with secondary and
tertiary alcohols have been related to various side reactions, for example,
lower aliphatic alcohols are rapidly attached by the IIX by-product:
ROH + HX-^RX + M:,0
Where as water in the reaction medium leads to polymerization of the
product: , . • , .
( R O ) , : - Si - ORi- RO Si (OR)., __JJ2O____.._ _ •;IIX act as catalyst
OR ORI I
RO - S i - O S i - O R I 2 ROM, etc.' • • • I - I
OR OR
l,n order to obtain higher yields -of monomer product, there have
been many attempts concerned with the removal of hydrogen chloride, for
example, hydrogen chloride can be driven oil by blowing a stream of dry
air or nitrogen through the reaction- mixture, alternatively hydrogen
chloride acceptors such as pyridine or dimethyamine can be introduced
causing the hydrogen chloride to be precipitated and then removed by
filtration. Also the yield is increased by addition of small amount of an
organic solvent such as chlorinated hydrocarbon and followed by
neutralisation with sodiuni ethoxide (C\l IsONa) before distillation'729'10'.
Tetramethoxysilane has been prepared'7'4" by treatment of an
ethered slurry of sodiuni methoxid.e (which reduce the solubility of MCI
37
in the reaction mixture) with an alky] chlorosilane and for which yields of
50 - 70% have been reported'7*. Partial hydrolysis products are formed in
considerable a mount due to, hydrogen chloride which liberated in the
primary step.
Other alkylorthosilicates including ethyl, n-propyl, n-butyl, n-amyl,
iso-butyl,; secrbutyl and 2-chloroethylortho-silicates were prepared using
tetrachlorosilane and the corresponding alcohol and their properties were
studied'7'. It was noted that the vigor of the reaction was greatest for
alcohols containing +1 or —I inductive effect and decreased for normal
alcohols as carbon lengthened.
Due to steric reasons it was found that1'12' it is difficult to synthesize
tetra-t-butoxysilane from chlorosilane and the corresponding alcohol, but
in the presence of pyridine, tri-butoxychlorosilane results;
A similar process was reported using silicon tetraehloride and
sodium-t-butoxide in refluxing petroleum ether, again the end product
was tri-butoxychlorosilane. However, the fourth chlorine can be replaced
by an ethoxy or isopropoxy group. Also tetra-t-butoxysilane has been
prepared,1'13', using tetrafluorosilane and sodium-t-butoxide or tertbulyl
alcohol. .
Other tetraalkoxysilanes were prepared by using tetrafluorosilane instead
of tetrachloro, according to the equation:
4ROH-I Sir, :>Si(OR), -I 41II-
(R - C21U, Bu, C(111,,, HLIC21 I3 CM CII2 and ph).
The synthesis of tetraethoxysilane by the reaction between ethanol
and tetrachlorosilane was a perfect method* "' for over a century in
laboratory and industry as illustrated by the following equation:
Si Cl., i-.4C\I Is ()l 1 T'y'M11^ Si (OC2I Uh -I- 411C1
38
The yield of tetraethpxysilane was 79% or 85% when the reactants
were added at very low temperature (0 - 2 C). It was found that MCI can
react at the beginning of the reaction with the product which can lower
the yield, this reaction is catalysed by higher temperature not by higher
concentration of HC1(741);
Si(OQ>H5j., + 1 ICl -» Si(OC,H.0.i CI+C21 -f5OI I
Tetraethoxysilane can be produced by a continuous method. The
continuous feeding of ethanol and tetrachlorosilane in automatic column
was described1451, the resulting product was 85-87%). Similarly vaporised
ethanol (82 - 85 C), and tetrachlorosilane were introduced in the glass
column at opposite points, the hydrogen chloride escaped through the
reflux, system, tetraethoxysilane is collected, in a receiver (82% yield).
Also the reaction can be done in a stream of nitrogen gas<7>.
The purity of tetraethoxysilane produced by the continuous method was
described. Tetraethoxysilane having < 10 ppm I ICl, or with no halogen
contents, with < 5%'di, tri and tetra content was produced.
The resulting product depends on the ethanol-tetrachlorosilane
ratio, thus esterification of SiCl4 with ethanol in 1:1, 1:2, 1:3 and 1:4
ratios at 100, 130, 145 and 155 C respectively, gave 90%, C2Il50SiCI:,,
95%). (C2H5O)2 SiCU 80% (t\II,OJ., SiCl and 82% (C2II,O), Si
respectively'' '*.
A procedure in which cthyltetraethanoate is used insteatl of ethanol
to react with tetrachlorosilane is described1''. This procedure gives a.high
yield of telratilhoxysilane (.88 - 91%) with high purity (0.02 - .0 .1
chlorine content, and with 3.1 -4.6%) polysiloxanes). Studies have been
.V)
carried out to elucidate the mechanism of the reaction:
, .i" .. SiCl4.t4RQH->S.i(OR)4 + 4HClt
Two mechanisms appeared to be possible, both involving the
addition of the lone pair of alcoholic oxygen to tetrachlorosilane, which
increases the electron density at silicon atom, and weakens the silicon
halogen link. These two mechanisms are shown as:
R Cl Cl II
\ \ x ,xO >S\< O ->Si(OR)4 + 4HCl
X . x \ \H Cl Cl R
R Cl Cl II
X . \ x x '^ O — > Si< O 4 Si (OR)4 I SiO, -I 4RCI -I- 211,0
/ ; • • X \ A . ;
II ' Cl Cl R
Both mechanisms occur when normal alcohols are used.
The preparation of alkoxysilanes from glycol monoethers
(IIOCH.CTbpR) has been reported1719' as shown below:
41IOCII2CII2OR * SiCL, ~> Si(OCI I2CH2OR)t ^ 411CI
(R = Me, Pr1 and n Hu).
Similarly phenols react with tetrachlorosilane to produce
tetraphenoxysilanes, phenol do not react with hydrogen chloride like
ulcohols.
A methocl for the preparation of polyhalophenoxysilanes has been
described'7'191 by reacting jihehol in the presence of amiiie catalyst, the
reaction can be illustrated as follows:
n c . r M . . . . . . . , , , , C a t a l y s tR n S i ( 1 , , , 1 - 4 - 1 RnS. o-<0
X X
(R - alkyl or aryl, X - halogen, n ~ 0, 1, 2).
The reaction is carried out at elevated temperature with reactants in the
molten state or, dissolved in tin inert solvent such as diethyl .ether or
t o l u e n e . •' < J' i: .' '' ;'.'' : ' .
1.4.2. The Exchange Route to Tetraalkoxysilanes:
The, name implies an interchange of an alkoxy group of a lower
alcohol with an alkoxy group of a higher alcohol or phenol.
Tetraalkoxysilanes are often prepared by exchange reactions between an
alkoxysilanes and alcohols(f7V
Si(OR)4 + nROI 1 -> (RO).,.llSi(OR)n + nROI 1
Where R and R - alky!, aryl, R ± R :. ,
The reaction is known as alcoholysis or displacement and some times
transesterifi.cation. Usually higher alkyl or aryl alcohols are used together
with tetramethokysilane or (etraethoxysilanes so that the volatile alcohol
liberated can be distilled out of reaction vessel to assist in forming the
desired product.
The reaction is usually accelerated by heat and/or catalyst, for
example acids or bases. The reaction is useful for the synthesis of
tetraalkoxysilanes containing reactive groups such as tertiary alcohols
which are rapidly attached by hydrogen chloride liberated in the initial
step of the reaction. The process is also suitable for the preparation of
letraalkoxysilanes from alcohols containing a group which reacts with
hydrogen chloride such as aminoalcohols. The alkoxy-exchange reaction
can also occur between two different alkoxysilanes.
Thus in a study of the reaction between tetramethoxysilane and'
tetraethoxysilaiie all possible exchange products were detected'7 ':
(CI hO),Si + (C21 l5O)4Si ^ - = ^ (Cl hOh SiOC2l I3 i
• • ' ' • ( C l h O ) : Si '(CM I^O)5+ CM
41
Some alkoxysilanes were prepared by using tetraethoxysiiane, also
the mechanism of. the reaction is postulated1"12'which indicates that the
Si - O link is broken as shown below:
R H8+
H5C2O O .OC2H5\ 4; / •
' . S i , • • ' • • . • • • .
. / \
The alkoxy-exchange route to tetraalkoxysilanes is significant,
especially with those reagents, which react with alcohols. :•
1.4.3. The Direct Synthesis:
The halosilane route to tetranlkoxysilanes is a source of many
difficulties concerning control of pollution'291, chlorine which is used in
the manufacture of silicon tetrachloride, is a hazardous material. So also
is silicon tetrachloride. Both can cause severe pollution. The hydrogen
chloride by-product decreases the overall yield by initiating a series of
side reactions,
However, an alternative recent route has been developed for the
production of tetraalkoxysilanes, by a direct method which is based on
reacting elementary silicon with a corresponding alcohol using suitable
catalyst. The reaction can be represented as follows:
Si t- 4ROII _ -_£ iU^L> Si(OR)i + 21 i2ts o l v e n t • ' ••
•If the alcohol used.is elhanol, the product is telraethoxysilane, which can
be converted to elhylpolysilicafes by hydrolysis and condensation-
polymerisation, using a limited a mount of water' to control the
hydrolysis •• . .
With the diScoxcry in 1940 of the direct synthesis, the problems of
large-scale production of organosilicon halides have been solved to great
, A 1.50.51)
extent
42
1.4.3.1. The Elemental Route Using Mctalie and Metal Salt Catalysts:
•Alkali, alkaline earth metals, and aluminum react(l3) with alcohols
to jgive metal alkoxides. The speed of the reaction depends both on the
metal and on the alcohol, increasing with increasing electropositivity and
decreasing with length and branching of the chain. Thus sodium reacts
strongly with ethanol, but' slowly with tretiary butanol. The reaction with
alkali metals is sometimes carried out in ether, benzene, or xylene. Some
processes use the metal amalgam or hydride instead of the free metal.
Alkaline earth metals and aluminum are almost always covered wilh an
oxide film. Slight etching with iodine or mercuric chloride breaks the film
and facilitates the reaction. So a metalic and metal salt catalysts is usually
needed to activate the metal surface. The reaction of alcohols with silicon
metal using metalic or metal salt catalysts wasstudied"9 '^^ ' .
It was found that of the several lower alcohols, only mcthanol'
reacted readily to form recognizable product using copper metal as
catalyst at elevated temperature in the range of 250-300 C,
( ' u • • ^
4011,011 I Si —--; > S i ( ( )n i , ) r t 21 h t250-300 ('
Telramethoxysilane was found to be in 40 •-• 45% yi.eld where as
tetraethoxysilane is produced in only. 10% yield'7'.
Similarly tetramethoxysilane in a high yield (85%) was produced using
CuCl2 as catalyst solvated in alky! benzene' '.
•The lower primary alcohols'such • as ethanol, l-propanol and iso-
butyl alcohol react to give small yields mainly of triaikoxysilane, where
as isopropyl, n-butyl, sec-butyl and t-butyl alcohols did not react'7'"'.
Reaction of elhanol with silicon at 23OC catalysed with Cu.(l) chloride
using aromatic hydrocarbon having 1-4 alkyl groups as solvent was
* described, giving (75%) HSi(OC2H5).i. (C2H5O).,Si was produced using. . * • • • • • • • • • . • • • • • • • . ' • ,
, CuCl, NiCL and mixed polyeyclic hydrocarbons (280 450 C).
. - • . - . , " . . *
Also tetraethoxysilane was prepared by contacting Me2Nl'l witli
activated silicon.
1.4.3.2. The Elemental Route Using Metal Alkovides Catalysis:
The use of metal alkoxides as catalysts iiv production of
tetraethoxysilanes from silicon metal and the corresponding alcohol has
been reported"9'. The preferred metal alkoxides are those of alkali metals
particularly those of sodium and potassium, alkaline-erath metals, and
even alkoxides of aluminum. Other possible catalyst is the reaction
product of sodium ethoxide and 2-ethoxyethanol. Tetramelhoxysilane
.with high purity and high yield (95%) was prepared by the reaction of
silicon with Cil.iOII and NaOCH, as catalyst'7'. Si(QR)., (R = C,..,) are
prepared from the corresponding alcohol ROM and silicon in the presence
of NaOR and Si(OR).| as a solvent. In another experiment KO2CII,
NaO2CHt, N'aOBu or K( )/\c are used as catalysts'7 '".
A method is described for production of tetraethoxysilane as
follows: Stir suspension of silicon powder in a large volume of catalytic
solution pre-heated to 1.50- 160C, dry elh'anol is then added batch-wise.
The catalytic solution has1""'1 sufficient thermal capacity to maintain the
temperature Catalysed the reaction, and to,discharge tetraethoxysilane as
vapour together with ethanol and hydrogen gas. The thermal capacity can
be maintained by step-wise addition of reactants.
Also BuOCI LCI LOCI LCI I.?OK has been used""1 as a catalyst for the
production of tetraelhox) silane.
'I'etraethoxysilane-iSU.7% yield'was prepared by the reaction of
ferro-silicide usins.' K'.()(' .1 Is or NaOl\l l.s 35% solution in CNH.sOI I, but a
44
15% NaOC2H5-C2HsOH solution gives 39.5% and 5% LiOC2H5-C2H5QI 1
•'•gives only 25% (C2H5Q).,Si(7). •
Telraethoxysilane 98% pure was prepared from silicon powder or
metalsilicide slurried with Si(OC2H5).( then C2II5OCH2CI I2ONa was
added, the mixture was heated to 1 30 C then ethanol was added.
Tetraethoxysilane 70.2% yield was prepared by the reaction of
ethanol with a mixture of silicon or sjlicide in the presence of Si(OC2l 15)|,
NaOC2H5 and C2H5OCH2CH2OH to act as 3-component catalyst.
The reaction between elemental silicon and ethanol does not take place
without using any catalyst. Magnesium ethoxide was found(7) to be an
effective catalyst for the reaction.
Also the reaction of elemental silicon in presence of tin ethoxide
was described'" '. It is found that tin ethoxide can act as a catalyst for this
reaction.
However, the direct synthesis of tetraethoxysilane by a direct
reaction of silicon with alcohol has many advantages compared lo the
previous methods, so a considerable effort has been expended1 9) to
develop the direct synthesis for production oftetraethoxysilane due lo its
industrial uses or applications.
Other methods lor the production of organic silicates which have
specialized significance were described' ', these include alcoholysis or
oxidation of organometalic compounds and carbides and reduction of
esters. . •
A new route which offers a path to alkoxysilanes that is simple and '
versatile has been developed1'''1". It enables the production of Si(()C2I I0i
from Ca,(SiO))O and C'a2Si0.|. Also it opens a way to alkoxysilanes that
are not easily available by-other means, it uses readily accessible starting
materials and gives non-toxic by-products.
45
Chapter Two
Experimental
E Experimental:ter* • • - , . •
§Lt. Materials:
Absolute eihauoi v v s - ^ / W ) Jus lDecr» t.voo. V»MV\V\\*V. <*\ . V , .
Cliemicals, and industrial spirit was dried using calcium chloride which
was put in the oven at 100- 120 c for two days - which was added to and
closed in a flask and left for a week in the hood, then dried ethanol, after
confirmation, was decanted and protected from atmospheric moisture
until used.
2.1.2. Magnesium:
•The grey ribbon (Supplied by Hopkin and Williams) was cleaned
by scratching to give a bright ribbon which was then used.
2.1.3. Mercury (I) Chloride:
A white powder (Supplied by BDI1 Chemicals) was used as a
catalyst,
2.1.4. Silicon Powder:
The silicon pure blue-grey powder (Supplied by Hopkin and
Williams) was used. .
2.1.5. Magnesium Ethovide:
The white solid magnesium ethoxide was prepared and identified
(see section 2.3.2.) then used immediately in the reaction.
2.1.6. Anhydrous Tin I etrachloride:
The volatile colourless liquid (Supplied by BD11 Chemicals) was
used as a catalyst.
2.1.7. Tin(ll) Oxide: r
% Pure black crystals^:(Supplied by Hopkin and Williams) was used
as a catalyst. : ' .
2.2. Equipment and Apparatus:
2.2.1. General:
Quick-fit Apparatus were used all through the study.
Figure 2.1. represents the reaction system for experimental apparatus*719'.
2.2.2. Infra-red Spectrometer:
'•Infra-red spectra were recorded on A Perkin-Blmer (157 sodium
chloride) spectrometer at ambient temperature with spectra range
4000cm"1 to 600cm"1. Liquid sample was spotted on two sodium chloride
plates which were pressed together to give a thin film, solid sample were
mulled with potassium bromide to give a thin disk. Absorption
frequencies are given in cm" .
2.2.3. Gas-Liquid Chromatographic Systein(GLC):
A pye unicam gas chromatography wi(h (lame ionisation detector
(Hydrogen was 30cm7min., air at 300em7niin.) 1'itted at 150 C was used
to obtain chromatograms. The carrier gas was nitrogen at 20cm7min, and
the column used is OV1 7 glass which is 2.0m long X4mm (I.D.), lilted at
140 C. Chart-speed was lem/min. '
Also A Hewlett Packard (series 5848) gas chromatograph with
(lame ionization detector was used.
The carrier gas was nitrogen at 30cm /min, and the column used is () V I 7
glass which is 6ft long x 2mm (1.1).). Injection temperature is 150 C and
oven temperature 140 (\
47
Water inlet
hermometer
Guard lube
(Containing CaC
Water Outlet
Condenser
Round-bottom iiask
1 lot plate
magnetic stirrer
rij». 2.1. Reaction Sysleni for experinRMital Apparatus
2.3. Experimental Procedure:* • - •
2.3.1: General: . ,
Chromic acid was used for cleaning apparatus, rinsed with tap
water, distilled water and dried in the oven at 120 C the most important
features of these reactions are:
(i) Moisture must be excluded.
(ii) Dry ethanol or commercial redistilled is used.
(iii) The reaction system is closed carefully and left inside the
hood during the night.
The experimental section includes two parts:
Part one: includes preparation of magnesium ethoxide using dry ethanol,
and mercury (I) chloride as a catalyst.
Part two: includes preparation of silicon ethoxide, using the direct
reaction between ethanol and silicon in presence of magnesium ethoxide,
tin tetrachloride and tinnous oxide as catalysts.
2.3.2. Preparation of Magnesium Ethoxide Catalyst:
Using the reaction system (Fig. 2.1.) lor experimental apparatus;
ethanol (100cm', 1.72mol) was put in a two-necked round bottom flask,
then cleaned magnesium metal (1.2.g, 0.05mol)-was:cut into small pieces,
and added in one portion-in addition to a tinny amount of mercury(l)
chloride. In one side neck a thermometer was fitted, and at the main neck
a vertical condenser ended with a guard tube containing calcium chloride
to avoid atmospheric moisture. ;
The reaction mixture was relluxed for about 14 hours and stirred
magnetically using effective magnetic stirrer where the leading of the
thermometer was steady at 78 c. .
, At the end a, white precipitate \vas formed which was separated by
filtration. :1 The. product obtained was analysed using IR spectra. This experiment
was summarised.in Table 2.1.
Table 2.1. Summary of the Reaction Between Ethnnol and
Magnesium Catalysted by Mercury(I) Chloride:
Reflux time/hours
Ethanol /cm
Magnesium /g
Merciiry(I) chloride
Magnesium ethoxide /g :
14.0
100
1.2
Tinny a mount
2.9
2.3.3. Direct Synthesis of Tetraethoxysilane:
2.3.3.1. Catalysed by Magnesium Ethoxide:
Using similar apparatus and procedure as in experiment (2.3.2), dry
ethanol (100cm, 1.72mol) was added to silicon powder (lg, O.()4mol) in
a two necked round bottom flask, then the catalyst magnesium ethoxide -
prepared in experiment 2.3.2. - (0.1 g, 0.001 niol) was added to.
In the side neck, a thermometer was fitted, and at the main neck a vertical
condenser ended with a guard lube containing calcium chloride to avoid
atmospheric moisture was fitted.
The mixture was refluxed .for a bout 4.6 hours continued-with stirring
using effective magnetic stirrer. The reading of the thermometer was
steady at 78 c. •
At the end of the experiment, the final reaction mixture was filtered to
separate unreacted silicon. •
A colourless'•'liquid remained was then fractionated to separate the
product at its boiling temperature. The product was analysed using 1R
spectra arid.GLC.
This experiment was summarised in Table 2.2.
Table 2.2. Summary of the Reaction Between Ethanol and Silicon
Catalysted by Magnesium Ethoxide:
Time /hours • 46.0
Ethanol / cm3
Silicon/ g
Magnesium ethoxide / g
Product/cm"
100
0J
1.9
2.3.3.2. Catalysed by Tin Tetrachloridc:..
Using similar apparatus and procedure as in experiment (2.3.2), dry
ethanol (100cm , \J2mo\) was added to silicon powder (lg,0.04mol) in
a two necked round bottom flask, then the catalyst tin tetraehloride (lew
drops) Was added using a dropper.
In the side neck, a thermometer was fitted, and at the main neck a vertical
condenser ended with a guard tube containing calcium chloride to avoid
atmospheric moisture was fitted.
The mixture was refluxed for a boul 46 hours continued with stirring
using effective magnetic stirrer. The reading of the thermometer was
steady at 78 c.
At the end of the experiment, the final reaction mixture was filtered to
separate uiireaeted silicon. ; ' •
A pale yellow filtrate which,was obtained, was then fractionated to
separate the product at iis boiling temperature. The product was analysed
using IK, spectra ami ( i l \ . \
This experiment was summarised in Table 2.3.
51
Table 2.3. Summary''of the Reaction Between Ethanol and Silicon
Catalysted by Tin Tetrachloride:
Time / hours
Ethanol / cnv
Silicon / g
Tin Tetrachloride /drop
Product/cm
46.0
100
1
5
2.7
2.3.3.3. Catalysed by Tin(ll) Oxide:
Using similar apparatus and procedure as in experiment (2.3.2), dry
ethanol (100cm, 1.72mol) was added to silicon powder (0.5g, ().02mol)
in a two necked round bottom flask, then the catalyst tin oxide (tinny
amount) was added.
In the side neck a thermometer was fitted, and at the main neck a vertical
condenser ended with a guard tube containing calcium chloride - to avoid
atmospheric mojsture - w a s fitted. •
The mixture was re fluxed for about 46 hours continued with stirring
using effective magnetic stirrer.
The reading of the thermometer was steady nt 7<S e. At the end of the
experiment, the final reaction mixture was filtered to separate unreacted
silicon.
A colourless filtrate which was obtained was fractionated to separate the
product at its boiling temperature. The product was analysed using IR
spectra and Gl.('.
,52
This experiment was summarised in Table 2.4.
i,- • • - . ' •
• * * • • • , ' • •
Table 2.4.. Summary'of the Reaction Between Ethanol and Silicon
Catalysfed by Tin oxide:
Time / hours
Ethanol / cm
Silicon/g
Tin oxide
Product / cm
46.0
100
0.5
Tinny a mount
1.3
53
Chapter Three
Results
3. Results: *. • • • • • > j
3.1. Theoretical and Experimental Yields:• i
Yields of tetraethoxysilane obtained using different catalysts were
summarised in the following table:
Table 3.1. Yields of Tetraethoxysilane Obtained Using Different
Catalysts:
The catalyst
••Mg(OC2H5)2
SnCl4
SnO
Experimentalyield /g
1.7872.4831.192
Theoreticalyjeld /g
2.9713.7142.229
Yield %
60.1566.8653.48
3.2. Magnesium Ethoxide Catalyst :
3.2.1. IR Spectrum of the Solid Product:
The white solid magnesium ethoxide obtained was analysed using
the equipment and procedure given.in section (2.2.2.). '
Results are- shown in fable (3.2.). and Fig. (3.1.).
Table 3.2. 'Infra-red Spectra of the Solid Product of the Reaction
Between Ethanol and Magnesium Catalysted by Mercury(I)
Chloride:
Frequency/em'1
3350-3020 b
2790\v1 8 5 0 s . •' •• :i -,
1470s1430s1275w '95 3 w
,i = broad.s = sharp.w = weak.
Assignments
C - 11 Stretch
C - 11 StretchC r-11 StretchC - O - M StretchC - O - M Stretch
• C - C - 0 StretchC-O Stretch
3.2.1. IR Spectrum of the Liquid Product:
IR .spectra -ot the filtrate of reaction 2.3.2. w -,\s oruiued
Results are shown m Table (,3.3.) and Fie. i3'.2\.
Table 3.3. Infra-red Spectra of the Liquid Product of the Reaction
Between Etiianot and Magnesium Catalysted by Mercury (I)
Chloride:
Frequency/cm"
3350-3030 b
2800-2700 b
1350-1330 b
1060 s
875 s
b = broad.
s = sharp.
vv = weak.
Assignments
O - H Stretch
C - 11 Asym. Stretch
C — H Bending
C - C T T T TClh Rocking
55
.4000
106
tot-
2.0 ooW a v e n u m b e r (cm" )
1SCG- " sooo SCO
Wavelength (Microns)
Fig. (3.1.) Infra-Red Spectra of the Solid Product of the ReactionBetween Ethanol and Magnesium (see Table 3.2.)
2OO0Wavenumber (cm" )
19 00 v 1000 900 &D0 loo
r-
. . . Wavelength (Microns)
Fig. (3.2) Infra-Red Spectra of the Liquid Product of the ReactionBetween Ethanol and Magnesium (see Table 3.3. ;
•33. Silicon Ethoxide: '
3.3.1. Catalysed by Mg(OC2H5)2: * v
3.3.1.1. IR Spectrum:
The IR spectra results for tetraethoxysilane obtained using
magnesium ethoxide catalyst are shown in Table (3.4.) and Fig. (3.3.).
Table 3.4. Infra-red Spectra of the Prodiict of the Reaction Between
Ethanol and Silicon Catalysted by Magnesium Ethoxide:
Frequency / cm"
3400 b
2900 m
1500-1350 b
110s
1065 s
895 b
ssiunments
O - H Stretch
C - II Asym Stretch;
C - H Bend
Si - O Asym stretch
•C - O Stretch
SiO.| Asym stretch
b = broad.
s = sharp.
in = moderate.
.58
2000Wavenumber (cm'1)
[coo SCO 700
•o
7 S ? toWavelength (Microns)
iz
Fig. (3.3.) Infra-Red Spectra of the Product of the Reaction BetweenEthanol and Silicon Using Mg(OC2H5)2 Catalyst (see Table 3.4.)
3.3.1.2. GI Ghromatograph:
Results' obtained using GL chromalography are summarised in
Table (3.5.) and Fig. (3.4.).
Table 3.5. GLC Analysis of the Product of the Reaction Between
Ethanol & Silicon Catalysed by Magnesium Ethoxide:
(On Hewlett Packard)
Retention Time (RT)
0.58
0,63
10.15
12.80
Area
29120000
359,90000
20280
37020
Area %
44.685
55.227
0,03 1
0.057
Assignment
Rthanol
Tetrnelhoxysilane
1 Iexaethoxydisiloxane
60
- )—-I I—
2 1.3 0
Fig. (3.4.): Cas-liquid Chromafogrnph of the Product of.the
Reaction Between Ethanol and Silicon Using
I\'ig(OEt)2 Catalyst (On Apye IJnicain).
61
3.3.2. Catalysed by S11CI4: »v. , ' • • • • • • . ; , . . , •• • ;
3.3.2.1. IR Spectrum:
T h e IR spect ra results for t e t rae thoxys i lane obta ined us ing tin
te t raohlor ide ca ta lys t are shown in Tab le (3.6.) and Fig. (3 .5 . ) .
Table 3.6. Infra-red Spectra of the Product of the Reaction Between
Etha'nol and Silicon Catalysted by Tin Tetrachloride:
Frequency/cm"'
3150s
2870 s
1 4 5 0 - 1 4 3 0 b ••
1410-1370 b
1350 m
1210 m
1100-1080 b
1050s
890 s
718 in'
D =.broad.
s s= sharp.
nl ^ moderate.
Assignments
O-
C-
Si-
c -c -
Si
~sT-
-H Stretch
- H A Sym.Stretch
- O Stretch
-H Bend
-II Bend
- () A sym stretch
-OStrelch
62
\z \l- <r S 6 : 7 . S ;• T ^ . ^
Wavelength (Microns)
Fig. (3.5.) Infra-Red Spectra of the Product of the Reaction BetweenEthanol and Silicon Using SnCl* Catalyst (see Table 3.6.)
15
3.3.2.2, GL Chroniatograph
,*. Results obtained using GL chromatography are summarised in
Table (3.7.) and Fig. (3.6.). • • • , . .
Table 3.7. GLC Analysis of the Product of the Reaction Between
Ethanol & Silicon Catalysted by Tin Tetrachloride;
(On Hewlett Packard)
Retention Time (RT)
0.06
0.91
1.35
18.10
Area
121200
481500000
207200
32100
Area %
0.025
99.925
0.043
0.007
Assignment
—
Ethanol
Tetraethoxysilane
1 Iexaethoxydisiloxnne
64
0.91
0
Fig. (3.6.): Gas-liquid Chronialograph of (he Product of (he Keaclion
Behveen Ethanol and Silicon Using S11CI4 Ca(alys(.
(see Table 3.7.).
33.3. Catalysed by SnO: '
3.3.3.1. IR Spectrum::.
T h e IR s p e c t r a resu l t s for t e t r a e t l i o x y s i l a n e o b t a i n e d u s i n g t in ( I I )
o x i d e c a t a l y s t a re s h o w n in T a b l e (3 .8 . ) a n d Fig . ( 3 .7 . ) .
Table 3.8. Infra-red Spectra of the Product of the Reaction Between
Ethanol and Silicon Catalysted by Tin Oxide:
b = brand.
s = sharp,
w = weak.
Frequency cm"'
3 3 5 0 s ; : ; • / •••;•..
2880 w
1450-1370 1)
Tf[0w~
1060 s
890 w 7 '
Assignments
C-
c-
Si-
"si"-
Si-
- H Stretch .
- 11 Stretch
- 0 Stretch
0 A sym Slrelch
- O Stretch
C) Stretch
66
Wavenumber (cm"1)
0 3 7 T 9 io
Wavelength (Microns)
Too
15
Fig. (3.7.) Infra-Red Spectra of the Product of the Reaction BetweenEthanol and Silicon Using SnO Catalyst (see Table 3.8.)
3.3.3.2. GL Chroniatograph:
Results obtained using GL chroniatography are summarised in
Table (3.9!) and Fig. (3.8.)..
Table 3.9. GLC Analysis of the Product of the Reaction Between
Ethano! & Silicon Catalysed by Tin Oxide:
(On Hewlett Packard)
Retention Time (RT)
0.05
0.53
0.62
0.81
7.88
Area
13850
396
456
563200000
392400
Area %
0.002
0.000
0.000
99.928
0.070
Assignment
__
—
lilhanol
Tetraethoxysilane
68
O.Si
- f -
0
Fig. (3.8.): Gas-liquid Chromatograph of Hie Product of the
Reaction Between Elhanol and Silicon Using SnO
Catalyst.
(see Table 3.9.).
:< However*' results obtained from all these experiments are
sequentially discussed in details in the following discussion section.
70
Chapter Four
Discussion & Conclusion
4. Discussion and Conclusion:
4.1. Discussion;This discussion section is concerned With tire .preparation of some' ''
metal alkoxides, magnesium ethoxide and tetraethoxysilane using the
direct synthetic procedure in presence of different catalysts: magnesium
ethoxide, tin tetrachioride and tin oxide.
This procedure was carried out to confirm results and gain experience and
scope of the reaction.
This discussion chapter contains two main parts, firstly the
preparation of magnesium ethoxide by the direct method. The product
obtained have been used as a.catalyst in the second part, which deals with
the synthesis of tetraethoxysilane by the direct method using dry ethanol
and silicon powder in presence of different catalysts.
4.1.1. .Preparation of Magnesium Ethoxide by the Reaction of
Magnesium and Ethanol:
Magnesium ethoxide in 50.9% yield was obtained as a white solid
using the direct synthetic procedure as represented by the equation'7':
Mg + 2CH3a-I2OH .JiM^i ^ Mg(OC2Hi)2 -I- I J2t
The IR characteristic bands of magnesium ethoxide compared with
literature values are shown in the following table:
Table 4.1. •Reported and Obtained 1R Clmraeterislic Bands for
Mg(OC2H5)2:
Clccc-
Assigiiinenls
\)-C a sym. def.-O-lvl stretch- 0 stretch- 0 stretch
Frequency/em"1
Obtained
147014301275
?53_ :
Literature
1470-14351430.1275950
Reference
IK57167
71
These IR results and physical characteristic (state, colour and in.p.)
confirm the .formation of magnesium ethoxide.
4.1.2. Preparation of Tetraethoxysilane by the Reaction o f Silicon
and Ethanol:
Tetraethoxysiiane in 60.15%, 66.86% and 53.48% yields was
prepared using the direct synthetic procedure in presence of Mg(OC2I I.O2,
SnCl.) and SnO catalysts respectively as represented by the equation'7':
Si + 4CH3CH,OH - C a t a i y lL> Si (OCHOi + 2M,tA
The IR characteristic bands of tetraethoxysiiane obtained in these
reactions were compared with the reported values in the following tables:
Table 4.2. Reported and Obtained IR Characteristic Bands for
Si(OC2H5)4 Prepared Using Mg(OC2H5)2 Catalyst:
Assignments
Si - O a syin. Stretch
• C - 0 stretch
Si - O a syni. Stretch
Obtained
Frequency/em"
Literature
1110 .
1065 •
895
1135- 1090 .
1110
065
900
Reference
57
72
Table 4.3. Reported and Obtained IR Characteristic Bands ton
.Si(OC2Hs).» Prepared Using SnCI4 Catalyst:
Si
Si
Si
Assignments
- O Stretch
- O a sym. stretch
- 0. Stretch
Frequeney/cnf'
Obtained
1450-
1100-
1050
890
718 •
-1430
-1080
Literature
1430- 1425
1135-1090
1130- 1000
1090 - 1020
880 - 720
780 - 640
'Reference
57
57
7
17
Table 4.4. Reported and Obtained IR Characteristic Bands for
Si(OC2H5)4 Prepared Using SnO Catalyst:
Si -
Si -
S i -
S i -
Assignments
O Stretch
(.) a sym. stretch
O. Stretch
O. Stretch
Obtain
1450-
1110
1060
890
Frequency/cm '
ed
1370
Literature
1430- 1425
1 1 3 5 - 1090
1 130- 1000
1090 - 1020
800 720
Reference
57
57
17 •
18
7
These 111 results and boiling point confirmed the formation of
letraethoxysilane using Mg(OC\I h)i, SnCl.i and SnO catalysts.
Also the, (iL ehromatograph main beaks - agree with the reported
ones""'1 '.- confirmed the formation of tetraethoxysilane mixed with
hexaethoxydisiloxane which may formed due to hydrolysis by
atmospheric moisture or water which present as minor contaminant in
ethanol.
73
Tetraethoxysiiane could be,obtained using Mg(OC7H5)2, SnCl.i and Sn()
catalysts. . ' '
The efficiency of theses catalyst in accelerating-this reaction is
attributed to the availability of the empty 3d-orbital of silicon and
the unshared pair of electrons at the oxygen of Mg(OC2ll5)2 and SnO
and chlorine of S11CI4 which was in agreement with the proposed
mechanism for this reaction*7'.
S11CI4 was found to be the most effective catalyst, because it is the
most electrons dense so if easily attacks (lie empty 3d- orbital of silicon.
74
4.2. Conclusion
' . . ' • • * '
Tetraethoxysilane has been prepared from, the reaction of elemental
silicon and absolute ethanol using the direct synthetic procedure
according to the following equation:
Si + C2H5OH J ^ l ^ L ^ Si (OC2H5)4 + 21 l2tA
The reaction does not proceed without using any catalyst due to the silica
layer, which covers the silicon surface.
The efficiency of different catalysts was examined under the standard
conditions which was described.
Magnesium ethoxide, tin tetrachloride and tin oxide were found to be
effective catalysts in the direct synthesis of tetraethoxysilane.
The products obtained were characterised using infra-red spedroscopy
and gas-liquid chromatography.
The direct synthesis of tetraethoxysilane from elemental silicon and
ethanol permits the isolation of a relatively high purity product in
comparison with other methods.
The route offers a path to alkoxysilanes and alkoxysiloxanes that is
simple and versatile. In addition, it opens a way to alkoxysiloxanes that '•
are not easily available by "other methods. In many instances it uses
readily accessible starling materials and gives non toxic by products.
Some of the alkoxysilanes and alkoxysiloxanes to which it leads may be
of interest as ceramic precursors.
75
References* T
L. Rochow, E.-G.; Silicon and Silicones; P.I, 40 - 50; Springer
Verlag; 1986.
2. Bradley, D. C , Mehrotra, R.C. &. Gaur, D.P.; Mela! Alkoxides,
P. 1 - 55, 74 - 81; Academic Press INC, (London) Ltd.; 1978.
3. Kirk - Othmer; Encyclopedia of Chemical Technology; Vol. 20;
3rtl Edition; P. 912; John Wiley & Sons, (New York); 1982.
4. Bradley, D.C.; Progress in Inorganic Chemistry, Vol. 2, P. 3, 357;
Interscience, (New York); 1960.
5. Finar, I. L.; Organic Chemistry; Vol. 1, 6lh Edition; P. 181 182;
Continental Printing Co. Ltd., (Hong Kong); 1973.
6. Akhmetor, N.S.; General and Inorganic Chemistry, P. 412 - 4 1 3 ; .
Mir Publishers; 1987.
7. Sulayman, N.M.; M.Sc; Thesis; P. 3 -33 , 78; U. ofK. (Sudan);
1995.
8. Purccll, K.F. & Kotz, J.C.; Inorganic Chemistry; P. 3 19, 477; 1 loll
- Saimders International Editions; 1977.
9. Cotton, F.A. & Wilkinson, G,; Advanced Inorganic Chemistry; 3"'
Edition; P. 309 ~ 333; John Wiley & Sons (New York); 1972.
10. Aylett, B.J.; Organometalic Compound's; Vol. 1, Part Tow;'4 t h
Edition; P. 8; John Wiley & Sons, (New York); 1970.
11. Mineral Gallery -The Silicate Class; P. 1; Amethyst galleries Inc.;
1998.
12. Chang, P.S. & Ikiese, M.A.; .1. Am. Chcm. Soc; Vol. 115; P.
11475 84; 1993,
13. Brel/.inger, • I.)... ^ Josten, • W.; Encyclopedia of. Chemical
Technology.; Vol. 2; 3rd Edition; P. 1-17; John Wiley & Sons,
(New York'); 1 ^82.
. 76
14. Bradlecy, D.C., Mehrotra, R.C., Swanwick, J.D. & Wardlaw, W.;J.
' Chem.Soc.; Vol. 2; P. 2025-30; 1953.
15. Bradlecy, D C ; Advanced Inorg.Chem. Radio Chem.; Vol. 15;
P. 259; 1972.
16. Aleksa, V., Klaeboe, P., Nielsen, C.J., Tanevska, V. & Guirgis,
G.A.; Vib Spectrsc; Vol 17(1); P. 1 - 30; 1998.
17. Smith, A.L.; Analysis of Silicones; Vol.,41; P. 217-242,271;
Wiley & Sons, (London); 1974.
18. Jones, R.A. & Cross, A.D.; An Introduction to Practical Infra-red
Spectroscopy; 3rd Edition; P. 66 - 67, 72 - 96; Butter Worth,
(London); 1969.
19. Omer, O. Y.; Ph.D. Thesis; P. 40; University of Manchester; 1989.
20. Peetre, I.B;J. of Chromatography; Vol. 90( 1); P. 35 - 55; 1974.
21. Ahmed, A.Y.; M.Sc. Thesis; P. 81; U. of K. (Sudan); 1997.
22. Gpel, S.C., Chiang, M.Y. & Bulwo, W.E; Inorg. Chem.; Vol. 29;
•',.., P. 4640-46; 1990.
23. Bidell, W., Doring, J., Hans, W.B., Hund, U., Plappert, E. & Berke,
IT; Inorg. Chem.; Vol. 32; P. 502 - 507; 1973.
24. Ingham, R.K. & Hurtstman, W.D.; CA; Vol. 79; P. 508-509;
1973.
25. I lays, D.S. & Fu, G.C.; J. Org. Chem.; Vol. 61; P. 4; 1996.
26. Shah, G.B.; J. Appl. Polym. ScL; Vol. 70(1 1); P.2235 - 3 9 ; 1998.
27. Lammcrtink, R.G., llempenius, M.A. & Vancso, G.J.; Macromol.
Chem. Physics; Vol. 199(10); P. 2141 - 4 5 ; 1998.
28. Yang, IT, Aspiund, M.C., Kotz, K.T., Wilkens, M.J., Frei, 11. &
. Harris, C.B.; J. Am. Chem. Soc; Vol. 120(39); P. 10154 65;
1998.
29. Emblem, H.G.; Materials Chemistry & Physics; Vol. 8; P. 2 6 5 -
277; 1983.
.. '• 7 7 ' ' '•
Aelion,-R.,. Loebel, A. & Eirich, F.; j . Am. Chem. Soc.; Vol. 72,
P. 5-705- 11; 1950. •
Yoldas, B.E.; J. of 'Polymer. Science; Vol. 24( 12); P. .3475 - 78;
1986.
Paul, A.; Chemistry Of Glasses; 2"d Edition, P. 52 - 53; Champan
& Hall Ltd. (London); 1990.
Wijk, ML, Norrestam, R. & Westin, G.; Inorg. Chem.; Vol. 35;
P. 1077-79; 1996.
Kepler, C.L., Londergan, T.M., Lu, J.Q., Pa'ulasaari, J. & Weber,
W.P.;J. Polymer; Vol. 40(3); 1999,
Greenwood, N.N. & Earnshow, A.; Chemistry of the Elements;
P. 3 i 7 - 319, 380 - 382; Butler Worth - I Icinemann 1 ,td.; 1984,
Wideman, T., Fazen, P.J., Su, K., Remesen, E.E., Zank G.A. &
Sneddon, L.G.; Appl Organometai Chem; Vol. 12; P. 681 - 693,
725-734; 1998.
Hench, L.L. & -West,. J.K.; Chem. Rev.; Vol. 90(1); P. 33 - 72;
1990.
Good, G.B. & kenney, M.E.; Inorg. Chem.; Vol. 29; P. 1216; 1990.
Ojima, .1., Fracchiolla, D.A., Donovan, R.J.& Banerji, P.; J. Org.
Chem.; Vol. 59; P.7594 - 95; 1994.
Saucier, R.; CA; Vol. 56; P. 3357c; 1962.
Seylerth, 1). ^. Rochovv, E.G.; J. Org. Chem.; Vol. 20; P. 250 -
255; 1955. ' •. , ' '.
Ridge, D. & 1odd,M.; J; Chem. Soc'.; Pt. 4; 2637; 1949. :
Hyde, -J.F. & Curry, J.W.; .1. Amr. Chem. Soc; Vol. 77;
P. 3140-41;; 1955.
Okavvara,. P.; CA; Vol. 47;'P. 4478h; 1953.
Okawara, P.'/Numa, V. & Wastase, T.; CA; Vol. 49; P. 1154b;
1955.
78
46. Fukiikawa, S.; C M Vol. 49; P. 11315; 1953.
47. Seif, C.A.V -Bosch^i, R.H. & Holder, J.'P.; CA; Vol. 72; P. 78391 j ;
1970. , , .. . '
48. Bhule, R.S. & Swasamban, M.A.; CA; Vol. 76; P. 85383u; 197).
49. . Ivanoi, V. 1., Monsalyskii, V.N., Samolilov, S.JVI. & Tsvetkov,
O.N.; CA; Vol. 83; P. 590213; 1975.
50. Newton, W.E. & Rochow, E.G.; Inorg. Chem.; Vol. 9(5); P. 1071 -
76; 1970.
51. Rochow, E.G.; J. Amr. Chem. Soc; Vol. 67; P. 963 - 965; 1945.
52. Baumgarten, R.L.; Organic Chemistry - A Brief Survey; P. 1 6 8 -
169; John Wiley &'Sons (London); 1978.
53. Rochow, E.G.; J. Amr.Chem. Soc; Vol. 70; P. 2170 - 2179; 1948.
54. Montle, J.F., Markouski, II.J., Lodewyck, P.I). & Schneider, D.F.;
CA; Vol. 97; P. 56006b; 1932.
55. Magee, W.L.•& Telshow, J.E; CA; Vol. 95; P. 21977x; 1982.
56. Okawara, R., Hatta, S. & Shirma, T.; Bull. Chem. Soc. Japan; Vol.
28; P. 541; 1955.
57. Man;, G. & Pockett, B.W.; Practical Inorganic Chemistry.; P. 97;
Van Nastrand Reinhold Company (London); 1972.
58. The Perkin Elmer Catalogue of Gas Liquid Chromatography;
Perkin Elmer Supplies; P. 56; 1984.
79