chemistry. organic compounds containing oxygen - iii session
TRANSCRIPT
Chemistry
Organic Compounds Containing Oxygen - III
Session
Session Objectives
1. Properties of phenols
2. Reaction of phenols
3. Preparation of ethers
4. Properties and reactions of ethers
5. Some useful ethers
6. Crown ethers
Acidity of phenol
Phenol is more acidic than aliphatic alcohols because conjugate base is stabilized by resonance.
O
– O
–O
–
O
–
Reactions of phenol
Electrophilic aromatic substitution
—OH group is ortho, para- directing group and activates the benzene rings.
Chemical reaction of phenol
Fries rearrangement
Distillation with Zn dust :
6 5 6 6C H OH C H + ZnO
Nitration
OH
Conc. HNO3
Conc. H2SO4
OH
NO2O2N
NO2
With dilute HNO3, it gives ortho and para-isomers which can be separated easily by distillation.
O H
Dil. HNO3
O HN O2
O H
N O2
+
With concentrated HNO3 phenol is converted to 2,4,6-trinitrophenol.
Bromination of phenol
OHaqueous medium
Br23
CHCl3/CCl4
OH
BrBr
Br
2, 4, 6 tribromophenol
Br2
OH
Br
p-Bromophenol
OH
Br
o-Bromophenol(minor product)
(major product)
+
+ 3HBr
Kolbe’s reaction
ONa
CO2
OH
COONa H+
OH
COOH
+
Salicylic acid
400 K
4 –7 atm
Reimer-Tiemann Reaction Mechanism OH
CHCl3
NaOH/H2Oheat
C
O
H
OH
salicylaldehyde
CHCl3 + OH- CCl3- + H2O
CCl3- CCl2 + Cl-
a carbene
+ OH-
OOH O
H
O
H CCl2+
O
H
CCl2 CHCl2
O-
CHCl2
O-
H2O
heat
OHC
O
H+ HCl2
Overall:
Reimer Tiemann Reaction
O
CHO
OH OH
CHOCHCl3
O
CHCl2
H+
Salicylaldehyde(main product)
aq. NaOH, 70°C
The mechanism involves dichlorocarbene as an intermediate
3OH CHCl 2 3 2H O : CCl Cl : CCl
On treating phenol with chloroform in presence of sodium hydroxide, a —CHO group is introduced at ortho position of benzene ring.
Fries rearrangement
Esters of phenols yield phenolic ketones on treatment with anhydrous aluminium chloride.
O H
3
O HO
+(CH CO ) O3 2
C O C H3
AlC l
O H
O C O C H 3
O C O C H 3
Coupling Reaction
N N Cl OH+
N N OH
–OH
p-hydroxy azo benzene
Phenol Reactions: A Summary OH
NaOH
or
Na
O- Na+ 1. NaOH
2.RX (primary)
OR
RCOCl
or
(RCO)2O
OC
O
R
AlCl3
OH
C
O
RArN2
+
OH
N
NAr
CO24-7 atmheat
OH
C
O
O-Na+
CHCl3
O-
C
O
H
NaOH
Na+
Ethers
• Formula R-O-R where R is alkyl or aryl.• Symmetrical or unsymmetrical• Examples:
O CH3CH3 O CH3
O
Introduction
Structure and Polarity
• Bent molecular geometry
• Oxygen is sp3 hybridized
• Tetrahedral angle
Hydrogen Bond Acceptor
Ethers cannot H-bond to each other.
In the presence of -OH or -NH (donor), the lone pair of electrons from ether forms a hydrogen bond with the -OH or -NH.
Solvent properties
• Nonpolar solutes dissolve better in ether than in alcohol.
• Ether has large dipole moment, so polar solutes also dissolve.
• Ethers solvate cations.
• Ethers do not react with strong bases.
• Grignard reagents
O B
H
H
H
+ _
BH3 THF
Ether complexes
• Crown ethers
• Electrophiles
Nomenclature
CH3CH2 O CH2CH3
diethyl ether orethyl ether
CH3 O C
CH3
CH3
CH3
t-butyl methyl ether ormethyl t-butyl ether =>
• Alkyl alkyl ether
• Current rule: alphabetical order
• Old rule: order of increasing complexity
• Symmetrical: use dialkyl, or just alkyl.
• Examples:
Common name
IUPAC Names
CH3 O C
CH3
CH3
CH3
2-methyl-2-methoxypropane
O CH3
Methoxycyclohexane
• Alkoxy alkane
• Examples:
Preparation
By dehyration of alcohols
CH3CH2OH
C2H5OC2H5
CH2=CH2
H2SO4
410 K
H2SO4
443 K
Williamson’s Process
ROH + Na – +RO Na 21
H2
–R – O – R` X– –RO R`—X 2
NS
0
0 0
Must be 1
2 and 3 Alkene will be the major
product
But R`X .
R`X
Important laboratory method for the preparation of symmetrical and unsymmetrical ethers.
Williamsons ProcessCH3
CH3
– +3H C O Na
|
|– C – + 2 5C H Br Path 1
CH3
CH3
3 2 5H C OC H|
|– C –
CH3
CH3
3H C|
|– C – Br–
2 5C H O Na + Path 2
H2CCH3
CCH3
(Major)
CH3
CH3
3 2 5H C OC H|
|– C –
(Minor)
Best results are obtained if the alkyl halide is primary. If tertiary alkyl halide is used, an alkene is the only reaction product and no ether is formed.
Cleavage of Ethers
• Ethers are unreactive toward base, but protonated ethers can undergo substitution reactions with strong acids.
• Alcohol leaving group is replaced by a halide.
• Reactivity: HI > HBr >> HCl
CH3 O CH3 H Br CH3 O CH3
H_
Br_
++
Br_ +
CH3 O CH3
H
Br CH3 + H O CH3
Mechanism
Alcohol is protonated, halide attacks, and another molecule of alkyl bromide is formed.
Phenyl Ether Cleavage
O CH2CH3HBr
OH
+ CH3CH2 Br
• Phenol cannot react further to become halide.
• Example:
Alkyl aryl ethers are cleaved at the alkyl oxygen bond due to the low reactivity of aryl-oxygen bond.
Electrophilic substitution in alkyl aryl ethers
The alkoxy group(-OR) is ortho, para directing and activate the aromatic ring towards electrophilic substitution in the same way as phenol.
O R
O R
–
–
–+ O R
+ O R
+
Helogenation
Anisole undergoes bromination with bromine in ethanoic acid even in absence of iron(III) bromide catalyst.
O C H3
A n iso le
Br2
O C H 3
B r
Friedel Craft reaction
O C H3
A n iso le
O C H3
+ C H C l3
3AlC l
O C H3
C H 3
C H 3+
Alkylation
Acylation O C H
3
+ CH CO Cl3
AlC l
O C O C H 3
O C O C H 3
3 O C H3O C H3
Nitration
O C H
HNO
N O2
N O2
+4
3
3 O C H3O C H3
H SO2
Obtained mixture of ortho and para isomers.
Illustrative Example
Give the major products that are formed by heating each of the following ethers with HI.
C H — C H — C H — C H — O — C H C H 323 22
C H3
C H C H C H O C C H C H
C H 3
3 2 2 2 3
(i)
(ii)
Solution
C H — C H — C H — C H O H 323 22
C H 3
C H C H I+(i)
C H C H — C — I
C H 3
3 22 23
C H3
C H C H C H O H +(ii)
Crown ethers
Cyclic polyethers containing four or more ether linkages in a ring of twelve or more atoms.
Crown ethers bind certain metal ions depending on size of the cavity
O
OO
O O
Na+
In this reaction crown ether is host and metal ion is guest.Crown ethers allow inorganic salts to dissolve in non-polar solvents.
O
OO
O O
Na+
Inclusion compound
Uses of ethers
As solvent and inhalation anaesthetic.
A number of naturally occurring phenol and ethers are used as flavourings and in perfumes of their pleasant odour.
OH
CHO
OCH3
Vanillin
Thank you