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TRANSCRIPT
1-What is benzene?
2-What is Electrophilic Substitution
Reactions?
3- Addition reaction
4- Types of addition reaction
5- Mechanisms of substitution and
addition reaction
*
*Benzene : Resonance Description
Had Alternating Double And Single Bonds Thus
These Double Bonds Are Described As
Conjugated Bonds.
Primary analysis revealed benzene had...
a molecular mass of 78
a molecular formula of C6H6
Kekulé suggested that benzene was. . PLANAR
and CYCLIC
To explain the above, it was suggested that the structure oscillated between
the two Kekulé forms but was represented by neither of them. It was a
RESONANCE HYBRID ( average of two structures that differ only in the
placement of the valence electrons).
Benzene : Resonance Description
However, all bond lengths in benzene to be equal and intermediate between single bond and double bond lengths (1.39 Å) and the ring is more stable than expected.
7
one way to overlap
adjacent p orbitals
delocalised pi
orbital system
another
possibility 6 single bonds
Benzene : Resonance Description
Aromatic compounds are compounds that resemble benzene in chemical behavior
thus they tend to react by substitution rather than by addition
* electron cloud delocalized all over the ring
* the resonance picture this helps to explain lack of reactivity of benzene
(substitution not addition )
Benzene : Resonance Description
Electrophilic Substitution Reactions
Electrophilic substitution happens in many of the reactions of compounds
containing benzene rings – the arenes.
Benzene, C6H6, is a planar molecule containing a ring of six carbon atoms each
with a hydrogen atom attached
Kekule
Structures
Resonance
Energy = 36
Kcal / mole
All bonds are equivalent (The ring is symmetric. Bond lengths are between a
single and a double bond)
Very Stable (Less reactive than other groupings of atoms)
The “Double Bonds” in a Benzene Ring Do Not React Like Others
Alkene Benzene
RClH
R
H
Cl
ClH+ + no reaction
RCl
2
R
Cl
Cl
Cl2+ +
no reaction
RBr
2
R
Br
Br
Br2+ +
no reaction
R R ORCO
3H RCO
3H+ +
no reaction
E+SLOW
E
HH
E
H
E
H
resonance stabilized cation
E
H
"base"
re-aromatizeE
+ HB
delocalizedcation
:B
H
E
E
H
E
:X
restores
ring
resonance
intermediate
benzenium ion or a benzenonium ion
(+)
(+)
All of the reactions follow the same pattern of mechanism. The
reagents combine to form a strong electrophile E+ ,and
its partner (:X ), which react as follows:
Reaction Mechanism
Benzene is treated with a mixture of concentrated nitric acid and concentrated
sulphuric acid at a temperature not exceeding 50°C. As temperature increases
there is a greater chance of getting more than one nitro group, -NO2, substituted
onto the ring.Nitrobenzene is formed.
H2SO4
heat
The Nitration Of Benzene
If you are going to substitute an -NO2 group into the ring, then the
electrophile must be NO2+. This is called the "nitronium ion" or the "nitryl
cation", and is formed by reaction between the nitric acid and sulphuric acid
The Halogenation Of Benzene
Benzene reacts with chlorine or bromine in an electrophilic substitution reaction, but
only in the presence of a catalyst. The catalyst is either aluminium or ferric chloride (or
aluminium (ferric) bromide if you are reacting benzene with bromine) or iron.
FeCl3
As a chlorine molecule
approaches the benzene ring, the
delocalised electrons in the ring
repel electrons in the chlorine-
chlorine bond
It is the slightly positive end of the chlorine molecule which acts as the
electrophile. The presence of the ferreic chloride helps this polarisation.
Named after Friedel and Crafts who discovered the reaction.
Reagent : normally the acyl halide (e.g. usually RCOCl) with
aluminum trichloride, AlCl3, a Lewis acid catalyst.
The AlCl3 enhances the electrophilicity of the acyl halide by
complexing with the halide.
Friedel-Crafts Acylation of Benzene
Electrophilic species : the acyl cation or acylium ion (i.e. RCO+ ) formed by
the "removal" of the halide by the Lewis acid catalyst, which is stabilised by
resonance as shown below.
Some Substitution Reactions of Benzene
Cl2
AlCl3
Cl
CH3Cl
AlCl3
CH3
CH3
CCl
O AlCl3
C CH3
O
OH N
O
O
H2SO
4
N O
O
S
O
OH
OOH S
O
O
OHSO
3
+
+
+
+
+
Halogenation
Friedel-Crafts
Alkylation
Friedel-Crafts
Acylation
Nitration
Sulfonation
+ +
-
-
Question: Explain WHY?
then show the mechanism
R OH + HBr R-Br + H2O
R OH + HBr
H+
no reaction
HW
*
Addition Reaction
A + B AB
• An addition reaction is a reaction in which two molecules join together to make
a bigger one. Nothing is lost in the process. All the atoms in the original molecules
are found in the bigger one.
• In an addition reaction, new
groups X and Y are added to the
starting material. A bond is
broken and two bonds are
formed.
• Addition and elimination reactions are
exactly opposite. A bond is formed in
elimination reactions, whereas a bond
is broken in addition reactions.
The double bond dissolves back to single bond and new bonds reach out to A and
B whose bond is also dissolving
C C C C
BA
C C
A B
A-B can be :H-H H-OH H-X OH-OH OH-X
Addition Reaction
Types of addition reactions
Addition Reaction
1) Electrophilic Addition (Reactions Of Alkenes)
1) Nucleophilic Addition (aldehydes (RCHO) and ketones
(RCOR)
Electrophilic Addition
An electrophilic addition reaction is an addition reaction in which molecule
has a region of high electron density is attacked by another molecule, atom or
group carrying some degree of positive charge
Electrophilic Addition
Electrophilic addition happens in many of the
reactions of compounds containing carbon-carbon
double bonds (the alkenes e.g. ethene).
Electrophiles are strongly attracted to the exposed electrons in the π bond
H2C CH2+ HX CH3CH2X
X = Cl, Br, I
a. H2C CH2 H X+ H3C CH2 + X-
b. H3C CH2 + X- H3C CH2Xfast
slow
Mechanism
Electrophilic Addition
H-X is polarized; H is +, X is -
rates depend upon acid strength:
stronger the acid, faster the rate
HI > HBr > HCl>>>>>>>>HF
Addition of HX (HI, HBr, HCl)
Addition of Hydrogen Halides
When the reaction forms the carbocation intermediate,
the most highly substituted carbocation is favored :
tertiary > seconday > primary.
Step 1: H+ adds to C=C double bond Step 2: Br- ion adds to carbocation
Addition of Hydrogen Halides
However, if the double bond carbon atoms are not structurally equivalent, i.e.
unsymmetrical alkenes as in molecules of 1- propene, 1-butene, 2-methyl-2-butene and 1-
methylcyclohexene, the reagent may add in two different ways to give two isomeric
products. This is shown for 1-propene in the following equation.
Only one product is possible from the addition of these strong acids to symmetrical
alkenes such as ethene, 2-butene and cyclohexene. A
AA
A
+ HX
AA
H X
AA
(x= Cl or Br or I)
+ HClH3C
CH3
Cl
H
+ HI
H
I
Electrophilic Addition
Markovnikov’s rule stats that : In addition of unsymmetrical reagent to
unsymmetrical alkenes the positive ion adds to the carbon of the alkene that
bears the greater number of hydrogen atoms and the negative ion adds to the
other carbon of the alkene.
However when the addition reactions to such unsymmetrical alkenes are carried out,
it was found that 2-bromopropane is nearly the exclusive product. Thus it said the
reaction proceeded according to Markovnikov’s rule
+ HCl
H3CCH3
CH3
H3CCH3
CH3Cl
Electrophilic Addition
Mechanistic interpretation of Markovnikov’s rule: The reaction proceeds through
the more stable carbocation intermediate.
CH3 CH CH2
CH3 CH CH3
CH3 CH2 CH2
H BrBr
Br
Br Br
2º carbocationmore stable
1º carbocationless stable
Addition of HCl to 1-Propene.
It is a regioselective reaction, follow Markovnikov`s rule.
Addition of HBr to 1-Propene in presence of peroxide.
In the presence of peroxides (chemicals containing the general structure
ROOR'), HBr adds to a given alkene in an anti-Markovnikov fashion
Regioselective: One of the possible products is formed in larger amounts than
the other one(s).
Regiospecific: Only one of the possible products is formed (100%).
Electrophilic Addition Addition of Hydrogen Halides
Anti-Markovnikov addition
Only one product is possible from the addition of H2O in presence of acids as
catalysts to symmetrical alkenes such as ethene and cyclohexene.
However, addition reactions to unsymmetrical alkenes will result in the formation
of Markovonikov’s product preferentially.
CH3 CH3
OH
H
+ H2O
H
Unsymmetrical akenes
Symmetrical akenes
A
AA
A
+ H2O
AA
H OH
AA
H3CCH3
OH
H
+ H2O
H
H
Electrophilic Addition
Addition of H2O: Hydration
* ADDITION OF H2O
HBr and HCl easily add to alkenes. Since water also is a molecule of the type HX
which can donate a proton, H2O should be able to add to alkenes in the same way as
HBr, for example, resulting in the hydration of an alkene. However, for the addition
of H2O to alkenes to occur acid catalysts are required.
2. Nucleophilic Addtion
It is the most common reaction of aldehydes (RCHO) and ketones (RCOR)
e.g. The reaction of aldehydes and ketones with hydrogen cyanide
hydroxynitriles.
Classify each of the following as either substitution, elimination or addition
reactions. OH
Br
b)
c) OH
a)
HW
Draw the product of each of these examples of A-B when they add to 1-propene.
C C
H
H H
CH3
H-H H-OH H-X OH-OH OH-X
Hw
Characteristics of Aromatic Compounds
To be classified as aromatic, a compound must have
1-Cyclic structure
2-Coplanar structure.
3-Each atom of the ring must have a p orbital to
form a delocalized π system i.e. no atoms in the
ring can be sp3 hybridized instead all atoms must be
sp2 hybridized (N.B. carbocation and carbanions are
sp2 hybridized
4-Fulfill Huckel rule i.e. the system must have 4n + 2 pi electrons :
thus by calculating n value it will be an integral number i.e. n=0, 1, 2, 3,
Conjugated (C=C-C=C-C=C)
Examples of aromatic compounds
Examples of non aromatic compounds
N
O
n=1 n=1 n=1 n=0 n=1 n=1
sp3 C n=1/2 n=1/2 sp3 C n=1/2
Examples
35
Examples
10
6
a. IUPAC Names
They are named as derivatives of benzene. One side group is named as a prefix in front of the word benzene. No number is needed for mono-substituted benzene.
tert-Butyl-benzene Ethyl-benzene Nitro-benzene Chloro-benzene
C(CH3)3 CH2CH3 NO2 Cl
Nomenclature of Aromatic Compounds
1. Monosubstituted Benzenes
Benzene ring has priority over :side chains with alkyl, alkoxy groups, halogens, double and triple bonds
In some cases the side chains on aromatic
ring contain functional groups of higher
priorities (NH2, OH, CHO,C=O, COOH,
COOR) thus in this case the aromatic ring will
be considered as a substituent and the side
chain will be used to give the root name. Two
aromatic radials are known
CH2
Benzyl group
(C6H5-)
phenyl group
Vinyl-benzene Allyl-benzene Ethynyl-benzene Butyl-benzene
C CHOCH3
Methoxy-benzene
Nomenclature of Aromatic Compounds
b. Common Names Of Monosubstituted Benzenes
Toluene Styrene Phenol Benzaldehyde Benzoic acid Aniline
CH3 CH=CH2OH NH2H O HO O OCH3
Anisol
Nomenclature of Aromatic Compounds
All disubstituted benzenes (two groups are attached to benzene), can give
rise to three possible positional isomers.
When the substituents are different, they are of equal priorities they will
should be listed in alphabetical order.
2. Nomenclature of Disubstituted and polysubstituted Benzenes
X
Y
X X
Y
YCommon:
IUPAC:orth- meta para
1,2- 1,3- 1,4-
1-Chloro-2-ethylbenzene 1-Bromo-3-nitrobenzene 1-Fluoro-4-iodobenzene
C2H5
Cl
NO2
Br
o-Chloroethylbenzene m-Bromonitrobenzene p-Fluoroiodobenzene
IUPAC:
Common:
FI
Nomenclature of Aromatic Compounds
If one of the substituents is part of a parent compound, then the di-substituted
or poly-substituted benzene is named as a derivative of that parent compound
i.e. priorities determine the root name and substituents.
CH3 CH3 CH3
CH3
CH3
CH3Common:
IUPAC:o-Xylene m-Xylen p-Xylene
1,2-Dimethyl-benzene 1,3-Dimethyl-benzene 1,4-Dimethyl-benzene
OH
Cl
COOH NO2
Br
CH3
Common:
IUPAC:o- Chlorophenol m-Bromobenzoic acid p-Nitrotoluene o-Methoxybezaldehyde
2-Chlorophenol 3-Bromobenzoic acid 4-Nitrotoluene 2-Methoxybezaldehyde 2,4,6- Trinitrotoluene
CHO
OCH3
CH3
NO2
NO2O2N
Nomenclature of Aromatic Compounds
* 1-Electrophilic Aromatic Substitution Reactions
COR
RCOCl, AlCl3
Acylation
Reactions of Aromatic
42
Meta directors Ortho , para directors
-NO2 -SO3H -COOH, -COOR -CHO, -COR -CN
-OH, -OR -NH2, -NHR, -NR2 -C6H5 -CH3, -R (alkyl) -F, -Cl, -Br, -I
Reactions of Aromatic Alkyl groups and groups with lone pairs (electron donating groups) direct new groups to ortho-, para-
positions and speed-up the reaction (i.e. o & p directors and activating groups).
Halogens direct new groups to ortho-, para- positions but they slow down the
reaction (i.e. halogens are o & p directors and deactivating groups).
Electron withdrawing groups such as nitro, nitrile, and carbonyl direct new
groups to the meta-position and slow the reaction down (i.e. i.e. m directors and deactivating groups).
Thus the order of reactivity of benzene and monosubstituted benzene derivatives in E.Ar.sub. is as in the
following chart.
Substituted benzene with
o,p directors > Benzene > Halobenzene derivatives > Substituted benzene with m- directors
1)Halogenation a) alkyl side chain(using UV)
CH3
Br2
UV
CH2Br
HBr
CH2CH3
Cl2/ UV
CHClCH3CH2CH2Cl
Major Minor
Or
b) Substituted benzene (with CCl4, AlCl3,FeCl3 (two products)
OH
Br2/
CCl4
OH OH
+
Br
Br
Reactions of Aromatic
2-Side-Chain Reactions of Aromatic Compounds
OH
HNO3 / H2SO4
OH OH
+
NO2
NO2o-Nitrophenol 53 % p-Nitrophenol
47 %NO2
SO3 / H2SO4
NO2
m-Nitrobezenesulfonic acid
SO3H
Reactions of Aromatic
2. Nitration
Reactions of Aromatic
3.Oxidation
CH3KMnO4
Toluene
COOH
Benzoic acid
CH2CH3KMnO4
COOH
Benzoic acid
+ + OH2CO2
Q1: What is the empirical formula of the following compound: (p-methyl-Toluene):
a) C8H10 b) C8H12 c) C8H14 d) C6H14
Q2: What is the final product of the following reaction?
a) o-chlorobenzaldehyde b) m-chlorobenzaldehyde c) p-chlorobenzaldehyde d) a,c
Q3:Which one of the following compounds has aromatic
character?
*