Side chain oxidation
Lab 2
Side chain
It’s the aliphatic portion of the alkylbenzene.
CH3
Methylbenzene
(toluene)
Ethylbenzene
Isopropylbenzene
(cumene)
Phenylethen
(styrene or
vinylbenzene)
Benzylic Radicals and Cations
• Free Radicals:
• An atom or group of atoms that has at
least one unpaired electron ,so:
• Unstable
• Highly reactive
Benzylic Radicals and Cations
• Benzyl radical: is a specific name for the
radical produces from the Hydrogen
abstraction from the methyl group of
methylbenzene (tolune).
CH2
H CHS
R
- RH
Methtlbenzene
(toluene)
The benzyl
radical
Benzylic Radicals and Cations
• Benzylic radical: is the general name applies to
all radicals that have unpaired electron on the
side- chain carbon atom that attached directely
to the benzene ring.
C
A benzylic
radical
Benzylic Radicals and Cations
• Benzylic hydrogen atoms: is the hydrogen
atom of the carbon atom that directly attached
to the benzene ring.
C
Benzylic
carbon
H
A
benzylic
hydrogen
Benzylic Radicals and Cations
• Benzylic cation: is the departure of a leaving
group (LG) from a benzylic position.
C
LG
C
- LG-
A benzykic cation
Benzylic Radicals and Cations
• They have approximately the same stabilities,
this stability can be explained by resonance
theory.
C
C
C
C
C
C
Benzylic radicals are
stabilized by resonance
Benzylic cation are
stabilized by resonance
C
C
Halogenation of the side chain
Ring halogenation
Side- chain halogenation
•The electrophiles are positive
ion or they are Lewis acid
complexes with positive charge
•Need catalyst e.g. Lewis acid
•These positive electrophiles
attack the ∏ electron of the
benzene ring.
•Stabilized by Arenium ions
•the aromatic substitution takes
place.
•It take place when the reaction
is carried out in the absence of
lewis acid
•Require condition that favor
the formation of the radicals.
•The halogens dissociate to
produce halogen atoms
•The halogen atoms initiate
chain reaction by abstracting
hydrogens of the methyl group.
Bromination
O
O
CH3
N
Br
+
Br
Light
N
+
CCL4
O
O
NBS
Benzyl bromide
( a- bromotoluene)
(N- bromosuccinimide)
Chlorination
CH3
CH2CL
CL2
CL2
heat or
light
CL2
heat or
light
Benzyl
chloride
CCL3
CHCL2
Dichloromethlbenzene
heat or
light
Trichloromethylbenzene
H
Benzylic halogenation
Chain Initiation
Step 1
X
peroxides,
X
heat, or light
2X
Peroxides, heat, or light cause halogen
molecules to cleave into radicals.
Chain propagation
H
Step 2
C6H5
C
H
H
H +
X
C6H5
C
+ H
H
Benzyl radical
A halogen radical abstracts a benzylic hydrogen atom, forming a benzylic
radical and a molecule of hydrogen halide.
X
Benzylic halogenation
Step 3
H
H
C6H5
C
+ X
X
C6H5
C
X + X
H
H
Benzyl radical
Benzyl halide
The benzylic radical reacts with a halogen molecule to form benzylic halide
product and a halogen radical that propagates the chain.
Chain Termination
C6H5CH2 +
Step 4
X
and C6H2CH2 + CH2C6H5
C6H5CH2
X
C6H5CH2
Various radical coupling reaction terminate the chain.
CH2C6H5
Oxidation of the side chain
• The oxidation can be carried out by the action
of hot alkaline potassium permanganate.
O
CH3
OH
(1) KMnO4, OH , heat
(2) H3O
Toluene
Benzoic acid
Oxidation of the side chain
• Oxidation of the side- chain take place initially
at the benzylic carbon; alkylbenzenes with
alkyl group longer than methyl are ultimately
degraded to benzoic acids:
O
CH2R
OH
(1) KMnO4, OH , heat
(2) H3O
An alkylbenzene
Benzoic acid
Oxidation of the side chain
• Reaction:
COOH
CH3
+
KMnO4
NaOH
+ MnO2 +
H2O
Procedure:
• Dissolve 1 gm NaOH in 75 ml of distal water
in round flask (500ml).
• Add 7 gm of potassium permanganatr
KMnO4, and 4 ml of toluene. And heat it on
hot plate for 3 hr.
Procedure:
• In case if the KMnO4 color don’t change, add
few drops of ethanol until the violet color
disappear.
• Filtre by Buchner, take the filtrate and heat it
until the volume reduced
to 10- 20 ml.
Procedure:
• Cool the filtrate on a iced cold water, then add
HCL drop wise until the ppt stop.
• Fitre by Buchner.
Thank you