Lab 2

It’s the aliphatic portion of the alkylbenzene.
CH3
Methylbenzene
(toluene)
Ethylbenzene
Isopropylbenzene
(cumene)
Phenylethen
(styrene or
vinylbenzene)
 Terms you should
know :
Free Radicals
 Benzylic radical
 Benzylic hydrogen atom
 Benzylic cation

An atom or group of atoms that has at
least one unpaired electron ,so:
 Unstable
 Highly reactive


Is the general name applies to all radicals
that have unpaired electron on the carbon
atom attached directly to the benzene
ring.

Is the hydrogen atom of the carbon atom
that directly attached to the benzene ring.

Is the departure of a leaving group (LG) from a
benzylic position.
Ring 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.
Side- chain halogenation




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
hydrogen's of the methyl
group.
Chlorination
CH3
CH2CL
CL2
CL2
heat or
light
CL2
heat or
light
Benzyl
chloride
CCL3
CHCL2
Dichloromethlbenzene
heat or
light
Trichloromethylbenzene
Chain Initiation
Step 1
X
X
peroxides,
2X
heat, or light
Peroxides, heat, or light cause halogen
molecules to cleave into radicals.
Chain propagation
Step 2
H
H
C6H5
C
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
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
Step 4
C6H5CH2 +
and C6H2CH2 +
X
CH2C6H5
C6H5CH2
X
C6H5CH2
Various radical coupling reaction terminate the chain.
CH2C6H5

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 take place initially at
the benzylic carbon; any 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

Reaction:
COOH
CH3
+
KMnO4
NaOH
+ MnO2
+
H2O
Thank you