Side chain

It’s the aliphatic portion of the alkenylbenzene .

CH3

Methylbenzene (toluene)

Ethylbenzene Isopropyl- benzene (cumene)

Phenylethen (styrene or vinylbenzene)

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).

CHSHR

CH2

- RH

Methtlbenzene The benzyl (toluene) 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 H

Benzylic carbon

A benzylichydrogen

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

Benzylic radicals are stabilized by resonance

C C C C

Benzylic cation are stabilized by resonance

Halogenation of the side chain Side- chain halogenation Ring halogenation

•It take place when the reaction is carried out in the absence of lewis acid and under condition that favor the formation of the radicals.

•The halogens dissociate to produce halogen atoms and then halogen atoms initiate chain reaction by abstracting hydrogens of the methyl group.

•The electrophiles are positive chlorin or bromine ion or they are lewis acid complexes that have positive halogen.

•These positive electrophiles attack the ∏ electron of the benzene ring, the aromatic substitution takes place.

Bromination

Chlorination

CH3

+

O

O

N Br Light

CCL4

Br

+ O

O

N H

NBS

(N- bromosuccinimide)

Benzyl bromide ( a- bromotoluene)

CH3 CH2CL CHCL2 CCL3

CL2 CL2 CL2

heat orlight

heat orlight

heat or light

Benzyl Dichloromethl- Trichloromethyl-chloride benzene benzene

Benzylic halogenation

Chain Initiation Step 1

Chain propagationStep 2

X X 2 X peroxides,

heat, or light

Peroxides, heat, or light cause halogenmolecules to cleave into radicals.

C6H5 C H + X

H

H

C6H5 C + H X

H

H

A halogen radical abstracts a benzylic hydrogen atom, forming a benzylicradical and a molecule of hydrogen halide.

Benzyl radical

Benzylic halogenation

Step 3

Chain Termination Step 4

C6H5 C + X X C6H5 C X + X

H

H

H

HBenzyl radical Benzyl halide

The benzylic radical reacts with a halogen molecule to form benzylic halide product and a halogen radical that propagates the chain.

C6H5CH2 + X C6H5CH2 X

and C6H2CH2 + CH2C6H5 C6H5CH2 CH2C6H5

Various radical coupling reaction terminate the chain.

Oxidation of the side chain

• The oxidation can be carried out by the action of hot alkaline potassium permanganate.

CH3 OH

O

(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:

CH2R OH

O

(1) KMnO4, OH , heat(2) H3O

An alkylbenzene Benzoic acid

Oxidation of the side chain

• Reaction:

+ KMnO4 NaOH

COOHCH3

+ 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