Benzene and Alkyl Benzene - Result Question 35
####39. Give reasons for the following :
(i) tert-butylbenzene does not give benzoic acid on treatment with acidic $KMnO _4$.
(ii) Normally, benzene gives electrophilic substitution reaction rather than electrophilic addition reaction although it has double bond.
(2000)
Show Answer
Solution:
- (i) Oxidation of side-chain alkyl group of benzene occur through free radical mechanism and it initiates at $\alpha-C$ as.
$$ Ph-\stackrel{C}{\mid}-H \longrightarrow Ph-C _{\mid}^{\bullet}+H^{\bullet} $$
In tertiary butyl benzene, there is no $\alpha-H$, required to initiate free radical oxidation reaction, hence not oxidised with $KMnO _4$.
tert butyl benzene (no $\alpha-H$ )
(ii) Pi-electrons of double bonds are involved in aromatic delocalisation (aromaticity), hence electrophilic addition do not occur as it would destroy aromatic stability. However, electrophilic substitution do not destroy aromaticity.
