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)

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Solution:

  1. (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.



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