Question

Give reasons for the following observations:

(i) p-dichlorobenzene has higher melting point than those of o and m –isomers.

(ii) Haloarenes are less reactive than haloalkanes towards nucleophillic substitution reaction.

(iii) The treatment of alkyl chloride with aqueous KOH leads to the formation of alcohol but in the presence of alcoholic KOH, alkene is the major product,

Answer 1

(i) It is due to the symmetry of para-isomers that fits in the crystal better as compared to ortho and meta-isomers.

(ii) Resonance effect / Difference in hybridization of carbon atom in C-X bond / Instability of phenyl cation / because of the repulsion, it is less likely for the electron-rich nucleophile to approach electron rich arenes .

(iii) Alkoxide ion present in alcoholic KOH, is not only a strong nucleophile but also a strong base.