Answer to Question #86311 in Organic Chemistry for Gayatri

Question

Explain the following:
i) In the electrophilic substitution reactions, 1-position of naphthalene is more reactive than its 2-position. (Draw all possible resonance structures).

ii) Chlorination of ethylbenzene gives 1-chloro-1-phenylethane as the major product.

Answer 1

i) The most efficient stabilization of the intermediate carbocation produced by electrophilic attack on naphthalene comes from those resonance forms which retain one fully benzenoid ring. In the case of 1-substitution, two such structures can be drawn (A, B). For 2-substitution there is only one structure (F). The intermediate from attack at the 1-position is more stable and therefore the 1-substituted product is favoured in the reactions of electropholic substitution of naphthalene.

ii) The stability of the product of radical chlorination is determined by stability of intermediate radical:

1) tetratry>secondary>primary;

2) we should take into acoount possibility of conjugation, which makes intermediate radical more stable

Chlorination in β-position gives primary intermediate radical and there is no conjugation

Chlorination in α-position gives secondary intermediate radical and there is conjugation (resonance structures)

We can see that chlorination in α-position gives more stable intermediate radical, therefore formation of 1-chloro-1-phenylethane is favoured.