Question

Predict the major products when the following compounds are irradiated by light and treated with (1) 1 equivalent of Br₂ and (2) excess Br₂.

(a) isopropylbenzene

(b) tetralin

Short answer

(a) isopropylbenzene

(b) tetralin

Step-by-step solution

Side-Chain Halogenation.

Alkylbenzenes undergo free-radical halogenation much more easily than alkanes because the abstraction of a hydrogen atom at a benzylic position gives a resonance stabilized benzylic radical. For example, ethylbenzene reacts with chlorine in the presence of light to give a a- chloroethylbenzene. Further chlorination can occur to give a dechlorinated product.

Subpart (a) Isopropylbenzyl irradiated by light and treated with bromine.

1. When isopropyl benzene is reacted with 1 equivalent of Br₂ , only one hydrogen is replaced by bromine.

2. When isopropyl benzene reacted with excess Br₂, the product is:

Subpart (b) Tetralin irradiated by light and treated with bromine.

1. When tetralin reacted with 1 equivalent of Br₂ , only one hydrogen is replaced by bromine:

2. When tetralin reacted with excess Br₂, all four benzylic hydrogens are replaced: