19. Reactions of Aromatics: EAS and Beyond

For each compound, predict the major product of free-radical bromination. Remember that bromination is highly selective, and only the most stable radical will be formed.

(e)

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)

Propose a mechanism for the bromination of ethylbenzene shown below.

What are the products of the following reactions?

c.

Predict the major products of the following reactions.

(f) p-methylanisole + Br₂ , light

Predict the product of the following benzylic bromination reactions.

(b)

Show how you would convert (in one or two steps) 1-phenylpropane to the three products shown below. In each case, explain what unwanted reactions might produce undesirable impurities in the product.

The solvent tetrahydrofuran (THF) is often sold with a small amount of BHT added. Provide a mechanism that explains why this might be so.

A halogenation intended to make compound A formed B instead.

(b) Suggest a mechanism that rationalizes the formation of B.

A halogenation intended to make compound A formed B instead.

(d) Without looking it up, would you expect C–H_a or C–H_b to have the lower bond-dissociation energy?

In the second propagation step in the bromination of toluene, Br₂ is only attacked by a radical on the substituent carbon. Why?

(c) Based on what you know about the relative stabilities of alkyl radicals and benzylic radicals, predict the product of addition of HBr to 1-phenylpropene in the presence of a free-radical initiator.

(d) Propose a mechanism for this reaction.