Question

Benzene derivative X has the empirical formula C9H12. Its bromination in the light leads to the formation of two monobromo derivatives in approximately identical yield. Bromination in the dark in the presence of iron also gives two monobromo derivatives. If the reaction is carried out to a higher degree, the formation of four dibromo derivatives may occur.

Suggest the structure for compound X and for the bromination products. Write schemes for the reactions.

Answer 1

The compound with the empirical formula C9H12 can be:

Under the action of bromine in the light without catalysts, bromination of the aliphatic portion will occur, predominantly on the carbon atoms bonded to the aromatic nucleus. When the reaction is conducted in the dark in presence of iron, the latter is converted to FeBr3 and catalyzes the bromination of the aromatic ring.

Compound X cannot be I (as then only one monobromo derivative would be formed in the light); it cannot be one of the isomers IIIa, IIIb either.

IIIa - Only one monobromo derivative is possible in the bromination of the CH3 groups.

IIIb - Three different monobromo derivatives are possible under the same conditions.

Thus, selection must be made from the following four structures:

The condition that two monobromo derivatives can be formed in the dark, rules out structures IIa and IIb. The condition of the possibility of four dibromo derivatives rules out structure IIIc.Hence, the only possible structure of compound X is IIc. The scheme of the bromination reaction (next page):