SolutionsHydrolysis of Geminal Dihalides
1. From ethylbenzene to acetophenone
What is obtained when ethylbenzene is treated with two equivalents of NBS (light or peroxides) and then with aqueous NaOH?
Free-radical bromination takes place at the benzylic position, the only one that gives a resonance-stabilized radical. With two equivalents (1,1-dibromoethyl)benzene is obtained. Hydroxide replaces one bromine; the resulting geminal bromohydrin loses HBr and gives acetophenone.
2. Aldehyde or ketone?
Which carbonyl compound is obtained by this method from toluene? And from 3-methylcyclohexene?
Toluene gives (dibromomethyl)benzene and, after hydrolysis, benzaldehyde. In 3-methylcyclohexene the most reactive allylic position is C3 (tertiary), but it has only one hydrogen and cannot be halogenated twice; the double halogenation must take place at C6 (secondary allylic CH2) and, after hydrolysis, gives 4-methylcyclohex-2-enone (the ketone ends up conjugated with the double bond). Because the allylic radical is delocalized, other isomers are also obtained in practice.
The method is only useful at benzylic or allylic positions with two hydrogens: a CH2 gives a ketone and a CH3 an aldehyde.