Rearrangement during reaction of halogen compounds

Similarly, on reaction with o-phenylenediamine, 2/ ,4/ -dibromo-5/ -cho-lestan-3-one gives the quinoxaline derived from the 3,4-diketone.64b 

Thus considerable care is needed when attempting to deduce the structure from reactions of halo carbonyl compounds. 

A rearrangement that sometimes has preparative use occurs on hydrolysis of an 1,2-alkylene dibromide where one bromine atom is attached to a tertiary carbon atom. The dibromide may be obtained from the alkene or by bromina-tion of the corresponding tertiary alcohol. Production of isobutyraldehyde is a particularly smooth reaction 73 

2-Methylpropylene dibromide (2 moles) is heated with water (21) in a flask fitted with a fractionating column (90 cm in length). Heating is arranged so that at first the column acts only as a reflux condenser, but later the temperature is increased. When the temperature at the column head reaches 59-64° it is isobutyraldehyde that distils. After 50 h the dibromide layer in the reaction flask has disappeared and much distillate (185 g) has collected. Redistillation gives isobutyraldehyde (108 g, 75 %), b.p. 58-65°. The hydrolysis can be accelerated considerably by stirring. 

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