Regioselectivity of Alkane Halogenation

The chlorination of higher-molecular-weight alkanes yields a mixture of isomeric monochlorinated products. For example, the chlorination of butane or 2-methyl-propane, which have nonequivalent hydrogen atoms, yields significant amounts of isomeric monochlorinated derivatives. 

In contrast to chlorination, the bromination of alkanes is highly regioselective. For example, in the photochemical bromination of 2-methylpropane, more than 99% of the product results from substitution of bromine for the tertiary hydrogen atom. 

The halogenation of an alkane occurs by a free radical mechanism. The first step is the formation of a halogen radical, which subsequently abstracts a hydrogen atom from the alkane. These steps yield a carbon radical, which reacts with the halogen molecule to give a halogenated product. 

Let s consider the chlorination of butane again. If the propagation step generates a primary radical, the product is a primary halide. Thus, the -butyl radical gives 1-chlorobutane. 

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