Free radical substitution. Part 38. The effect of solvent on the atomic chlorination and bromination of 2-substituted butanes and the importance of steric effects

The relative selectivity of atomic halogenation of 2-substituted butanes is influenced by the phase and by solvents. There are solvents which increase the selectivity compared with the gas phase and solvents which decrease the relative selectivity. However the most striking feature of the halogenation (especially the bromination) of 2-substituted butanes is the high reactivity of the 2-position notwithstanding very unfavourable polar effects. This reactivity is attributed to the release of steric compression associated with the abstraction of the tertiary hydrogen atom. The halogenation of butan-2-ol esters is associated with some decomposition of 2-butyl radical [CH₃CH ₂C(OCOR)CH₃] and the chlorination of 2-phenylbutane with the formation of olefins 2-phenylbut-1-ene and 2-phenylbut-2-ene.