Mono-chlorination proceeds through free radical mechanism under sunlight and dry atmosphere. 2-methylpropane forms free radicals by breaking of C ─ H bond while in the influence of a free radical nucleophile that supports homolytic bond fusion of double bond.

There are 9 primary hydrogen but all are bonded to similar bonded carbon so there is only one type of 1° free radical shown above. This radical is stabilised by tertiary hydrogen from neighbouring atom by hyperconjugation thus giving one hyperconjugating structure.

The 3° free radical is stabilised by 9 hyperconjugating structures and thus has more stability as compared to 1° free radical.