JEE Advanced · 2021 · 03 Oct 2021 · Paper 1 · Hydrocarbons

Free-radical allylic bromination of 1-methylcyclohex-1-ene must be traced through every allylic radical the alkene and its allylic $\mathrm{C}-\mathrm{H}$ bonds can furnish. Abstraction from the ring methyl, from $C3$ and from $C6$ each gives a delocalised allylic radical, and each resonance terminus can capture bromine. The achiral exocyclic bromomethyl product is unique, while every ring carbon that becomes a new stereocentre splits into an $R$ and an $S$ form. Counting the single achiral product together with six pairs of enantiomers gives a maximum of $13$ distinct species including stereoisomers.