JEE Main · 2023 · 11 Apr 2023 · Shift 2 · Haloalkanes and Haloarenes

A halide precipitates $\mathrm{AgBr}$ with $\mathrm{AgN}{\mathrm{O}}_3$ only when the carbon-bromine bond ionises readily. Benzyl bromide gives a benzylic cation, cinnamyl bromide an allylic cation, and bromocyclopropene the aromatic cyclopropenyl cation, so each of these ionises and forms a precipitate. In $1$-bromocyclohexene the bromine is attached to a doubly bonded carbon; overlap of the bromine lone pair with the $\pi$ bond gives the carbon-bromine bond partial double-bond character and prevents ionisation. This vinylic bromide, $1$-bromocyclohexene, gives no precipitate with $\mathrm{AgN}{\mathrm{O}}_3$.