Sn1 Reactivity Order

The slow, rate-determining step of unimolecular substitution is the ionisation of the carbon-halogen bond to give a carbocation, so the reaction is fastest when that carbocation is most stable. Carbocation stability increases from methyl to primary to secondary to tertiary because alkyl groups donate electron density through inductive and hyperconjugative effects, spreading out the positive charge. tert-Butyl bromide forms a tertiary carbocation, the most stabilised of the set, and therefore undergoes SN1 fastest. The methyl-bromide option about least hindrance describes the SN2 preference, not the SN1 trend, since methyl gives the least stable cation. The ethyl-bromide claim is wrong because a primary carbocation is poorly stabilised. The isopropyl-bromide option is incorrect because, although secondary cations are reasonably stable, they are still less stable than the tertiary cation from tert-butyl bromide. The very steric crowding that makes tert-butyl bromide the slowest substrate toward SN2 simultaneously makes it the fastest toward SN1, since once the bulky groups are relieved by ionisation they help stabilise the resulting flat cation. This neat inversion of the reactivity order between the two mechanisms is a frequently tested idea in JEE problems. This follows the NCERT order of carbocation stability. Sanity check: more alkyl substitution gives a more stable cation, so tert-butyl reacts fastest by SN1.