MHTCET 15 Apr 2026 Shift 2 Paper

Concept:The $\mathrm{SN}^{2}$ mechanism is a one-step, concerted bimolecular reaction.Explanation:In the $\mathrm{SN}^{2}$ mechanism, the nucleophile attacks the carbon from the backside while the leaving group departs simultaneously.Bond formation and bond breaking occur together in a single step.During this transition state, the carbon is partially bonded to both the incoming nucleophile and the outgoing leaving group.Thus, the transition state has a pentacoordinated carbon:$$[\mathrm{Nu}\cdots R\cdots L]^{\ddagger}$$This matches Option B.Option A is wrong because $\mathrm{SN}^{2}$ gives mainly inversion of configuration, not racemization.Option C is wrong because no carbocation intermediate is formed in $\mathrm{SN}^{2}$.Option D is wrong because an $\mathrm{sp}^{2}$ hybridized carbocation intermediate is characteristic of the $\mathrm{SN}^{1}$ mechanism.Answer:Option B — The $\mathrm{SN}^{2}$ mechanism includes a transition state with a pentacoordinated carbon.