Q.The most easily hydrolysed molecule under SN1 condition is a) allyl chloride b) ethyl chloride c) isopropyl chloride d) benzyl chloride

Two mechanisms carry out nucleophilic substitution, distinguished by kinetics, stereochemistry and which substrates favour each. SN2 (bimolecular): Rate = k2[RX][Nu-], second order, ONE step. The nucleophile attacks from directly opposite the leaving halogen (backside attack) through a single transition state, inverting the carbon's configuration exactly as an umbrella flips in a strong wind (Walden inversion) -- so an optically active substrate with the leaving group ON the stereocentre gives a single inverted, still optically active product. SN2 favours LESS hindered carbons: methyl > primary > secondary >> tertiary (tertiary essentially never goes SN2, since three alkyl groups block the required backside approach). SN1 (unimolecular): Rate = k[RX] only, independent of nucleophile concentration, first order, TWO steps -- (1) slow, rate-determining heterolysis of C-X to a planar carbocation (sp2, empty p-orbital with two equal lobes), then (2) fast attack by the nucleophile from EITHER face. Because both faces are attacked with roughly equal probability, a chiral SN1 substrate (halogen on the stereocentre) gives a racemic (optically inactive) mixture. SN1 favours MORE substituted carbons: tertiary > secondary > primary > CH3X, since more alkyl groups (inductive donation plus hyperconjugation) stabilise the in …