Q.Choose the compound from the following that will react fastest by SN1 mechanism.
a. 1-iodobutane
b. 1-iodopropane
c. 2-iodo-2-methylbutane
d. 2-iodo-3-methylbutane

Three factors decide whether a given alkyl halide reacts by SN1 or SN2. Nature of the substrate: SN2's crowded, pentacoordinate transition state is disfavoured as substitution increases, so SN2 reactivity follows primary>secondary>tertiary; SN1's planar carbocation intermediate has no steric crowding and is instead stabilised by the +I effect and hyperconjugation from alkyl substituents' alpha-hydrogens, so SN1 reactivity follows the opposite order, tertiary>secondary>primary. Primary allylic and primary benzylic halides are important exceptions that react readily by SN1 despite being primary, because their carbocations are resonance-stabilised (the positive charge delocalises through the adjacent double bond or aromatic ring). In practice tertiary halides react by SN1, primary halides by SN2, and secondary halides by either or a mixture, depending on conditions. Nucleophilicity of the reagent: a stronger nucleophile (better Lewis base; a negatively charged species beats its own conjugate acid; nucleophilicity falls left-to-right across a period but rises down a group) speeds up SN2, because the nucleophile is a reactant in SN2's rate-determining step, but has no …