The correct order of increasing reactivity of C-X bond towards nucleophilic substitution reaction (\( S_N1 \)) in the following compounds is
(A) Chlorobenzene
(B) Benzyl Chloride
(C) Tertiary Butyl Chloride
(D) 1-Chlorobutane
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A), (D), (C), (B)
An SN1 reaction goes in two steps, and the slow step is the ionisation of the C–X bond to a carbocation. So the reactivity follows the stability of the carbocation formed: the more stable the carbocation, the faster the reaction.
(A) Chlorobenzene: the C–Cl bond has partial double bond character from resonance, and the phenyl cation would be very unstable (the positive charge in an sp2 orbital, not delocalised). It practically does not react: the least reactive.
(D) 1-Chlorobutane: it would give a primary carbocation, which is poorly stabilised; slow.
(C) Tertiary butyl chloride: it gives the tertiary carbocation \( \ce{(CH3)3C+} \), stabilised by the +I effect and hyperconjugation of three methyl groups; fast.
(B) Benzyl chloride: it gives the benzyl carbocation \( \ce{C6H5CH2+} \), in which the positive charge is delocalised over the ring by resonance; the fastest.
In increasing order of SN1 reactivity:
\( \displaystyle \text{chlorobenzene} \lt \text{1-chlorobutane} \lt \text{tert-butyl chloride} \lt \text{benzyl chloride} \)
Shortcut: Chlorobenzene is the least reactive, so the order starts with (A): Options (1) and (2). A primary halide is less reactive than a tertiary one, so (D) comes before (C), which leaves Option (2).