The species which does not show high reactivity towards \(S_N1\) reaction is:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Primary halide
An SN1 reaction goes through a carbocation, so its rate depends on how stable that carbocation is. Allylic and benzylic carbocations are stabilised by resonance; tertiary carbocations by the +I effect and hyperconjugation of three alkyl groups. A primary carbocation has neither, so it is the least stable.
\( \ce{CH2=CH-CH2-X -> CH2=CH-CH2^+ + X^-} \)
\( \ce{CH2=CH-CH2^+} \leftrightarrow \ce{^+CH2-CH=CH2} \)
Option (1) Primary halide — Not highly reactive towards SN1. Its carbocation is the least stable; a primary halide reacts mainly by SN2.
Option (2) Allylic halide — Highly reactive towards SN1. The allylic carbocation is stabilised by resonance with the C=C bond.
Option (3) Benzylic halide — Highly reactive towards SN1. The benzylic carbocation is stabilised by resonance with the benzene ring.
Option (4) Tertiary halide — Highly reactive towards SN1. The tertiary carbocation is stabilised by three alkyl groups.
NCERT (2026-27 edition):
Reactions of Haloalkanes – “… Step I is the slowest and reversible. It involves the C–Br bond breaking for which the energy is obtained through solvation of halide ion with the proton of protic solvent. Since the rate of reaction depends upon the slowest step, the rate of reaction depends only on the concentration of alkyl halide and not on the concentration of hydroxide ion. Further, greater the stability of carbocation, greater will be its ease of formation from alkyl halide and faster will be the rate of reaction. In case of alkyl halides, 3° alkyl halides undergo SN1 reaction very fast because of the high stability of 3° carbocations. … For the same reasons, allylic and benzylic halides show high reactivity towards the SN1 reaction. The carbocation thus formed gets stabilised through resonance (Unit 8, Class XI).”