The correct order of relative reactivity of alkyl halides towards \(S_N1\) reaction is:
(I) \(PhCH_2Cl\) (II) \(Ph_2CHCl\) (III) \(Ph_3C-Cl\)
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → III > II > I
An SN1 reaction goes through a carbocation; the more stable the carbocation, the faster the reaction. Benzylic halides react fast by SN1 because the benzyl cation is stabilised by resonance with the ring.
(I) \( \ce{PhCH2Cl} \) → \( \ce{PhCH2^{+}} \): positive charge spread over one benzene ring.
(II) \( \ce{Ph2CHCl} \) → \( \ce{Ph2CH^{+}} \): charge spread over two rings, more stable.
(III) \( \ce{Ph3C-Cl} \) → \( \ce{Ph3C^{+}} \): charge spread over three rings, the most stable.
NCERT (2026-27 edition):
Substitution Nucleophilic Unimolecular (SN1) – “… 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 …”