Question

Decreasing order towards \(S_N1\) reaction for the following compounds is:

• A. \(a \gt c \gt d \gt b\)
• B. \(d \gt b \gt c \gt a\)
• C. \(a \gt b \gt c \gt d\)
• D. \(b \gt d \gt c \gt a\)

Hint:

\(S_N1\) rate depends on carbocation stability. For para-substituted benzyl cations, +M groups (OMe) stabilise, \(-\mathrm{NO_2}\) and (to a smaller extent) Cl destabilise.

Answer 1

The Correct Option is D

All four compounds are benzylic chlorides, p-X-C₆H₄CH₂Cl (a: X = NO₂, b: X = OMe, c: X = Cl, d: X = H). In the \(S_N1\) rate-determining step they ionise to the benzyl carbocation, so the rate follows the stability of p-X-C₆H₄CH₂⁺.
b (OMe): strong +M (lone pair donation) greatly stabilises the cation - fastest.
d (H): parent benzyl cation - reference.
c (Cl): the -I effect outweighs +M, so the cation is slightly destabilised relative to H.
a (NO₂): strong -M and -I, strongly destabilises the cation - slowest.
Order: b > d > c > a.
Answer: (4) b > d > c > a