Arrange the following compounds in increasing order of \(S_N1\) reactivity
A. Primary alkyl halide
B. Tertiary alkyl halide
C. Vinylic halide
D. Secondary alkyl halide
E. Allylic halide
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → C, A, D, B, E
An SN1 reaction goes through a carbocation, and its slow step is the ionisation of the C–X bond. The more stable the carbocation, the faster the reaction.
C. Vinylic halide — first (least reactive). The C–X bond is on an sp2 carbon and gets partial double-bond character from resonance, and the vinyl cation is very unstable, so vinylic halides hardly react.
A. Primary alkyl halide — second. The primary carbocation has only one alkyl group to stabilise it.
D. Secondary alkyl halide — third. Two alkyl groups stabilise the cation by +I effect and hyperconjugation.
B. Tertiary alkyl halide — fourth. Three alkyl groups give the most stable alkyl carbocation.
E. Allylic halide — last (most reactive). The allyl cation is stabilised by resonance, the positive charge being spread over two carbons:
\( \displaystyle \ce{CH2=CH-CH2^+ <-> ^+CH2-CH=CH2} \)
NCERT (2026-27 edition):
Substitution Nucleophilic Unimolecular (SN1) – “SN1 reactions are generally carried out in polar protic solvents (like water, alcohol, acetic acid, etc.). The reaction between tert-butyl bromide and hydroxide ion yields tert-butyl alcohol and follows the first order kinetics, i.e., the rate of reaction depends upon the concentration of only one reactant, which is tert-butyl bromide. …
… 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.”