Arrange the following alkyl bromides in order of their increasing reactivity towards SN1 reaction.
A. 1-Bromobutane
B. Benzyl bromide
C. 3-Bromobut-1-ene
D. 2-Bromobutane
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
In an SN1 reaction, the slow step is the ionisation of the alkyl halide into a carbocation and a halide ion. So the more stable the carbocation formed, the faster the reaction. Carbocations are stabilised by:
Alkyl groups (+I effect and hyperconjugation): 3° > 2° > 1°.
Resonance: an allylic cation spreads its positive charge over two carbons, and a benzylic cation spreads it over the \( \ce{CH2} \) carbon and three ring carbons (ortho, ortho, para). Resonance-stabilised cations are more stable than simple alkyl cations.
Carbocation formed from each bromide:
A. 1-Bromobutane: \( \ce{CH3CH2CH2CH2^{+}} \) — primary, least stable.
D. 2-Bromobutane: \( \ce{CH3CH2CH^{+}CH3} \) — secondary.
C. 3-Bromobut-1-ene: \( \ce{CH2=CH-CH^{+}-CH3} \) — allylic, stabilised by two resonance structures.
B. Benzyl bromide: \( \ce{C6H5CH2^{+}} \) — benzylic, with the charge delocalised into the ring over four resonance structures.
\( \displaystyle \text{primary} < \text{secondary} < \text{allylic} < \text{benzylic} \)
Shortcut: The primary bromide A is the slowest and benzyl bromide B is the fastest. Only Option (2) starts with A and ends with B.
NCERT (2026-27 edition):
Reactions of Haloalkanes – “… 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.”