Question

Which of the following will undergo a faster $S_{N}1$?

• A. Neither I nor II
• B. Both undergo $S_{N}1$ at the same rate
• C. I
• D. II

Answer 1

The correct option is D

II

For $S_{N}1$, the ideal substrates are tertiary halides (X = Cl, Br, I) or tertiary alcohols ( under the right conditions). Here I is a primary alkyl bromide whereas II is a secondary substrate. Hence, the latter undergoes a faster $S_{N}1$ reaction. Do note that, ionization happens first in a slow, rate-determining step, leaving behind a carbocation. The carbocation then undergoes attack by a nucleophile (with or without a prior rearrangement – as the case may be).