Which statement about rearrangements in SN1 and SN2 mechanisms is true?

• A. SN1 never involves rearrangements; SN2 does.
• B. SN2 often involves rearrangements; SN1 does not.
• C. Neither SN1 nor SN2 involve rearrangements.
• D. SN1 can involve rearrangements of carbocation intermediates; SN2 generally does not involve rearrangements.

Answer: (D)

The key idea is whether a discrete carbocation forms during the reaction. In SN1, the leaving group departs first, creating a carbocation. That positively charged intermediate can undergo rearrangements—such as hydride or methyl shifts—to become a more stable carbocation before the nucleophile attacks. That’s why rearrangements are common in SN1.

In SN2, the process is concerted: the nucleophile attacks as the leaving group leaves, in one step. There isn’t a stable carbocation intermediate to rearrange, so rearrangements don’t occur in the typical SN2 mechanism. The reaction proceeds directly to the product with backside attack and inversion.

So the true statement is that SN1 can involve rearrangements of carbocation intermediates, while SN2 generally does not involve rearrangements.