In an SN1 reaction, when and why does carbocation rearrangement occur? I thought that it was related to the substrate, but I have worked through some problems where even though there is a methyl substrate rearrangment does not occur.
SN1 stands for a substitution, nucleophillic, unimolecular reaction. Unlike a bimolecular reaction (SN2), a unimolecular reaction depends only on a single substrate for its rate of reaction. This is usually because the reaction mechanism involves some type of ionic intermediate. The most common one is the initial loss of a leaving group, thus leaving a carbon based cation called a carbocation. This carbocation then forms a new chemical bond with the nucleophile in a second discrete step.
At the carbocation stage, molecular rearrangement can occur. Carbocation rearrangement occurs in order to change from a less stable cation to a more stable cation. Generally speaking, carbocation stability increases with more carbon-carbon bonds attached to the ionic carbon. So that means that the order of carbocation stability from most stable to least stable is tertiary>secondary>primary>>methyl. The methyl carbocation (CH3+) is the least stable since it has no carbon carbon bonds. The neopentyl group is the most stable since it is a tertiary carbocation (it has three carbon-carbon bonds). So if a secondary carbocation can rearrange to a tertiary carbocation during an SN1 reaction, then that arrangement will likely occur.