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...

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. 

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ncchemist | eNotes Employee

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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.

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