Substitution with accompanying alkyl shift
Description: 1,2-alkyl shifts can occur during substitution reactions if an unstable carbocation is formed adjacent to a quaternary carbon.
Notes: H–Cl is just an example here,, there are several other acids that can be used for this purpose (see below)
Notes: In the first and second examples a methyl group migrates over and the resulting tertiary carbocation is trapped by the nucleophile. Note that in the second example the Lewis acid is a silver salt, which makes halogens such as Cl a much better leaving group.
The third and fourth examples show ring expansion reactions. The third example shows a release of ring strain where it goes from a strained cyclobutane to a less strained cyclopentane.
In the fourth example a primary carbocation is less stable than the secondary carbocation that is formed through ring expansion.
Mechanism: Formation of a carbocation is achieved here through protonation of the alcohol with strong acid (Step 1, arrows A and B) followed by loss of water to give the carbocation (Step 2, arrow C). Next, migration of a methyl group from the adjacent quaternary carbon through a transition state such as that pictured (Step 3, arrow D) gives a tertiary carbocation which is then attacked by the nucleophile (Step 4, arrow E).