Description: When secondary (or primary) carbocations are formed adjacent to a quaternary carbon, 1,2-alkyl shifts can occur.
Notes: The acids in these reactions is often sulfuric acid (H2SO4) or tosic acid (TsOH) because the conjugate base of these acids are very poor nucleophiles.
Notes: In the first and second cases a methyl group shifts over by one carbon (in the second example, silver nitrate AgNO3 (-) is a Lewis acid that makes halides such as Br better leaving groups).
In the third example the cyclobutane ring expands to give the five-membered ring, which has less ring strain.
In the fourth example the ring expands to form a seven-membered ring from a six-membered ring (recall that primary carbocations are unstable relative to secondary carbocations).
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 undergoes deprotonation (Step 4, arrows E and F) to form the most substituted alkene (between C-2 and C-3)