Rearrangement of carbocations

2. methyl shift

In the previous page, we talked about rearrangements that could be either hydride shifts or alkyl shifts. There is a very simple way to find out for a specific carbocation whether it will rearrange by means of a hydride shift or an alkyl shift (and whether it will rearrange at all):

the shift that produces the most stable carbocation is the one that will actually occur.

In the example shown below, you can see how 3,3-dimethyl-butane-2-ol is protonated on the OH-group, prior to losing water. The resulting structure is a secondary carbocation. If a hydride were to migrate, the resulting structure would be a primary carbocation; since this is a less stable structure than a secondary carbocation, no hydride shift will occur.

What does occur, however, is a methyl shift. As a result of that, the secondary carbocation is converted to a tertiary one, which has a lower energy and hence is more stable. After the rearrangement, an attack of a nucleophile can occur; this is not shown in the animation.   Colour the shifting methyl group Frame 1: protonation of 3,3-dimethyl-butane-2-ol