What is an Electrophile?
This section will be focused on 3 questions “what is an electrophile?” “what is hyperconjugation?” and “what is a methyl shift?” But first, let’s define electrophile.
Electrophiles lack a lone pair of electrons and are, to some degree, electron deficient. Because one of the atoms doesn’t have all the electrons it wants, electrophiles often, but not always, carry a positive charge. For example, a carbocation, or a positive charge located on a carbon atom, is a common finding in organic chemistry electrophiles.
When dealing with a carbocation, there are two important concepts to know for mechanisms: hyperconjugation and hydride shifts.
What is Hyperconjugation?
We will now answer the question “what is hyperconjugation?” Hyperconjugation is the stabilization of a carbocation by neighboring carbon-hydrogen or carbon-carbon bonds. In general, we look at the number of carbon-hydrogen bonds on the carbon neighboring the carbocation to assess hyperconjugation. The more carbon-hydrogen bonds in this position, the more stable the carbocation.
It therefore follows that a tertiary carbocation is more stable than a secondary carbocation, which in turn is more stable than a primary carbocation.
Hydride shift (and methyl shift)
As we just saw, carbocations prefer to have as many neighboring carbon-hydrogen bonds as possible. In fact, this is such a strong preference that hydrogen atoms, and sometimes even whole methyl groups, will shift, moving the carbocation to a more stable position. If a hydrogen moves to better stabilize the carbocation, it is called a hydride shift. If it’s a methyl group that moves, it is called a methyl shift. Take a look at the molecule below:
The carbocation is on a secondary carbon, which is more stable than a primary carbocation but less stable than a tertiary carbocation. If a neighboring hydrogen moves via a hydride shift, the carbocation will get placed on a more stabilized tertiary carbon.
If there is no hydrogen available, then a methyl group can shift to try to move the carbocation to a more stable carbon.
The molecule prefers to make a hydride shift more than a methyl shift because of the large amount of energy that must be overcome for a methyl shift to occur. Therefore, methyl shifts only occur when no hydride shift is possible.