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.

What is an electrophile? This image shows 2 electrophiles. Note how each is electron deficient.

 

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.

What is hyperconjugation? As this image shows, it is the ability of neighboring carbon-hydrogen bonds to stabilize carbocations.

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:

Picture39

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.

This figure shows an example of a hydride shift.

 

Keep it Simple
When drawing a hydride or methyl shift, notice that the arrows are slightly different than typical curved arrows. In a hydride or methyl shift, the tail of the arrow cuts through the bond that is moving, indicating the electrons of the bond are also moving. Some professors are strict about this while others are not, so be sure to check in with yours!

 

If there is no hydrogen available, then a methyl group can shift to try to move the carbocation to a more stable carbon.

This figure shows an example of a methyl shift.

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.