Question

Why are alkyl groups said to be electron "releasing" (also known as electron "donating") compared to hydrogen when talking about carbocations?

Answer 1

This is in the context of a discussion on hyperconjugation stabilization.

For a carbocation, you can have either a methyl (`"CH"_3`), primary (`1^@`), secondary (`2^@`), or tertiary (`3^@`) carbocation.

They are ranked in stability like so:

You can see that from left to right the number of alkyl groups attached to the central positively-charged carbon increases (each alkyl group replaces a hydrogen), which correlates with the increase in stability.

So, it must be that the alkyl groups have something to do with it. In fact, there is an effect called hyperconjugation that describes what is going on here. This is one instance, but there are other kinds for other contexts.

In this case, the electrons in a `\mathbf(sigma)`-bonding orbital (here, it is of the surrounding methyl groups' `"C"-"H"` bonds) can interact with the adjacent empty `\mathbf(p)` orbital on the central positively-charged carbon.

The above image depicts a comparison between a primary carbocation and a methyl carbocation.

The carbon with the empty, purple `p` orbital is the positively-charged carbon, while the yellow `sp^3` `sigma`-bonding orbital is capable of donating the electrons into the empty purple `p` orbital.

This extends the molecular orbital to stabilize the carbocation and demonstrates the electron donating/releasing character of an adjacent methyl group.

We can see the stabilizing effect in this molecular orbital diagram:

The empty `p` orbital is stabilized after the adjacent filled `sigma` bond shares its electrons with the empty `p` orbital.

(It doesn't have to be empty though; it could be partially filled, like in a carbon radical compound.)