A carbocation is a positively charged species.  The positive charge is localised on an sp2 hybridised carbon atom.  Hence the carbocation itself is planar around this carbon.  The p orbital of the sp2 carbon is empty. Typically they are significantly stabilised if the positive charge can be delocalised across the molecule, and thus they form a part of our discussion of resonance.

As already noted, alkyl groups donate electron density, and hence the more of these groups surrounding the sp2 carbon, the more stable it will be.

Methyl < Primary <

Secondary

<

Tertiary

This helps us to explain Markownikow’s rule in more detail; it is actually just a disguised way of saying the most stable carbocation forms preferentially.

There are other ways to stabilise carbocations, for example, we can use the empty p orbital to spread the charge:

In this case, the empty p orbital (indicated with a “+”) is next to a pi bond, and can overlap with the p orbitals nearest it, thus spreading the positive charge over the whole of the molecule, which increases the stability.  This case is an example of an allylic carbocation.  This concept is closely linked to resonance, which we will look at in more detail later.

We can extend the concept above further, and spread the charge into a benzene ring in the same manner.

A curious example of an unlikely looking stabilised carbocation is:

This seven membered ring is actually aromatic, which is where it achieves its surprising stability.