Question

What determines radical stability?

Answer 1

Here's a method to figure out stability of intermediates.

Explanation:

This method will help you in figuring out stability of all types of intermediate be it carbocation, carbanion or carbon free radical.
Just think of carbon atom as a person who neither happy with more electron density nor with less. It just needs the right amount of electrons. A free radical has one electron less than Carbon atom. So it needs to compensate for the loss of one electron.

The increasing order for this stability order is

Benzylic carbon > Allylic carbon > Tertiary carbon > Secondary carbon > Primary carbon.