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## Key Questions

If the internal energy of the radical is high, the radical is unstable. It will try to reach a lower energy level.

If the internal energy of the radical is low, the radical is stable. It will have little tendency to react further.

Free radicals have only 7 electrons in their valence shell. They are higher in energy than atoms with 8 valence electrons.

Carbocations are also electron-deficient species. They are even higher in energy, because they have only 6 valence electrons.

Electron-donating alkyl groups stabilize carbocations. They also stabilize free radicals.

So the order of stability of free radicals is the same as for carbocations:

This means that tertiary radicals are most easily formed, and methyl radicals are least easily formed.

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.

• Radicals are most stabilized by two things:

1. Hyperconjugation, or

2. Resonance

The former delocalizes electron density from adjacent $C - H$ bonds, and the latter delocalizes the radical electron around a molecule.

I'll refer you to another one of my answers to explain the first concept, because students find it hard to understand, generally.

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