Radical Stability

Key Questions

  • Answer:

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


    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.


  • Radical stability refers to the energy level of the radical.

    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.