Radical Stability

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Radical Stability
2:47 — by Jules B.

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Key Questions

  • 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.

  • Let’s talk a bit about stability first, and then circle back to their structure. Being electron deficient, you might already have a hunch regarding factors that might stabilize free radicals. A considerable portion of organic chemistry can be explained simply by understanding that: 1) opposite charges attract (and like charges repel), and 2) the stability of charges increases if it can be spread out over a greater volume. These still apply here. Electron poor species are stabilized by neighboring atoms that can donate electron density. [“if you’re poor, it helps to have rich neighbors”]. The most common way to interpret “rich neighbors” here is the observation that increasing the number of alkyl groups on the carbon bearing the free radical increases its stability. Radical stability increases in the order methyl < primary < secondary < tertiary. [For a second, more conceptually complex example, see the bottom of the post]. **

  • generally the order of stability followed by the radicals is,

    allylic >#3^@#>#2^@#>#1^@#> vinylic