Free Radical Stability and The Three Radical Fates
A free radical is a species with at least one unpaired electron. When denoting the movement of one
electron, make sure to use a half curved arrow (Hardinger 129).
Radicals have similar properties to carbocations in terms of stability.
Degree of Substitution: Radicals are stabilized by electron-rich neighbors, such as CH3 groups. As a
result, a primary radical is less stable than a secondary radical, and a tertiary radical is the most stable of
the three. However, resonance stabilization dominates over substitution. For example, a secondary
radical with resonance is more stable than a tertiary radical without resonance. While CH3 + is too
unstable to exist, a methyl radical is permissible (Hardinger 133).
Resonance: A radical can also gain stability from pi bond resonance as this provides electron density to
the deficient radical (Hardinger 133-134). However, a radical does not gain resonance from adjacent lone
pairs.
There are three radical fates to consider.
1. Add to a pi bond, switching it out for a stronger sigma bond (Hardinger 130). Note: This diagram
comes from the Lecture Supplement as cited below.
2. Transfer an atom, which typically makes a weaker sigma bond into a stronger sigma bond (Hardinger
130). Note: This diagram comes from the Lecture Supplement as cited below.
3. Combine two radicals to make a bond between two atoms of the same type (Hardinger 131).
However, this fate is fairly uncommon, as it is somewhat rare for a radical to exist in a concentration that
is high enough to easily find a match before undergoing one of the other radical fates.
Hardinger, Steven. “Chemistry 14D Lecture Supplement” 4th Ed. Hayden McNeil. 2016. Print.