Any type of reaction that involves the pairing or unpairing of electrons. Free radical reactions (a) frequently occur in gas phase, (b) often show an induction period, (c) often proceed by chain reaction, (d) are often initiated by light, heat, or reagents which contain unpaired electrons (such as oxides or peroxide decomposition products), (e) are inhibited by materials such as hydroquinone which react rapidly with free radicals, (f) are rarely acid or base catalyzed, (g) often proceed at a rate independent of the polarity of the medium.
Example: