How does a free-radical substitution reaction differ from nucleophilic substitution?

Substitution reactions are categorized based on the nature of the substituent and the mechanism of the reaction. The primary difference between free-radical substitution and nucleophilic substitution lies in the electronic nature of the reagents and the species involved.1

Nucleophilic Substitution

In nucleophilic substitution, the substituent is an electron-rich species known as a nucleophile. This reagent provides an electron pair to form a new bond with the substrate. These reactions are often characterized by their interaction with saturated carbon centers and can proceed through either bimolecular or unimolecular mechanisms. Common nucleophiles include halogen anions, ammonia, and hydroxyl groups.12

Free-Radical Substitution

In contrast, radical substitution involves species that contain at least one unpaired electron. Unlike nucleophiles, which are defined by their ability to donate electron pairs, free radicals are highly reactive entities that typically arise from the homolytic cleavage of electron-pair bonds. These reactions often involve chain processes where the radical abstracts parts of a molecule to complete its own electron pair, simultaneously generating a new free radical.34

While nucleophilic substitution is driven by the attraction of electron-rich species to electron-deficient centers, radical substitution is driven by the high reactivity of unpaired electrons.13