Free Radical Substitution vs. Free Radical Addition — What's the Difference?

Applications and Occurrences: Free Radical Substitution reactions are common in halogenation reactions of hydrocarbons. In contrast, Free Radical Addition reactions are pivotal in the polymerization process of certain monomers. While both reactions can be harnessed for specific industrial purposes, their methods and outcomes are distinct.

Reaction Initiation: Both Free Radical Substitution and Free Radical Addition reactions are initiated by radicals, often formed when a bond is broken by energy, such as heat or light. However, while both reactions involve radicals, their mechanisms and final products differ substantially.

Mechanisms: The mechanisms of these two types of reactions involve multiple stages, including initiation, propagation, and termination. In both cases, the stability of the radical intermediates often dictates the course and the products of the reaction. However, the specific steps and intermediates for Free Radical Substitution differ from those in Free Radical Addition.

Free Radical Substitution: This is a type of chemical reaction wherein a radical replaces a specific atom or group of atoms in a molecule. The process of Free Radical Substitution typically requires a halogen and a hydrocarbon. When these react under specific conditions, usually initiated by heat or light, a hydrogen atom from the hydrocarbon is replaced by the halogen. Free Radical Addition, on the other hand, doesn't involve this sort of replacement.

Free Radical Addition: In contrast to Free Radical Substitution where an atom is replaced, Free Radical Addition involves radicals adding on to a molecule. Typically, this type of reaction is seen in the formation of polymers, where radicals play a crucial role in adding monomers together. Unlike Free Radical Substitution, no atoms are taken away in this process.