What type of reaction does a Lewis acid typically participate in?

A Lewis acid is defined as a species that can accept an electron pair to form a covalent bond. This characteristic makes Lewis acids highly effective in reactions where they can increase the electrophilicity of a substrate, facilitating electron flow.

In the context of electrophilic substitution reactions, a Lewis acid often enhances the reactivity of electrophiles by coordinating with them, thereby stabilizing positive charges that may develop or by increasing their electrophilic nature. This is common in aromatic substitution reactions, where a Lewis acid can interact with a substrate like an aromatic ring, facilitating the substitution of another group.

In contrast, nucleophilic addition and electrophilic addition reactions typically involve nucleophiles attacking electrophiles, which may not directly relate to the classic role of Lewis acids as electron pair acceptors in the context of substitution reactions. Free radical substitution is fundamentally different in mechanism, often involving radical species rather than the electron-pair mechanisms associated with Lewis acids.

Thus, the participation of Lewis acids is best characterized in electrophilic substitution reactions due to their ability to stabilize and activate electrophiles for substitution processes.