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Key Concepts

Electrophilic Addition Reactions

These are reactions in which an electrophile adds to a multiple bond (such as a double or triple bond) in a substrate. The process involves the attack of an electron-deficient species (electrophile) on the electron-rich ? bond, and can proceed through different mechanisms that determine the nature and stability of any intermediates formed during the reaction.

Carbocation Intermediate

In some electrophilic addition reactions, the initial attack of the electrophile on the ? system produces a carbocation intermediate. This positively charged intermediate is a key feature in two-step mechanisms and is typically stabilized by substituents adjacent to the positive center. Its formation plays a crucial role in determining the reaction's regiochemistry and stereochemistry.

Concerted Mechanisms

Certain electrophilic addition reactions occur in a single, concerted step without the formation of any isolated intermediates. In this mechanism, bond formation and bond breaking occur simultaneously, bypassing the generation of reactive intermediates such as carbocations. This pathway is often associated with a more synchronous reaction coordinate.

Three-membered Ring Intermediates

Some electrophilic additions proceed via the formation of a three-membered cyclic intermediate, such as a halonium ion, where the electrophile forms bonds with two atoms of the original ? system. This strained cyclic intermediate is transient and determines the stereo- and regioselectivity of subsequent nucleophilic attack, leading to the final addition product.

Five-membered Ring Intermediates

In certain electrophilic addition processes, particularly those involving intramolecular reactions, a five-membered cyclic intermediate can be formed. Five-membered rings are generally favored due to their minimal ring strain and the kinetic as well as thermodynamic stability they provide, influencing the course and outcome of the reaction.

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