Addition reaction (organic chemistry)

Mechanistic roles and basic distinctions

Mechanistically, addition reactions are often described in terms of electron flow: one component behaves as the electron donor (a nucleophile) and the other as the electron acceptor (an electrophile). The nucleophilic partner supplies the electrons to form new bonds, while the electrophile accepts electron density to complete the addition. For introductions to these roles see nucleophile, electron transfer and electrophile. Not all additions proceed by classic nucleophile–electrophile pairing; radical additions, pericyclic additions and concerted processes are also important. Addition reactions are conceptually the reverse of elimination reactions, in which a single reactant fragments to give a multiple bond and a small molecule.

Common types

• Electrophilic addition — typical for alkenes: protonation or other electrophile attack followed by nucleophile capture (e.g., hydrohalogenation, halogenation).
• Nucleophilic addition — common to polarized double bonds such as carbonyls: a nucleophile adds to the electrophilic carbon of a C=O group (carbonyl addition).
• Radical addition — involves free radicals adding across double bonds; important in polymer chemistry and some chain reactions.
• Hydration and hydrofunctionalization — addition of water or other protic species across a double bond (see hydration as an example).
• Hydrogenation — addition of H2 across unsaturated bonds under catalysis to saturate hydrocarbons.

These categories can overlap: for instance, hydroboration-oxidation of alkenes proceeds by a concerted addition with distinctive regiochemical and stereochemical outcomes.

Regioselectivity and stereochemistry

Additions often display predictable regiochemistry (where new groups attach) and stereochemistry (the spatial arrangement of substituents). Rules such as Markovnikov’s predict which carbon receives which atom in polarized additions to alkenes, while anti versus syn addition describes whether two groups add to opposite or the same faces of a double bond. Control of these aspects is central to synthetic planning because different regiochemical or stereochemical outcomes produce different isomers with distinct properties.

Applications and importance

Addition reactions are indispensable in laboratory synthesis and industry. They permit straightforward functional-group installation, construction of complex molecules and formation of polymer chains from simple olefins (for example, conversion of ethylene into polyethylene). Catalytic variants (acid, base, transition-metal catalysts) improve selectivity and turnover, making additions a mainstay of pharmaceutical, agrochemical and materials synthesis.

Notable facts and further reading

Because additions can be reversible or coupled to further transformations, they are often used within sequences of reactions to build complexity. For practical guides and mechanisms consult introductory resources on organic chemistry, mechanistic treatments at reaction overview and specialized discussions of electrophilic and nucleophilic processes at multiple bond chemistry and C–C bond formation. For laboratory techniques and safety see general procedure references at functional group chemistry, polar multiple bonds, and notes on reagent roles at nucleophiles, electron flow and electrophiles. Comparisons with elimination and examples such as hydration and carbonyl addition illustrate the breadth of the reaction class.