Addition Reaction
An addition reaction is a chemical reaction in which two or more molecules combine to form a single, larger product with no atoms lost as a by-product. Addition reactions occur most commonly at carbon–carbon multiple bonds (alkenes, alkynes) and at polar carbonyl groups, where a reagent adds across the unsaturation. They are fundamental to industrial synthesis — the hydrogenation of vegetable oils, the production of polymers like polyethylene, and the manufacture of alcohols from alkenes all proceed via addition reactions.
Types of Addition Reactions in Organic Chemistry
| Type | Reagent Added | Typical Substrate | Product | Example |
|---|---|---|---|---|
| Hydrogenation | H₂ (with Ni/Pt catalyst) | Alkene (C=C) | Alkane | Ethene + H₂ → Ethane |
| Halogenation | X₂ (Br₂, Cl₂) | Alkene | Dihalide | Ethene + Br₂ → 1,2-Dibromoethane |
| Hydrohalogenation | HX (HBr, HCl) | Alkene | Haloalkane | Propene + HBr → 2-Bromopropane |
| Hydration | H₂O (acid catalyst) | Alkene | Alcohol | Ethene + H₂O → Ethanol |
| Nucleophilic addition | Nu⁻ (CN⁻, RMgX) | Aldehyde/Ketone | Alcohol | Ethanal + HCN → Cyanohydrin |
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Related Terms
Elimination Reaction
An elimination reaction is an organic reaction in which atoms or groups are removed from adjacent carbon atoms, forming a new pi bond (typically a C=C double bond) in the product. The two principal mechanisms are E2 (concerted, bimolecular) and E1 (stepwise via carbocation). Elimination reactions often compete with nucleophilic substitution and are favoured by strong, bulky bases; high temperatures; and tertiary substrates — making them key in the synthesis of alkenes and alkynes in industry and the laboratory.
Nucleophile
A nucleophile is a chemical species that donates an electron pair to an electrophile to form a new covalent bond, acting as a Lewis base. Nucleophiles are characterised by the presence of a lone pair of electrons, a negative charge, or a region of high electron density that attacks electron-deficient centres (electrophilic carbons). Strong nucleophiles drive SN2 reactions and nucleophilic addition; weaker nucleophiles favour SN1 pathways — making nucleophilicity a key parameter in predicting organic reaction outcomes.
Aldehyde
An aldehyde is an organic compound containing a carbonyl group (C=O) in which the carbon atom is bonded to at least one hydrogen atom, giving the characteristic –CHO functional group at the end of a carbon chain. Aldehydes are more reactive than ketones because the carbonyl carbon is less sterically hindered and more electrophilic. They are important industrial chemicals — formaldehyde is used in resins, acetaldehyde in vinegar production, and benzaldehyde gives almonds their characteristic scent.
From Latin "additio" (adding to), from "addere" (to add). The term was formalised in organic chemistry during the 19th century to distinguish reactions that form a single product by combining reactants, as opposed to substitution or elimination reactions.