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.
Common Nucleophiles and Their Strengths
| Nucleophile | Formula | Type | Relative Strength | Typical Reaction |
|---|---|---|---|---|
| Hydroxide ion | OH⁻ | Anionic | Strong | Substitution/elimination with alkyl halides |
| Cyanide ion | CN⁻ | Anionic | Strong | Addition to carbonyls, SN2 |
| Iodide ion | I⁻ | Anionic | Strong (polarisable) | SN2 with primary alkyl halides |
| Water | H₂O | Neutral | Weak | SN1 solvolysis |
| Ammonia | NH₃ | Neutral | Moderate | Substitution to give amines |
| Thiolate ion | RS⁻ | Anionic | Very strong (soft) | SN2 with alkyl halides |
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Related Terms
Electrophile
An electrophile is a chemical species that accepts an electron pair from a nucleophile to form a new covalent bond, acting as a Lewis acid. Electrophiles typically carry a full positive charge, a partial positive charge (δ+), or an empty orbital that makes them electron-deficient and reactive towards electron-rich species. Electrophilic attack is the first step in many important organic reactions, including electrophilic aromatic substitution, electrophilic addition to alkenes, and Friedel–Crafts reactions used in industrial chemistry.
Nucleophilic Substitution
Nucleophilic substitution is a fundamental class of organic reactions in which an electron-rich species (nucleophile) attacks a carbon atom bearing a leaving group, displacing that leaving group and forming a new bond. There are two main mechanistic pathways: SN2 (concerted, bimolecular, inverts stereochemistry) and SN1 (stepwise via a carbocation intermediate, gives racemic mixture). These reactions are central to the synthesis of alcohols, ethers, amines, and other functional groups from alkyl halides.
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.
From Latin "nucleus" (kernel, core) + Greek "philos" (loving, fond of). The term was introduced by Christopher Ingold in the 1930s to describe species that are attracted to positively charged or electron-deficient centres, in contrast to "electrophiles" which seek electron-rich centres.