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.
SN1 vs SN2 Nucleophilic Substitution — Key Differences
| Feature | SN1 | SN2 |
|---|---|---|
| Mechanism | Two-step (via carbocation) | One-step (concerted) |
| Rate law | Rate = k[substrate] | Rate = k[substrate][nucleophile] |
| Substrate preference | Tertiary > secondary > primary | Primary > secondary; tertiary disfavoured |
| Stereochemistry | Racemisation at chiral centre | Inversion of configuration (Walden inversion) |
| Solvent preference | Polar protic (e.g. water, ethanol) | Polar aprotic (e.g. acetone, DMSO) |
| Leaving group | Good leaving group required | Good leaving group required |
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Related Terms
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.
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.
Stereoisomer
Stereoisomers are molecules that have the same molecular formula and the same connectivity (sequence of bonds) between atoms but differ in the three-dimensional arrangement of their atoms in space. There are two main types: enantiomers (non-superimposable mirror images, related by chirality) and diastereomers (stereoisomers that are not mirror images, including cis/trans and E/Z isomers). Stereoisomers can have profoundly different biological activities — the drug thalidomide is a classic example, where one enantiomer is therapeutic and the other teratogenic.
The term "nucleophilic substitution" was coined by English chemist Christopher Ingold in the 1930s. "Nucleophilic" derives from Latin "nucleus" (kernel, nucleus) + Greek "philos" (loving) — a nucleus-loving species. "Substitution" comes from Latin "substituere" (to put in place of).