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.
E1 vs E2 Elimination Reactions — Key Differences
| Feature | E1 | E2 |
|---|---|---|
| Mechanism | Two-step (via carbocation) | One-step (concerted) |
| Rate law | Rate = k[substrate] | Rate = k[substrate][base] |
| Base required | Weak base sufficient | Strong base required (e.g. KOH/EtOH) |
| Substrate | Tertiary favoured | All types, tertiary preferred |
| Stereochemistry | No stereospecificity | Anti-periplanar geometry required (anti elimination) |
| Regioselectivity | Zaitsev's rule (more substituted alkene) | Zaitsev's or Hofmann product depending on base |
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Related Terms
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.
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.
From Latin "eliminare" (to thrust out, banish), from "e-" (out) + "limen" (threshold). In chemistry, the term was adopted in the early 20th century by Christopher Ingold and co-workers to describe reactions where atoms are "expelled" from a molecule to form an unsaturated product.