An elimination reaction is a type of organic reaction in which a molecule loses two atoms or groups from adjacent carbon atoms, resulting in the formation of a multiple bond (usually a double bond or sometimes a triple bond).
Key Characteristics
- Two atoms or groups are eliminated from the starting compound.
- The most common result is the formation of an alkene (C=C) or an alkyne (C≡C).
- A base or heat is often needed to drive the reaction.
Common Types of Elimination
- Dehydrohalogenation – Removal of a hydrogen atom and a halogen atom (from an alkyl halide) to form an alkene.
- Dehydration – Removal of a water molecule (from an alcohol) to form an alkene.
- Dehydrogenation – Removal of two hydrogen atoms (from alkanes or alkenes) to form unsaturated compounds.
Mechanisms
- E1 (Unimolecular Elimination):
Happens in two steps — first the leaving group departs, then a proton is removed to form the double bond. - E2 (Bimolecular Elimination):
Happens in a single concerted step — a base removes a proton at the same time the leaving group departs.
Why They Matter
- Elimination reactions are a major method for synthesizing alkenes and alkynes, which are important intermediates in organic synthesis.
- They allow chemists to control product distribution (Zaitsev vs. Hofmann product, E vs. Z isomers).
- They are widely used in industry to make essential chemicals like ethylene, propylene, butadiene, and styrene, which are building blocks for plastics, rubbers, and fuels.
In short:
An elimination reaction is a reaction where atoms or groups are removed from a molecule, typically producing a double or triple bond, and plays a key role in both laboratory and industrial organic chemistry.