Elimination
The base removes a hydrogen from the carbon next to bromine. The C-H electrons form the C=C pi bond as bromide departs, so elimination removes H and Br without changing the two-carbon skeleton.
- Reagents
- KOH in ethanol
- Conditions
- Heat with ethanolic alkali
- Reaction class
- Elimination
- Equation
- CH3CH2Br + KOH -> CH2=CH2 + KBr + H2O
Overview
The base removes a hydrogen from the carbon next to bromine. The C-H electrons form the C=C pi bond as bromide departs, so elimination removes H and Br without changing the two-carbon skeleton.
Transformation
- Equation
- CH3CH2Br + KOH -> CH2=CH2 + KBr + H2O
- Reactants
- Bromoethane + Potassium hydroxide
- Products
- Ethene + Potassium bromide + Water
- Reagents
- KOH in ethanol
- Environment
- Heat with ethanolic alkali
- Reaction class
- Elimination
- Mechanism
- E2 elimination
- Evidence level
- source-backed molecular example
Scope and limitations
- Scope
- A beta hydrogen is removed as bromide leaves.
- Limitations
- Substitution competes, especially for a primary substrate; the equation gives the intended elimination branch.
Related reactions
- Hydration: Ethene → Ethanol
Ethene gives ethanol through hydration.
- Dehydration: Ethanol → Ethene
Ethanol gives ethene through dehydration.
- Hydrolysis: Bromoethane → Ethanol
Hydroxide attacks the saturated carbon bearing bromine while the C-Br bond breaks. This primary-substrate SN2 route replaces bromide with OH and preserves both carbon atoms; aqueous conditions favour this branch over elimination.
- Cyanide substitution: Bromoethane → Propanenitrile
The carbon end of cyanide attacks the primary carbon of bromoethane as bromide leaves. The new C-C bond joins cyanide’s carbon to the original chain, giving a three-carbon nitrile rather than acetonitrile.
- Ammonia substitution: Bromoethane → Ethylamine
Ammonia attacks the carbon bearing bromine to give an alkylammonium intermediate. A further ammonia molecule removes a proton to release ethylamine; the product can undergo additional alkylation if haloalkane remains.
References
- Pearson Edexcel International Advanced Level Chemistry Scheme of WorkPearson Education Limited · Pearson qualifications · 2018
Official Pearson teaching guidance specifying chloroalkane-relevant conditions: aqueous alkali, ethanolic potassium hydroxide, alcoholic ammonia, alcoholic potassium cyanide, and PCl5 alcohol chlorination.