Elimination Reactions of Halogenoalkanes (Cambridge (CIE) A Level Chemistry): Revision Note
Exam code: 9701
Elimination Reactions of Halogenoalkanes
An elimination reaction involves the loss of a small molecule from a larger organic molecule
In halogenoalkanes, this small molecule is usually a hydrogen halide (e.g. HBr or HCl)
The product is typically an alkene
The elimination reaction of bromoethane with ethanolic sodium hydroxide
Reaction Conditions for Elimination
Halogenoalkanes are heated with ethanolic sodium hydroxide (NaOH dissolved in ethanol)
Under these anhydrous conditions, elimination occurs:
The C–X bond (where X = halogen) breaks heterolytically
A halide ion (X⁻) is released
A double bond forms, producing an alkene
E.g. Elimination of bromoethane:
bromoethane + sodium hydroxide (ethanol) → ethene + sodium bromide + water
C2H5Br + NaOH (ethanol) → C2H4 + NaBr + H2O
One hydrogen atom and the bromine atom are eliminated
The carbon chain forms a C=C double bond to make ethene
Examiner Tips and Tricks
Reaction conditions are crucial in determining the product.
Elimination = hot, ethanolic NaOH
Substitution = warm, aqueous NaOH
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