Elimination Reactions of Halogenoalkanes (Cambridge International A-Level Chemistry)

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

Exam Hint Reaction conditions are crucial in determining the product.

Elimination = hot, ethanolic NaOH

Substitution = warm, aqueous NaOH