Elimination (AQA AS Chemistry): Revision Note

Mechanism: Elimination

The elimination reaction

• An elimination reaction is one in which a small molecule is removed from a larger molecule, forming a multiple bond (usually a C=C double bond)

• Halogenoalkanes can undergo elimination when heated under reflux with concentrated sodium hydroxide dissolved in ethanol (ethanolic NaOH)

• Under these conditions:

  • The C–X bond breaks heterolytically

  • The hydroxide ion acts as a base

  • A hydrogen atom is removed from a carbon adjacent to the carbon bonded to the halogen

  • A hydrogen halide (HX) is eliminated

  • An alkene is formed

• Overall, the reaction removes H and X from adjacent carbon atoms, producing a C=C double bond

  • Example: Bromoethane

CH₃CH₂Br + NaOH (ethanol, heat) → CH₂=CH₂ + NaBr + H₂O

• Bromoethane reacts with hot ethanolic sodium hydroxide to form ethene:

The elimination mechanism

• The mechanism is called elimination because a small molecule, hydrogen halide, is removed from the molecule

• The elimination mechanism:

  • Is a one-step reaction

  • The base removes a proton (H⁺)

  • The C–H bond breaks

  • The C–X bond breaks at the same time

  • The C=C double bond forms simultaneously

The importance of reaction conditions

• The reaction conditions can influence the reaction products for the same reactant and reagent

• In this case, the solvent and temperature determine whether substitution or elimination is favoured

  • With hot ethanolic NaOH → elimination (alkene formed)

  • With aqueous NaOH → substitution (alcohol formed)

    • This is a nucleophilic substitution reaction