Elimination reactions

• In an elimination reaction, a haloalkane forms an alkene and a metal halide
• In an elimination reaction, the ion attacks and then removes a hydrogen atoms from the carbon atoms next to the carbon atom of the carbon - halogen bond. A new C=C forms. The C-Br is then broken and a bromide ion is released
• during an elimination reaction with hydroxide ions, the ions act as a base
• During an elimination reaction with hydroxide ions, the reagent is potassium hydroxide or sodium hydroxide
• Elimination requires heat and for the reagent to be dissolved in anhydrous ethanol
• Primary haloalkanes favour substitution
• Tertiary haloalkanes favour elimination
• In secondary haloalkanes, elimination and substitution reactions occur at the same time
• If the base strength of the electron pair donor is increased, elimination is more likely to occur than substitution
• Aqueous NaOH has a weaker base of OH- than NaOH in ethanol, with the stronger base CH3CH2O-
• Higher temperatures favour elimination reactions
• For secondary haloalkanes you can alter the conditions to favour substitution or elimination:

  • substitution: warm aqueous ethanol
  • elimination: hot anhydrous ethanol