Elimination Reactions IB Chemistry Power Points Topic 20 Organic Chemistry www.pedagogics.ca Elimination Reactions

Elimination of HBr from halogenoalkanes Recall: nucleophilic substitution of a bromoalkane

Elimination of HBr from halogenoalkanes Recall: nucleophilic substitution of a bromoalkane

Elimination of HBr from halogenoalkanes Recall: nucleophilic substitution of a bromoalkane Reaction occurs when bromoethane is added to a dilute NaOH solution. If conditions change, the nature of the reaction will change

Elimination of HBr from halogenoalkanes An elimination reaction is one where a small molecule is removed from the reactant molecule. Substitution OH- + C2H5Br  C2H5OH + Br- Elimination OH- + C2H5Br  C2H4 + H2O + Br- Which reaction occurs depends on the reacting conditions.

Elimination of HBr from halogenoalkanes

Elimination reaction mechanisms E2 mechanism - similar to SN2 - bimolecular process transition state, no intermediates typical of secondary or tertiary substituted alkyl halides

Elimination reaction mechanisms E1 mechanism - similar to SN1 - unimolecular process carbocation intermediate typical of tertiary substituted alkyl halides

Elimination reaction mechanisms How the reactions happen – the role of OH- The hydroxide ion hits one of the hydrogen atoms in the alkyl halide and pulls it off. This leads to a cascade of electron pair movements resulting in the formation of a carbon-carbon double bond, and the loss of the halogen as R-.