2. Elimination reaction of halogenoalkanes Elimination: removal of small molecule (often water) from the organic molecule Elimination with alcoholic hydroxide ions Change in functional group: halogenoalkane alkene Reagents: Potassium (or sodium) hydroxide Conditions: In ethanol ; Heat Mechanism: Elimination Type of reagent: Base, OHC C H H H Br C H H H H + KOH C C + KBr + H2O H H H C H H H 1-bromopropane propene Note the importance of the solvent to the type of reaction here. Aqueous: substitution Alcoholic: elimination With unsymmetrical secondary and tertiary halogenoalkanes two (or sometimes three) different structural isomers can be formed C C C C Cl H C H H H H H H H H H H C C C C C H H H H H H H H H H C C C C H C H H H H H H H H H 2-methyl -2- chlorobutane can give 2-methylbut-1-ene and 2-methylbut-2-ene Often a mixture of products from both elimination and substitution occurs The structure of the halogenoalkane also has an effect on the degree to which substitution or elimination occurs in this reaction. Primary tends towards substitution Tertiary tends towards elimination
The concurrent substitution and elimination reactions of a halogenoalkane (eg 2-bromopropane with potassium hydroxide).
Students should be able to:
• explain the role of the reagent as both nucleophile and base
• outline the mechanisms of these reactions.