Elimination from halogenoalkanesAQA A-Level Chemistry: Mind map

Exam questions on Elimination from halogenoalkanes

1. Hydroxide ions, OH⁻, can react with a halogenoalkane in two different ways. Each oxygen atom in OH⁻ has lone pairs of electrons, and the ion can also accept a proton. Both reactions start with the polar carbon–halogen bond.
   Explain how the role of the hydroxide ion in substitution differs from its role in elimination.2 marks
2. A student warms 2-bromopropane, CH₃CHBrCH₃, with aqueous potassium hydroxide and obtains mainly propan-2-ol. She repeats the experiment using a solution of potassium hydroxide in ethanol and heats the mixture strongly. This time the main product is a gas that decolourises bromine water.
   Write an equation for the formation of the gas in the second experiment, using structural formulae for the organic species.2 marks
3. A technician needs to make propene from 2-bromopropane. A competing reaction, which gives propan-2-ol, takes place at the same time under some conditions, so the technician must choose the reagent and conditions with care.
   Describe, in words, the mechanism for the elimination of HBr from 2-bromopropane by hydroxide ions, referring to the movement of electron pairs.3 marks