A LevelChemistryAQARevision NotesOrganic ChemistryAlcoholsElimination Reactions of Alcohols

Elimination Reactions of Alcohols (AQA A Level Chemistry): Revision Note

Exam code: 7405

Stewart Hird

Written by: Stewart Hird

Reviewed by: Caroline Carroll

Updated on

Elimination Reactions

  • Alcohols can also undergo dehydration to form alkenes

  • Alcohol vapour is passed over a hot (600 oC) catalyst of aluminium oxide (Al2O3) powder or pieces of porous pot

    • Excess hot, concentrated sulfuric acid or phosphoric acid is used as a catalyst

  • Phosphoric(V) acid is used as an alternative dehydrating agent

Apparatus for dehydrating ethanol over hot aluminium oxide catalyst, collecting ethene gas over water.
Dehydration of ethanol using aluminium oxide as a catalyst forms ethene gas, which can be collected over water

  • This is an example of an elimination reaction

    • Elimination reactions involve a small molecule being formed as a by-product

    • The water molecule is formed from the -OH group of the alcohol and the H+ from the acid catalyst

    • The neighbouring hydrogen is then lost as an H+ ion, which regenerates the acid catalyst

  • The reaction and mechanism for the dehydration of propan-2-ol is shown below

Mechanism for the acid-catalysed dehydration of propan-2-ol, showing protonation of the OH group, loss of water, and formation of the C=C double bond
Dehydration of propan-2-ol mechanism

  • Alkenes produced by this method can be used to produce addition polymers without using monomers derived from crude oil

Related topics

Examiner Tips and Tricks

The most commonly penalised mix-up in this topic: students often confuse this acid-catalysed elimination of water from alcohols with the base-induced elimination of hydrogen halides from halogenoalkanes (drawing an OH⁻ ion attacking a hydrogen). These mechanisms often carry several marks, so it is worth investing the time to practice them.

 In this mechanism, any C–H bond that breaks must have the H drawn explicitly — the usual "stick" shorthand (bond drawn without the H) is penalised here even though it's often accepted elsewhere.

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Author
Reviewer
Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Content Creator

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.

Caroline Carroll

Reviewer: Caroline Carroll

Expertise: Head of Content Delivery

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about delivering high-quality resources to help students achieve their full potential.