Elimination Reactions of Halogenoalkanes (Oxford AQA International A Level Chemistry)

Revision Note

Philippa Platt

Written by: Philippa Platt

Reviewed by: Stewart Hird

Elimination Mechanism

  • In an elimination reaction, an organic molecule loses a small molecule

    • In the case of halogenoalkanes this small molecule is a hydrogen halide (e.g. HCl)

  • The halogenoalkanes are heated with ethanolic sodium hydroxide (no water present) causing the C-X bond to break heterolytically, forming an X- ion and leaving an alkene as an organic product

  • Under these conditions the OH- ion can act as a base removing an H+ ion from the halogenoalkane

Elimination of a halogenoalkane

Halogen Compounds Elimination, downloadable AS & A Level Chemistry revision notes
Hydrogen bromide is eliminated to form ethene

Elimination with OH-

  • There are three curly arrows in this mechanism

    • Curly arrow from lone pair on :OH- ion to H atom bonded to the adjacent carbon atom

    • Curly arrow from the C–H bond adjacent to the appropriate C–C bond

    • Curly arrow from C-X bond to Br (X) atom

 

Which product will form?

  • Note that the reaction conditions should be stated correctly as different reaction conditions will result in different types of organic reactions

Mechanism favoured

Conditions

Product

Nucleophilic substitution

NaOH in water

Room temperature

Favoured by primary halogenoalkanes

Alcohol

Elimination

NaOH in ethanol (no water present)

High temperature

Favoured by tertiary halogenoalkanes

Alkene

Isomeric alkenes

  • It is possible for elimination of halogenoalkanes to produce a mixture of alkenes which can also exhibit E / Z isomerism

elimination-to-form-alkenes-from-halogenoalkanes

Examiner Tips and Tricks

To help visualise the elimination mechanism you can think of a stair case as the shape is very similar. Double check you have drawn first curly arrow to the correct hydrogen atom.

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Philippa Platt

Author: Philippa Platt

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener

Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Lead

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.