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Reaction Mechanisms (OCR A Level Chemistry A): Revision Note

Exam code: H432

Richard Boole

Written by: Richard Boole

Reviewed by: Philippa Platt

Updated on

Bond fission

Homolytic fission

  • Homolytic fission is breaking a covalent bond so that each atom takes one electron from the bond

  • This forms two radicals

    • A radical is a reactive chemical species with an unpaired electron

  • Examples of radicals include: Cl•, Br•, CH3•

    • The dot (•) represents the unpaired electron and must be shown in radical mechanisms

Heterolytic fission

  • Heterolytic fission is breaking a covalent bond so that the more electronegative atom takes both the electrons from the bond

  • This forms a negative ion and leaves behind a positive ion 

Diagram explaining heterolytic and homolytic fission, showing covalent bond splitting into charged ions or radicals with labelled processes and outcomes.
The diagram shows heterolytic fission in which the most electronegative atom takes both electrons in the covalent bond and homolytic fission in which each atom takes one electron from the covalent bond

Curly arrows

  • Reaction equations tell you about the amount of reactants and products, including their stoichiometry, in a reaction

  • Reaction mechanisms tell you about how the reaction actually takes place

  • In a reaction mechanism, curly arrows show the movement of electrons

    • Curly arrows can be single headed or double headed

Single headed arrows

  • Single headed curly arrows are sometimes called fish-hook arrows or half arrows

  • They show the movement of a single electron

  • This can occur when:

    • A covalent bond undergoes homolytic fission to form two radicals

    • Two radicals terminate forming a covalent bond in the process

    • A radical and a covalent compound propagate as part of a reaction, as shown:

Chemical reaction showing chlorine radical abstracting a hydrogen atom from methane, forming hydrochloric acid and a methyl radical.
During this propagation step, the unpaired electron from chlorine and one of the electrons from the C-H bond react together to form a new covalent bond and the other electron from the C-H bond moves to form a methyl radical

Examiner Tips and Tricks

The use of single headed arrows is not required knowledge

Their main use is in free radical reactions which are usually represented using written equations rather than outlined mechanisms

Double headed arrows

  • Double headed curly arrows show the movement of a pair of electrons

  • This can occur when:

    • Covalent bonds break - a covalent bond undergoes heterolytic fission to form a positive and a negative ion

    • Covalent bonds form - a lone pair of electrons attacks a positive or δ+ centre and forms a new covalent bond, as shown:

      Chemical reaction diagram showing a bromide ion reacting with a carbocation, forming 2-bromopropane.
      During this step of an addition reaction, the lone pair from the bromide ion reacts with the positive carbocation (a carbon atom with a positive charge) to form a new covalent bond

Examiner Tips and Tricks

Examiners are looking for key points in reaction mechanisms:

  1. The curly arrow must be double headed (as shown above)

  2. The curly arrow must start from a lone pair of electrons OR a covalent bond

  3. The curly arrow must end at an area of positive charge

  4. Where appropriate, molecules should include:

    • Partial charges, δ- and δ+

    • Full charges, - and +

    • Lone pairs of electrons

  • Curly arrows are a feature of three main types of reaction:

    1. Addition reactions

      • Two reactants forming one product
        e.g. ethene + bromine → 1,2-dibromoethane

    2. Substitution reactions

      • An atom or group of atoms in a compounds is replaced by another atom or group of atoms
        e.g. bromoethane + OH- → ethanol + Br-

    3. Elimination reactions

      • A small molecule is removed from a larger molecule
        e.g. ethanol rightwards arrow with acid space catalyst on top ethene + water

  • More specific examples of expected curly arrow mechanisms include:

AS organic mechanisms

A Level organic mechanisms

Nucleophilic substitution of haloalkanes

Nucleophilic addition of carbonyls

Electrophilic addition

Electrophilic substitution

Elimination reactions of alcohols

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    Author
    Reviewer
    Richard Boole

    Author: Richard Boole

    Expertise: Chemistry Content Creator

    Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.

    Philippa Platt

    Reviewer: Philippa Platt

    Expertise: Chemistry Content Creator

    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