A-Level Chemistry May/June 2025 Q8(a)(ii): One of the isomers of L forms in much smaller amounts than the other two isomers. Draw…

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  Structure of the isomer:

  The isomer formed in the smallest amount is 3-bromomethylbenzene.

        Br
        |
     /-----\
    |       |
    |   O   |  
    |       |
     \-----/
        |
       CH₃
  

  Explanation:

  The methyl group (–CH₃) attached to the benzene ring is an activating group. It is ortho, para-directing (or 2,4-directing).

  This means the methyl group increases the electron density at positions 2, 4, and 6 of the ring, making these positions the most favourable for attack by the Br⁺ electrophile. Substitution at position 3 (the meta position) is therefore electronically disfavoured, resulting in the 3-bromo isomer being formed in the smallest amount.

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  How the marks are awarded

  • M1 — The M1 mark is awarded for correctly drawing or naming the structure of 3-bromomethylbenzene, which is the meta-isomer.
  • M2 — The M2 mark is awarded for the explanation stating that the methyl group is ortho, para-directing (or 2,4-directing), which makes the meta (3-) position the least likely site for substitution.

  Common mistakes

  • Drawing one of the major isomers, 2-bromomethylbenzene (ortho) or 4-bromomethylbenzene (para), instead of the minor meta-isomer.
  • Giving an incorrect explanation, for example, stating that the methyl group is meta-directing.
  • Providing an incomplete explanation based only on steric hindrance, without mentioning the primary electronic directing effect of the methyl group.
  • Drawing the product of a free-radical substitution reaction, (bromomethyl)benzene (C₆H₅CH₂Br), where the bromine is on the methyl group instead of the ring.

  Examiner tip: Memorise the directing effects of common substituents on the benzene ring for electrophilic substitution reactions, distinguishing between activating (o,p-directing) and deactivating (m-directing) groups.

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