A-Level Chemistry May/June 2025 Q5(b)(i): Deduce the number of sigma (ơ) bonds and pi (π) bonds present in Z. number of o bonds n…

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  The structure of Z is assumed to be ethyl acrylate, CH₂=CHCOOCH₂CH₃, which has the required number of bonds.

  To find the number of sigma (σ) and pi (π) bonds, we count them from the full displayed formula.

  1. Sigma (σ) bonds: Every single bond is a σ bond, and every double bond contains one σ bond.

  • Count all single bonds (C-H, C-C, C-O):
    • C-H bonds: $2 + 1 + 2 + 3 = 8
    • C-C$ single bonds: 2
    • C-O single bonds: 2
  • Count the σ component of each double bond:
    • One $C=C$ bond contributes 1 σ bond.
    • One $C=O$ bond contributes 1 σ bond.

  Total σ bonds = (C-H) + (C-C) + (C-O) + (σ from C=C) + (σ from C=O) Total σ bonds = 8 + 2 + 2 + 1 + 1 = 14

  2. Pi (π) bonds: Every double bond contains one π bond.

  • The molecule contains one $C=C$ double bond, which is 1 π bond.
  • The molecule contains one $C=O$ double bond, which is 1 π bond.

  Total π bonds = 1 + 1 = 2

  Final Answer:

  number of σ bonds 14

  number of π bonds 2

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  How the marks are awarded

  • B1 — Awarded for correctly deducing there are 14 sigma (σ) bonds by systematically counting all single bonds and one bond from each double bond in the molecule.
  • B1 — Awarded for correctly deducing there are 2 pi (π) bonds by identifying the two double bonds ($C=C$ and $C=O$) present in the molecule.

  Common mistakes

  • Miscounting the number of C-H bonds, especially on the ethyl group, leading to an incorrect sigma bond count.
  • Forgetting to include the sigma bonds within the $C=C$ and $C=O$ double bonds, leading to an answer of 12 σ bonds.
  • Incorrectly counting a double bond as two sigma bonds instead of one sigma and one pi bond.
  • Only counting the pi bond from the $C=C$ double bond and ignoring the one in the $C=O$ group, leading to an answer of 1 π bond.

  Examiner tip: To find the number of sigma and pi bonds, always draw the full displayed formula of the molecule and remember that every single bond is a sigma bond, every double bond is one sigma and one pi bond, and every triple bond is one sigma and two pi bonds.

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