OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2022: Question 3

1 mark · Medium difficulty · Multiple Choice

Determine the number of sigma bonds in a molecule of methylbenzene

Practise this question

Question

Multiple choice question 3 asking 'What is the number of sigma bonds in a molecule of methylbenzene?', with four options A (7), B (10), C (12), and D (15). Below the options is an answer box and the mark allocation [1] on the bottom right.
Question text

3 What is the number of sigma bonds in a molecule of methylbenzene?

A 7

B 10

C 12

D 15

Your answer

[1]

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer for question 3 is D, worth 1 mark, allowing the correct number of sigma bonds (15).

3 D 1 AO1.2 ALLOW 15 (correct number of sigma bonds)

How to answer it

Counting Sigma Bonds in Methylbenzene

What this question tests

This question assesses your understanding of covalent bonding structure, specifically the ability to distinguish between sigma (σ) and pi (π) bonds in aromatic organic molecules like methylbenzene (C₆H₅CH₃).

Question 3 (Multiple Choice)

Answer: D (15)

✅ Correct Answer

The correct option is D (15). A molecule of methylbenzene contains a total of 15 sigma bonds.

💡 Key Knowledge

  • Sigma (σ) bonds are formed by the direct head-on overlap of atomic orbitals between two atoms.
  • Every single covalent bond consists of exactly one sigma bond.
  • Every multiple bond (double or triple) contains one sigma bond and the remaining components are pi (π) bonds.
  • In the delocalized benzene ring, there are 6 carbon-carbon sigma bonds and 5 carbon-hydrogen sigma bonds. The methyl group adds another 4 sigma bonds (3 C-H and 1 C-C).

🧠 Exam Technique

Do not try to count these purely in your head. Quickly sketch out the full displayed or structural formula of methylbenzene on your rough paper. Count every single line (representing single bonds) explicitly, remembering that a benzene ring contains 6 C-C single bonds and 6 C-H bonds (though one C-H is replaced by the CH₃ group, leaving 5 aromatic C-H bonds).

❌ Common Errors

  • Forgetting pi bonds: Students often incorrectly count the delocalized electrons or π-system as extra sigma bonds. Remember, the delocalized ring contains 6 sigma C-C bonds and a system of π-bonds, but only the backbone single-bond framework counts towards the sigma bond total.
  • Miscounting methyl hydrogens: Forgetting that the methyl group ( -CH₃ ) contributes 3 C-H sigma bonds plus 1 C-C bond connecting it to the ring.
Mark Scheme Breakdown: 1 mark awarded for selecting D.

Topics

Module 6: Organic chemistry and analysis · 6.1 Aromatic compounds, carbonyls and acids

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.