OCR A-Level Chemistry AS Breadth in chemistry (01), June 2022: Question 14

1 mark · Easy difficulty · Multiple Choice

Determine the number of sigma bonds in the given organic molecule.

Practise this question

Question

Multiple choice question asking for the number of sigma bonds in a diene molecule shown as a skeletal formula, with options A (1), B (3), C (7), and D (9).
Question text

14 What is the number of σ-bonds in the molecule below?

A 1

B 3

C 7

D 9

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme indicates the correct answer is D, corresponding to 9 sigma bonds.

14 D 1 AO1.2 ALLOW 9

How to answer it

Counting Sigma Bonds in Unsaturated Hydrocarbons

What this question tests

This question assesses your ability to interpret skeletal formulas of organic molecules, recall structural definitions of covalent bonding (sigma vs. pi bonds), and count single and multiple bonds accurately.

Question 14 Analysis

Identifying Sigma (σ) and Pi (π) Bonds

✅ Correct Answer

Option D (9)

Mark Awarded: 1 / 1 mark (AO1.2)

💡 Key Knowledge

  • Every single covalent bond is a sigma (σ) bond formed by the direct overlap of orbitals.
  • In any double bond, there is one σ-bond and one pi (π) bond.
  • The displayed formula of the molecule shown (buta-1,3-diene, CH₂=CH-CH=CH₂ ) contains carbon atoms bonded to hydrogen atoms and neighboring carbons.

🧠 Exam Technique

Always redraw skeletal formulas into fully expanded displayed structures or count systematically before answering multiple-choice questions. Don't rush and miss terminal hydrogen atoms or multiple bonds.

❌ Common Errors

Students often forget to count hidden hydrogen atoms implied by the skeletal ends, or mistakenly count the second line of a double bond as an extra sigma bond instead of a pi bond.

📐 Step-by-Step Breakdown

  1. Identify the molecule: The skeletal structure represents buta-1,3-diene with molecular formula C₄H₆ .
  2. Count Carbon-Carbon bonds: There are 3 C-C bonds in total (two double bonds and one single bond). Each contains one σ-bond, giving 3 σ-bonds. (Note: the 2 extra lines in the double bonds are π-bonds, not σ-bonds).
  3. Count Carbon-Hydrogen bonds: There are 6 C-H single bonds, giving 6 σ-bonds.
  4. Total calculation: 3 (C-C σ-bonds) + 6 (C-H σ-bonds) = 9 σ-bonds.

Topics

Module 4: Core organic chemistry · 4.1 Basic concepts and hydrocarbons

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.