OCR A-Level Chemistry AS Breadth in chemistry (01), November 2021: Question 18

1 mark · Medium difficulty · Multiple Choice

Identify the general formula of the dienes homologous series given their structure containing two double bonds.

Practise this question

Question

Multiple choice question 18 showing the skeletal formula of a diene with two double bonds and asking for the general formula of the dienes homologous series among options A (CnH2n+2), B (CnH2n), C (CnH2n-2), and D (CnH2n-4).
Question text

18 The ‘dienes’ are a homologous series of non-cyclic compounds with two double bonds.

The simplest diene is shown below.

What is the general formula of the dienes homologous series?

A CnH2n+2

B CnH2n

C CnH2n–2

D CnH2n–4

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer for question 18 is C.

18 C 1 AO2.2

How to answer it

General Formula of Non-Cyclic Dienes

What this question tests: Understanding and applying the concept of homologous series, interpreting skeletal structures, and deriving general algebraic formulas for unsaturated hydrocarbons containing multiple functional groups (specifically non-cyclic dienes with two double bonds).
Question 18

Determining the General Formula of Dienes

AS Level Chemistry - Organic Chemistry Foundations

✅ Correct Answer

C ( CnH2n-2 )

The simplest diene shown has 4 carbon atoms and 6 hydrogen atoms (butadiene, C₄H₆ ). Substituting n = 4 into CnH2n-2 gives C₄H₈₋₂ which equals C₄H₆ .

💡 Key Knowledge

  • Alkanes: Saturated hydrocarbons with general formula CnH2n+2 .
  • Alkenes / Cycloalkanes: Contain one degree of unsaturation (one double bond or one ring) with general formula CnH2n .
  • Dienes / Alkynes: Contain two degrees of unsaturation (two double bonds, one triple bond, or a ring with a double bond) with general formula CnH2n-2 .

🧠 Exam Technique

If you forget the general formula under exam pressure, count the atoms directly from the provided structure:

  1. Count the carbon terminals and vertices to find n = 4 .
  2. Add in the implicit hydrogen atoms on a skeletal structure (remember carbon forms 4 bonds).
  3. Test each multiple-choice option by plugging in n = 4 until you match your hydrogen count ( 6 ).

❌ Common Errors

  • Selecting option B ( CnH2n ) by confusing dienes with standard mono-alkenes.
  • Forgetting that each additional degree of unsaturation (like a second double bond) removes 2 more hydrogen atoms from the saturated alkane formula ( CnH2n+2 loses 4 hydrogens total: -2 for the first double bond, -2 for the second).
Mark Scheme Allocation: 1 mark available (AO2.2 - Application of concept to unfamiliar homologous series). Answer: C.

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), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.