OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2023: Question 7

1 mark · Medium difficulty · Multiple Choice

Identify the systematic IUPAC name for the given polyene structure containing multiple double bonds and methyl substituents.

Practise this question

Question

A multiple-choice question showing a chemical structure of a branched, unsaturated hydrocarbon with three double bonds and two methyl groups attached. Four options (A, B, C, D) provide different systematic IUPAC names: A is 3,5-dimethylocta-1,3,6-triene, B is 3,5-dimethylocta-2,5,7-triene, C is 4,6-dimethylocta-1,3,6-triene, and D is 4,6-dimethylocta-2,5,7-triene.
Question text

7 What is the systematic name for the compound below?

A 3,5-dimethylocta-1,3,6-triene

B 3,5-dimethylocta-2,5,7-triene

C 4,6-dimethylocta-1,3,6-triene

D 4,6-dimethylocta-2,5,7-triene

Your answer

[1]

Mark scheme

Show the mark scheme The mark scheme table shows that question 7 maps to correct answer A, worth 1 mark.

7 A 1 AO1.2

How to answer it

Systematic IUPAC Nomenclature

What this question tests

This question assesses your ability to apply IUPAC rules of organic nomenclature (AO1.2) to a multi-functional group hydrocarbon containing multiple alkene (diene/triene) and branched alkyl (methyl) groups. You must successfully identify the longest carbon chain, number it to give the lowest possible locants, and correctly alphabetise and suffix the compound.

Question 7 Breakdown

Determining the Systematic Name of a Complex Alkadiene/Triene

✅ Correct Answer

A — 3,5-dimethylocta-1,3,6-triene

Awarded 1 mark for selecting option A.

💡 Key Knowledge

  • Longest Chain: The continuous carbon chain has 8 carbons, making the root octa- .
  • Suffix: There are three double bonds, making it a triene .
  • Numbering Direction: Number from right-to-left to give the lowest locants for the functional groups (1, 3, 6 rather than 2, 5, 7).

🧠 Exam Technique

  • Trace both directions: Numbering left-to-right gives locants 2, 5, 7 for the double bonds (Option B and D). Numbering right-to-left gives locants 1, 3, 6 (Option A and C). Always prioritise the lowest possible numbers for principal functional groups.
  • Check substituent positions: With the right-to-left numbering giving alkene positions at 1, 3, and 6, the methyl branches sit neatly on carbons 3 and 5.

❌ Common Errors

  • Choosing numbering from the wrong end, leading to higher locants like 2,5,7 (options B and D).
  • Miscounting the carbon chain length or misidentifying the position of the methyl branches (confusing positions 3,5 with 4,6).

Topics

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

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.