OCR A-Level Chemistry AS Breadth in chemistry (01), June 2018: Question 16

1 mark · Hard difficulty · Multiple Choice

Determine the number of tertiary alcohol structural isomers with the molecular formula C6H14O.

Practise this question

Question

Multiple choice question 16 asking: How many structural isomers of C6H14O are tertiary alcohols? Four options are given: A 1, B 2, C 3, D 4, with a box for the student's answer and [1] mark allocation at the bottom right.
Question text

16 How many structural isomers of C6H14O are tertiary alcohols?

A 1

B 2

C 3

D 4

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme for question 16 indicating the correct answer is C, with 1 mark awarded.

16 C 1 ALLOW 3

How to answer it

Structural Isomers of Tertiary Alcohols (C₆H₁₄O)

What this question tests

This question assesses your ability to define structural isomerism, recognize the functional group and carbon skeleton requirements of a tertiary alcohol (where the carbon bonded to the -OH group is attached to three other carbon atoms), and systematically count all unique non-superimposable structural isomers for the molecular formula C₆H₁₄O without missing any or double-counting duplicates.

Question 16

Determining the Number of Tertiary Alcohol Isomers

✅ Correct Answer

C (3)

There are exactly 3 structural isomers of C₆H₁₄O that are tertiary alcohols.

💡 Key Knowledge

  • Tertiary Alcohol definition: The -OH group is attached to a carbon atom that is directly bonded to 3 other carbon atoms.
  • Core feature: A tertiary alcohol with formula C₆H₁₄O must feature a central carbon bonded to -OH and three separate alkyl group branches adding up to a total of 6 carbons.
  • Systematic approach: Build the longest carbon chain containing the tertiary carbon or use alkyl branching variations around the central C-OH junction.

🧠 Exam Technique

To avoid missing isomers or counting duplicates, systematically alter the carbon chain length attached to the tertiary center:

  • Branch 1: Methyl, Methyl, Propyl groups attached to the carbinol carbon -> 2-methylpentan-2-ol.
  • Branch 2: Methyl, Ethyl, Ethyl groups attached to the carbinol carbon -> 3-methylpentan-3-ol.
  • Branch 3: Methyl, Methyl, Isopropyl (1-methylethyl) groups -> 2,3-dimethylbutan-2-ol.

❌ Common Errors

  • Including primary or secondary alcohols: Misidentifying the degree of the alcohol (e.g., counting a secondary alcohol like hexan-2-ol).
  • Double counting: Renaming the exact same molecule from the opposite end of the longest carbon chain.
  • Chain length errors: Accidentally drawing molecules with 5 or 7 carbons instead of the required 6 carbons specified by C₆H₁₄O .
Examiner Commentary: Top-level responses quickly sketched out skeletal or structural formulas for the possible branched frameworks. Candidates who lost marks usually confused tertiary structures with secondary branched isomers like 2-methylbutan-2-ol derivatives or failed to account for all distinct structural arrangements of the butyl/pentyl alkyl branches around the central carbinol carbon.

Topics

Module 4: Core organic chemistry · 4.2 Alcohols, haloalkanes and analysis

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.