OCR A-Level Chemistry Synthesis and analytical techniques (02), November 2021: Question 2

1 mark · Medium difficulty · Multiple Choice

Identify the type(s) of stereoisomerism shown by 2,4-dimethylhex-2-ene.

Practise this question

Question

Multiple choice question 2 asking which type of stereoisomerism is shown by 2,4-dimethylhex-2-ene. Four options are provided: A, E/Z isomerism only; B, Optical isomerism only; C, Both E/Z isomerism and optical isomerism; D, Neither E/Z isomerism nor optical isomerism. A box for the student's answer is shown at the bottom.
Question text

2 Which type(s) of stereoisomerism is/are shown by 2,4-dimethylhex-2-ene?

A E/Z isomerism only.

B Optical isomerism only.

C Both E/Z isomerism and optical isomerism.

D Neither E/Z isomerism nor optical isomerism.

Your answer

[1]

Mark scheme

Show the mark scheme Mark scheme showing the correct answer for question 2 is B, worth 1 mark.

2 B 1 AO1.2

How to answer it

Stereoisomerism in 2,4-dimethylhex-2-ene

📌 What this question tests

This question assesses your ability to analyse organic structures for types of stereoisomerism (specifically E/Z isomerism and optical isomerism). You must apply IUPAC naming rules, visualize or sketch molecular structures, and independently check each carbon atom in the chain for restricted rotation around a double bond or the presence of a chiral center (asymmetric carbon atom bonded to four different groups).

Question Analysis: Multiple Choice Part 2

Identifying Types of Stereoisomerism

✅ Correct Answer: B

The correct option is B: Optical isomerism only.

Compound 2,4-dimethylhex-2-ene exhibits optical isomerism due to a chiral center at Carbon-4, but it does not show E/Z isomerism because Carbon-2 is bonded to two identical methyl groups.

💡 Key Knowledge

  • E/Z Isomerism requirements: Restricted rotation around a C=C double bond AND each carbon of the double bond must be attached to two different groups.
  • Optical Isomerism requirements: A chiral center (an asymmetric carbon atom bonded to 4 completely different groups), leading to non-superimposable mirror images (enantiomers).

🧠 Exam Technique

  • Always sketch the full displayed or skeletal structure of the molecule before jumping to conclusions.
  • Systematically check every carbon in the double bond for E/Z eligibility.
  • Scan the remaining saturated carbons for any atom bonded to four distinct entities.

❌ Common Errors

Many students automatically assume that any molecule containing a C=C double bond will automatically display E/Z isomerism (choosing option C or A). Always check the groups attached to both ends of the double bond individually!

Mark Scheme Allocation: 1 mark total (AO1.2 - Demonstrating knowledge and understanding of organic stereochemistry principles). Answer: B.

Step-by-Step Structural Breakdown

Why does 2,4-dimethylhex-2-ene behave this way?

Step 1: Draw the Carbon Skeleton

Write out the main hex-2-ene chain with a double bond between C2 and C3: C-C=C-C-C-C

Add the methyl groups at position 2 and position 4: CH₃-C(CH₃)=CH-CH(CH₃)-CH₂-CH₃

Step 2: Check C=C for E/Z Isomerism

Look at Carbon-2 ( =C(CH₃)₂ ). It is bonded to two identical methyl groups.

Because a carbon in the double bond is attached to two identical groups, it cannot exhibit E/Z isomerism.

Step 3: Check for Chiral Centers

Look at Carbon-4 ( -CH(CH₃)- ).

C4 is bonded to four different groups: (1) a hydrogen atom -H , (2) a methyl group -CH₃ , (3) an ethyl group -CH₂CH₃ , and (4) the rest of the unsaturated chain -CH=C(CH₃)₂ .

Therefore, C4 is a chiral center, meaning the molecule shows optical isomerism.

Topics

Module 4: Core organic chemistry · Module 6: Organic chemistry and analysis · 4.1 Basic concepts and hydrocarbons · 6.2 Nitrogen compounds, polymers and synthesis

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.