AQA A-Level Chemistry Paper 3, 2017: Question 24

Stereoisomerism in Substituted Propanes and Propenes

What this question tests

This multiple-choice question assesses your ability to distinguish between compounds that show stereoisomerism (both E/Z geometric isomerism and optical isomerism) and those that do not. You need to draw and evaluate structural formulas to check for restricted rotation with two different groups on each double-bond carbon (E/Z) or a chiral centre (an asymmetric carbon atom bonded to four different groups).

Question 24 Analysis

Identification of the Molecule That Does NOT Exhibit Stereoisomerism [1 Mark]

✅ Correct Answer

Correct Option: D (1,3-dichloropropane)

Structure: Cl-CH₂-CH₂-CH₂-Cl
Contains only single C–C bonds (free rotation, so no E/Z isomerism).
Carbon 1 is bonded to: 2× H, 1× Cl, 1× alkyl chain. (Not chiral)
Carbon 2 is bonded to: 2× H, 2× identical -CH₂Cl groups. (Not chiral)
Carbon 3 is bonded to: 2× H, 1× Cl, 1× alkyl chain. (Not chiral)
Since there is no chiral centre and no restricted rotation with differing groups, it shows no stereoisomerism.

💡 Key Knowledge

Stereoisomers have the same structural formula but a different spatial arrangement of atoms. Two types are assessed:

E/Z Isomerism: Requires a C=C double bond (restricted rotation) and each carbon of the C=C must be bonded to two different groups.
Optical Isomerism: Requires a chiral centre (a carbon atom attached to four different groups), resulting in non-superimposable mirror images (enantiomers).

📐 Step-by-Step Option Breakdown

Option	Displayed / Structural Formula	Type of Stereoisomerism	Explanation
A: 1,2-dichloropropene	CH(Cl)=C(Cl)CH₃	E/Z Isomerism	• C1 is attached to -H and -Cl (two different groups). • C2 is attached to -Cl and -CH₃ (two different groups). Shows stereoisomerism (E and Z isomers).
B: 1,2-dichloropropane	CH₂Cl-CH(Cl)-CH₃	Optical Isomerism	• C2 is bonded to 4 different groups: -H, -Cl, -CH₃, and -CH₂Cl. It possesses a chiral centre and therefore shows optical isomerism.
C: 1,3-dichloropropene	CH(Cl)=CH-CH₂Cl	E/Z Isomerism	• C1 is attached to -H and -Cl (two different groups). • C2 is attached to -H and -CH₂Cl (two different groups). Shows stereoisomerism (E and Z isomers).
D: 1,3-dichloropropane	CH₂Cl-CH₂-CH₂Cl	None	• No C=C double bond (no E/Z). • C2 has two identical -H atoms and two identical -CH₂Cl groups. Cannot show stereoisomerism.

🧠 Exam Technique

Watch the word "NOT": Questions containing negative keywords are classic traps. Underline "not" as soon as you read it.
Look for both types: Remember that "stereoisomerism" is an umbrella term covering both geometric (E/Z) and optical isomerism.
Symmetry check for optical centres: In propane chains, always check Carbon 2. If the two arms are identical (like -CH₂Cl in option D), C2 cannot be chiral.

❌ Common Errors

Forgetting Optical Isomerism: Many students instantly rule out options B and D because they are alkanes (no double bond), mistakenly assuming stereoisomerism means only E/Z. Option B is chiral!
Misidentifying C=C substituents: Rushing to sketch 1,3-dichloropropene (C) and incorrectly thinking C2 has two identical groups, overlooking that the H atom and -CH₂Cl group are different.

Mark Scheme Note: Award 1 mark for option D. No partial marks are available for Section B / multiple-choice questions.

AQA A-Level Chemistry Paper 3, 2017: Question 24

Question

Mark scheme

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