OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2017: Question 8

Counting Chiral Centres in Complex Organic Molecules

What this question tests

This question assesses your ability to identify asymmetric carbon atoms (chiral centres) within a complex, multi-functional organic structure (specifically an antibiotic molecule containing a beta-lactam and thiazolidine ring system, derived from penicillin). You must systematically inspect carbon atoms to find those bonded to four completely different groups.

Question 8

Exam Breakdown & Analysis

✅ Correct Answer

The correct option is C (4).

There are exactly 4 chiral centres present in the provided molecular structure.

💡 Key Knowledge

A chiral centre (asymmetric carbon) is a carbon atom bonded to four different groups or atoms.
Always check carbons taking part in double bonds or the benzene ring off-limits immediately, as they have fewer than 4 single-bond attachments.
Look out for -CH₃ groups (methyl groups) and -CH₂- groups (methylene groups); they can never be chiral because they have multiple identical hydrogens attached.

🧠 Exam Technique

Scan complex rings methodically junction by junction.
Systematically place a mental checkmark on every saturated carbon with a single bond to 4 distinct branches.
Do not get intimidated by complex biological scaffolds like fused bicyclic rings—break them down carbon-by-carbon.

❌ Common Errors

Counting CH₃ carbons: The two methyl groups attached to the right-hand ring share the same carbon and are identical—neither is chiral.
Overlooking ring carbons: Carbon atoms within rings can easily be chiral if the two pathways around the ring differ in structure.
Benzene ring confusion: Carbons in aromatic rings are sp² hybridized and bonded to only three species, ruling them out instantly.

Examiner Commentary & Breakdown: This is a standard multiple-choice recall and application question worth [1 mark]. Top-level responses quickly isolate the four asymmetric carbons without getting distracted by the aromatic ring or amide/peptide linkages.

Step-by-Step Location of the 4 Chiral Centres

Step 1: The Side Chain Carbon

Locate the carbon in the side chain attached directly to the amino group ( -NH₂ ) and the benzene ring. It is bonded to:

-H
-NH₂
-C(=O)... (peptide chain)
-C₆H₄OH (phenol ring)

Result: Chiral Centre #1 (Found!)

Step 2: The Beta-Lactam Ring Carbon

Look at the 4-membered beta-lactam ring carbon attached to the secondary amide -NH- group. It is bonded to:

-H
-NH- (amide link)
The ring C=O carbonyl carbon
The bridgehead ring carbon bonded to sulfur and nitrogen

Result: Chiral Centre #2 (Found!)

Step 3: The Bridgehead Ring Carbon

Examine the carbon situated between the sulfur atom (S) and nitrogen atom (N) in the fused ring system. It is bonded to:

-H
Sulfur atom (-S-)
Ring Nitrogen atom (-N-)
The adjacent ring carbon in the 4-membered ring

Result: Chiral Centre #3 (Found!)

Step 4: The Thiazolidine Ring Carbon

Locate the bottom carbon of the 5-membered ring attached to the carboxylic acid group ( -COOH ). It is bonded to:

-H
-COOH group
Ring Nitrogen atom (-N-)
The quaternary carbon with the two methyl groups

Result: Chiral Centre #4 (Found!)