OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2018: Question 11

✅ Correct Answer

The correct option is B (2).

There are 2 straight-chain structural isomers of C₇H₁₅Cl that contain a chiral carbon atom.

💡 Key Knowledge

• Straight-chain: The carbon skeleton must be a continuous, unbranched chain of 7 carbon atoms ( CH₃-CH₂-CH₂-CH₂-CH₂-CH₂-CH₃ backbone).
• Chiral Centre: A carbon atom bonded to 4 completely different groups ( C*XYZW ).
• Halogenoalkanes derived from heptane can have the chlorine atom attached at different positions along the chain (carbon-1 through carbon-4).

🧠 Exam Technique

Systematically draw out the positions of the chlorine atom on a straight 7-carbon chain and check each carbon for chirality:

• 1-chloroheptane : No chiral C (C-1 has two H atoms).
• 2-chloroheptane : Chiral! C-2 is bonded to: -H , -Cl , -CH₃ , and -CH₂CH₂CH₂CH₂CH₂CH₃ (4 different groups).
• 3-chloroheptane : Chiral! C-3 is bonded to: -H , -Cl , -CH₂CH₃ , and -CH₂CH₂CH₂CH₃ (4 different groups).
• 4-chloroheptane : No chiral C (C-4 is bonded to two identical propyl groups -CH₂CH₂CH₃ ).

❌ Common Errors

• Ignoring constraints: Students often lose marks by including branched-chain isomers (e.g., methylhexanes) even though the question explicitly specifies straight-chain.
• Misidentifying symmetry: Forgetting that 4-chloroheptane is symmetrical, meaning carbon-4 is bonded to two identical alkyl chains and is therefore achiral.