OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2017: Question 4

✅ Correct Answer

The correct option is B.

💡 Key Knowledge (CIP Rules)

• Assign priority to the two groups attached to each carbon of the C=C bond based on atomic number (higher atomic number = higher priority).
• Z-isomer (usammen / zusammen / together): The two higher-priority groups are on the same side of the double bond.
• E-isomer (entgegen / opposite): The two higher-priority groups are on opposite sides of the double bond.

🧠 Exam Technique

• Split the C=C bond down the middle vertically to look at the left carbon and right carbon independently.
• For Molecule B:
  • Left carbon: Cl (atomic number 17) beats H (atomic number 1). Higher priority = Cl (top-left).
  • Right carbon: CH₃ (Carbon has atomic number 6) beats F (atomic number 9)... Wait! Let's check fluorine!
  • Fluorine ( F ) has atomic number 9, whereas the carbon in CH₃ has atomic number 6. Therefore, F has a higher priority than CH₃ on the right carbon! Higher priority = F (top-right).
• Both high-priority groups ( Cl on the left, F on the right) are pointing upwards (the same side of the horizontal reference line through the double bond). Hence, it is the Z -isomer.

❌ Common Errors & Misconceptions

• Assuming CH₃ beats F: Students often incorrectly assume carbon chains automatically outrank single halogen atoms due to larger overall molecular mass, forgetting that CIP priority on a given atom is decided strictly by the single highest atomic number directly attached. Fluorine (atomic number 9) beats Carbon (atomic number 6).
• Confusing E/Z with cis/trans: Relying on simple "cis/trans" logic fails when there are four different groups attached to the alkene carbons, making strict CIP priority assignment essential.