OCR A-Level Chemistry Synthesis and analytical techniques (02), November 2020: Question 1

✅ Correct Answer

A ( E-3-bromopent-2-ene )

💡 Key Knowledge

• Longest Carbon Chain: The longest continuous chain containing the C=C double bond has 5 carbon atoms (pent-).
• Double Bond Locant: Numbering from right to left gives the double bond at carbon-2 ( pent-2-ene ) rather than carbon-3.
• Substituent Location: The bromine atom is attached to carbon-3 ( 3-bromo ).

🧠 Exam Technique

• Step 1 (Locant numbering): Number the carbon chain from the end that gives the C=C double bond the lowest possible number (Right to left: C1 is the rightmost CH₃ of the CH₂CH₃ group, C2=C3, C4-C5).
• Step 2 (Assign CIP priorities on C2): Compare H (atomic number 1) vs CH₃ (carbon has atomic number 6). Higher priority = CH₃ .
• Step 3 (Assign CIP priorities on C3): Compare Br (atomic number 35) vs CH₂CH₃ (carbon has atomic number 6). Higher priority = Br .
• Step 4 (Determine E/Z): The higher priority groups ( CH₃ on left, Br on right) are on opposite sides of the C=C double bond, making it the E isomer (entgegen).

❌ Common Errors

• Wrong chain numbering: Starting from the left gives the double bond at carbon-3, leading to incorrect names like option B or D ( pent-3-ene ). Always prioritise giving the alkene functional group the lowest number.
• Confusing E and Z: Mixing up the priority groups or incorrectly assuming E means 'same side' (remember: E = Epposite sides, Z = zusammen / same).