OCR A-Level Chemistry AS Breadth in chemistry (01), November 2021: Question 15

✅ Correct Answer

D: 2,2,3-trimethylbutane

All given options are structural isomers with the same molecular formula ( C₇H₁₆ ), meaning they have the same number of electrons. However, 2,2,3-trimethylbutane is the most branched structure, giving it a nearly spherical shape with the smallest surface area of contact.

💡 Key Knowledge

• London forces arise from temporary, fluctuating dipoles caused by the random movement of electrons.
• Greater surface area = more points of contact between molecules = stronger London forces.
• Increased branching creates a more spherical/compact molecular shape, reducing the surface area of contact and decreasing the strength of London forces.

🧠 Exam Technique

When comparing isomers with the same molecular formula, ignore the molar mass (it is identical for all choices!). Instead, look directly at the IUPAC names to determine the amount of branching. The isomer with the most methyl/alkyl branches packed onto the shortest main carbon chain will have the weakest intermolecular forces.

❌ Common Errors

Students often incorrectly assume that longer names or more substituents mean stronger forces overall. Some mistakenly confuse London forces with permanent dipole-dipole interactions or hydrogen bonding, forgetting that non-polar alkanes only experience induced dipole-dipole interactions.