JEE Main 2024 · 8 April · evening · Single correct · +4 / −1
IUPAC name of following hydrocarbon(X) is :
CH₃, -, CH, -, CH₂, -, CH₂, -, CH, -, CH, -, CH₂, -, CH₃; |, |, |; CH₃, (X), CH₃, CH₃
- (a) 2,5,6-Trimethyloctane
- (b) 2-Ethyl-2,6-diethylheptane
- (c) 3,4,7-Trimethyloctane
- (d) 2-Ethyl-3,6-dimethylheptane
Answer and solution
Answer (a) 2,5,6-Trimethyloctane
Solution
Chemistry ma’am · 3 steps
Find the longest chain first, then number from the end that reaches a methyl soonest.
- Longest chain. The horizontal chain runs straight through 8 carbons, from the left CH₃ to the right CH₃. No other path is longer, so the parent is octane and the three vertical CH₃ groups are methyl substituents.
- Number both ways. Number the chain from each end and write down where the three methyls fall. Then compare the two sets term by term and stop at the first difference. The lower number there wins.
Numbering from Methyl positions First term Left end 2, 5, 6 2 Right end 3, 4, 7 3 - So. The first point of difference is 2 against 3, so the left-end numbering is correct. The name is 2,5,6-trimethyloctane, option (a).
The idea to keep. Pick the longest carbon chain even if it bends or hides an "ethyl" at its end. Then compare locant sets at the first point of difference, not by their sum.
Answer from the NTA final answer key. Solution by PrepWiser.