Q.Which of the following represents the correct IUPAC name for the compounds concerned?
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Start your 14-day free trial to unlock the full solution →The key idea is to apply IUPAC nomenclature rules — longest chain, lowest locants, alphabetical order, and functional group priority — to choose the correct name from each pair. The correct answers are: (a) 2,2-Dimethylpentane,
(b) 2,4,7-Trimethyloctane,
(c) 2-Chloro-4-methylpentane,
(d) But-3-yn-1-ol.
Let’s go through each pair one by one. The goal is not just to pick the right name, but to understand why one is correct and the other is wrong.
1. Pair (a): 2,2-Dimethylpentane vs 2-Dimethylpentane
The first name, 2,2-Dimethylpentane, tells us there are two methyl groups attached to carbon number 2 of a pentane chain. The second name, 2-Dimethylpentane, is ambiguous — it doesn’t specify how many methyl groups are at position 2. The prefix “di-” is missing.
A common mistake is to think “2-Dimethylpentane” is acceptable shorthand. But IUPAC requires the multiplier prefix (di-, tri-, etc.) to be explicitly stated. “Dimethyl” without a number means two methyl groups, but the locant must be repeated if both are on the same carbon: hence 2,2-.
So the correct name is 2,2-Dimethylpentane.
2. Pair (b): 2,4,7-Trimethyloctane vs 2,5,7-Trimethyloctane
Here we have an octane chain with three methyl substituents. The rule is: when numbering the chain, we choose the direction that gives the lowest set of locants at the first point of difference.
Number the octane chain from left to right: carbons 1 to 8. If methyl groups are at positions 2, 4, and 7, the locant set is (2,4,7). If we number from the other end, the same groups would be at positions 2, 5, and 7 — that set is (2,5,7). Compare the two sets: at the first number where they differ (4 vs 5), 4 is smaller. So (2,4,7) is the lower set.
The “first point of difference” rule: compare the two locant lists in increasing order. The set with the smaller number at the first difference wins. Here, 4 < 5, so 2,4,7 is correct.
Thus the correct name is 2,4,7-Trimethyloctane.
3. Pair (c): 2-Chloro-4-methylpentane vs 4-Chloro-2-methylpentane
The parent chain is pentane. We have a chloro substituent and a methyl substituent. The rule: number the chain to give the lowest locants to the substituents, considered as a set. If we number from one end, chloro is at 2 and methyl at 4 → set (2,4). From the other end, chloro would be at 4 and methyl at 2 → set (2,4) again — same set! So both numberings give the same locant set.
Now, when locants are tied, we apply alphabetical order: “chloro” comes before “methyl” in the alphabet. So the name should list chloro first. That gives 2-Chloro-4-methylpentane.
Some students think alphabetical order only matters when locants are different, but it applies regardless. Also, note that the prefixes “chloro” and “methyl” are not modified by multipliers (no di-, tri- here), so they are compared directly.
The second name, 4-Chloro-2-methylpentane, violates alphabetical order (methyl before chloro). So the correct name is 2-Chloro-4-methylpentane.
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