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Which is a more correct name, according to IUPAC nomenclature: pent-2-ene-4-yne, or pent-3-ene-1-yne?

Trans-Pentenyne

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According to R-3.1.1 of IUPAC, Commission on Nomenclature of Organic Chemistry:

Numbers as low as possible are given to double and triple bonds as a set, even though this may at times give "-yne" a lower number than "-ene". If a choice remains, preference for low locants is given to the double bonds. Only the lower locant for a multiple bond is cited except when the numerical difference between the two locants is greater than one, in which case the higher-numbered locant is cited in parentheses (see R-0.1.4)

Numbering from the right side of the molecule as seen in the question figure will assign the locants (2,4) to the double and triple bonds respectively. On the other hand, following a similar numbering process from the left side of the molecule as seen in the diagram will yield (3,1) as the locants for the double and triple bonds respectively.

Now, as per IUPAC Recommendations 1:

P-14.3.5 Lowest set of locants

The lowest set of locants is defined as the set that, when compared term by term with other locant sets, each cited in order of increasing value, has the lowest term at the first point of difference; for example, the locant set ‘2,3,5,8’ is lower than ‘3,4,6,8’ and ‘2,4,5,7’.

Therefore, since (1,3) is a lower set of locants as compared to (2,4) as per the above definition, the IUPAC name of the compound should be pent-3-en-1-yne

Note that this molecule can also exhibit stereoisomerism by the virtue of it's double bond, so depending on whether the hydrogens are cis or trans to each other, the complete name of the molecule could be (3 Z)-pent-3-en-1-yne or (3​E)-pent-3-en-1-yne, respectively.

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    $\begingroup$ Note that this is a slightly outdated version of the IUPAC Recommendations (from 1993). The newest version is from 2013: pubs.rsc.org/en/Content/eBook/978-0-85404-182-4 Anyway, the corresponding rule is not changed very much, so the preferred IUPAC name is essentially the same. But the stereodescriptors should be written as (3Z)- or (3E)-. $\endgroup$ – orthocresol May 10 at 15:05
  • $\begingroup$ @orthocresol Ok thanks. Edited accordingly $\endgroup$ – Yusuf Hasan May 10 at 15:09

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