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This question already has an answer here:

Is the IUPAC name of $\ce{CH3CH=CH-C#CH}$ pent-3-en-1-yne or pent-2-en-4-yne?

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marked as duplicate by andselisk, Wildcat, Todd Minehardt, Pritt Balagopal, Jon Custer Aug 1 '17 at 13:08

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Subsection P-44.4.1.10.1 in Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) reads

For the endings 'ene' and 'yne' lower locants are assigned first to the endings as a set without regard to type and then to 'ene' endings.

Since the locant set "1,3" is lower than the locant set "2,4" (see here for a discussion of what that means), the appropriate choice of locants for the above compound leads to the name pent-3-en-1-yne.

pent-3-en-1-yne


As an aside, the configuration of the double bond, if unspecified, should be labelled with the symbol Ξ (P-91.2.1.2.1).

When the configuration is not known or must remain unspecified for lack of configurational homogeneity, the italicized symbol 'ξ' or 'Ξ' is used, preceded by the required locant. The symbol 'ξ' (small Greek letter 'xi') replaces noncapitalized CIP stereodescriptors such as 'r', 's', 'm', 'p'. The symbol 'Ξ' (capital Greek letter 'xi') replaces capitalized CIP stereodescriptors such as 'R', 'S', 'M', 'P', and nonCIP stereodescriptors 'E', 'Z', 'seqCis', and 'seqTrans'.

The preferred IUPAC name (PIN) for the compound with unspecified double bond geometry is therefore (3​Ξ)-pent-3-en-1-yne.

Of course, if the configuration is specified, then the (E)/(Z) labels should be used instead.

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  • $\begingroup$ So basically the "smallest sum of locants" again?! $\endgroup$ – Martin - マーチン Aug 1 '17 at 5:00
  • $\begingroup$ @Martin-マーチン Please note that the widely-cited "sum of locants" rule is incorrect. See: IUPAC nomenclature: “Smallest sum of locants”? and the answers therein. $\endgroup$ – orthocresol Aug 1 '17 at 6:39
  • $\begingroup$ Alkenes are preferred for lower numbering over alkynes then there is sum locant rule. So which rule is overtaken by the other? Is it always applied when the same(grou ps) are present together in a compound? $\endgroup$ – Abdul Khan Aug 1 '17 at 7:21
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    $\begingroup$ I should not have tried to joke about this... $\endgroup$ – Martin - マーチン Aug 1 '17 at 7:50
  • $\begingroup$ @AbdulKhan as the passage I quoted indicates: first you seek to minimise the locant set. Note that it is not the sum of locants that you minimise; see the link in my comment above. Only if the two possible locant sets are equivalent, do you assign a lower locant to the alkene. $\endgroup$ – orthocresol Aug 1 '17 at 11:37

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