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Subsection P- 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.


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

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.

  • $\begingroup$ So basically the "smallest sum of locants" again?! $\endgroup$ Aug 1, 2017 at 5:00
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    $\begingroup$ @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. $\endgroup$ Aug 1, 2017 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? $\endgroup$ Aug 1, 2017 at 7:21
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    $\begingroup$ I should not have tried to joke about this... $\endgroup$ Aug 1, 2017 at 7:50
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    $\begingroup$ @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. $\endgroup$ Aug 1, 2017 at 11:37

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