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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1$\begingroup$ Related: In nomenclature, does double or triple bond have higher priority?, Why is it -en-yne and not yn-ene? What is the explanation for this IUPAC specification?, Why doesn't the double bond get preference for suffix in IUPAC naming of an hydrocarbon when both double bond and triple bond are present together?, … $\endgroup$– user7951Commented Aug 1, 2017 at 5:33
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1$\begingroup$ … Why do these two IUPAC naming examples not begin numbering the chain closest to the alkyne?, What is the order of nomenclature priority among an alkyl side group, a double bond, and a triple bond? $\endgroup$– user7951Commented Aug 1, 2017 at 5:34
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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.
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 (3Ξ)-pent-3-en-1-yne.
Of course, if the configuration is specified, then the (E)/(Z) labels should be used instead.
answered Aug 1, 2017 at 3:54
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$\begingroup$ So basically the "smallest sum of locants" again?! $\endgroup$– Martin - マーチン ♦Commented Aug 1, 2017 at 5:00
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1$\begingroup$ @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. $\endgroup$ Commented Aug 1, 2017 at 6:39
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$\begingroup$ 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? $\endgroup$ Commented Aug 1, 2017 at 7:21
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4$\begingroup$ I should not have tried to joke about this... $\endgroup$– Martin - マーチン ♦Commented Aug 1, 2017 at 7:50
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1$\begingroup$ @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. $\endgroup$ Commented Aug 1, 2017 at 11:37