The preferred IUPAC name for the compound above is hept-1-en-6-yne. I understand that the numbers are chosen in order to give the alkene the lower locant, as explained in Should double or triple bonds have preference for low locants in IUPAC nomenclature?.

However, I'm wondering why the name has the form -en-yne instead of -yn-ene. In other words, why is it not "hept-6-yn-1-ene" instead?

Furthermore, if this is an IUPAC specification, is there any rationale for why en-yne should be preferred to yn-ene?

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    $\begingroup$ Anecdotally, some nomenclature decisions were made to make names easier to pronounce. enyne is easier than ynene. $\endgroup$
    – Ben Norris
    Apr 3 '15 at 21:08
  • $\begingroup$ The name is given according to alphabetical order like in the cases of prefixes, bromo preceds chloro, similarly, -ene preceds -yne. (E>Y) . $\endgroup$ Nov 5 '19 at 5:56

The current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) reads as follows:

P- The presence of one or more double or triple bonds in an otherwise saturated parent hydride (…) is denoted by changing the ending ‘ane’ of the name of a saturated parent hydride to ‘ene’ or ‘yne’. Locants as low as possible are given to multiple bonds as a set, even though this may at times give ‘yne’ endings lower locants than ‘ene’ endings. If a choice remains, preference for low locants is given to the double bonds. In names, the ending ‘ene’ always precedes ‘yne’, with elision of the final letter ‘e’ in ‘ene’. 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 locant is enclosed in parentheses.

Since, in names, the ending ‘ene’ always precedes ‘yne’ (with elision of the final letter ‘e’ in ‘ene’), the given compound is a ‘hept-x-en-y-yne’.

Since the lowest sets of locants in ‘hept-1-en-6-yne’ and ‘hept-6-en-1-yne’ are equivalent (1, 6), preference for low locants is given to the double bond. Therefore, locant 1 is assigned to the double bond and locant 6 is assigned to the triple bond.

Thus, the correct name is hept-1-en-6-yne.


  • $\begingroup$ thank's for your thoughtful and helpful answer. Certainly, I understand that this is the IUPAC Blue Book states that this would be the correct ordering, but I'm mostly interested in why that is the case. Specifically, you've highlighted a line "...The ending 'ene' always precedes 'yne'..." but why is this the case? $\endgroup$ Apr 3 '15 at 20:21
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    $\begingroup$ There has to be a rule and this happens to be it. Some choices are necessary but arbitrary: e.g. which side of the road to drive on. All countries have made a choice but it is a pity that they did not all make the same one. $\endgroup$
    – badjohn
    Sep 30 '18 at 10:25

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