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My textbook, Organic Chemistry (6th edition) by Brown, Foote, Iverson, and Anslyn, states:

Alkynes in which the triple bond is between carbons 1 and 2 are commonly referred to as terminal alkynes.

Is not this definition incomplete? I can imagine scenarios where we have a terminal triple bond that does not reside between carbons 1 and 2 according to IUPAC rules. E.g., $\ce{CH2=CH-C#CH}$. Here, the terminal triple bond is between carbons 3 and 4 (according to IUPAC rules, doubles bonds take priority when numbering).

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You are correct about the absolute numbering of $\ce{CH2=CH-C#CH}$. By the IUPAC priority rules, the alkene has the lower absolute numbering.

However, numbering can also be relative. In your case, the relative numbering 1,2 refer to either of the following:

  • relative position from the end of the chain, where the chain end is #1
  • relative position from the functional group you care about (where that group is #1)
  • suggesting that a reaction occurs on neighboring positions in a molecule.

That being said, the definition given by Brown et al. is imprecise. A better definition is one of the following:

A terminal alkyne is an alkyne in which one (or both) of the two alkyne carbons is at the end of a chain.

A terminal alkyne is an alkyne $\ce{RC#CR}$ in which one or both $\ce{R}=\ce{H}$

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  • $\begingroup$ I like the second one better, because the first one implies there is only one alkyl functional group in the molecule; there are many polyynes, the simplest being diacetylene with a similar structure but one fewer hydrogen than vinylacetylene. $\endgroup$
    – KeithS
    Apr 30, 2013 at 0:48
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You're right; the definition in your book is simplistic. A terminal alkyne would be better defined as "an alkyne with the formula $\ce{RC_2H}$", differentiating from an internal alkyne, which would have the formula $\ce{RC_2R'}$ and the added stipulation that neither R nor R' can be a hydrogen. Equivalently stated as in Wikipedia, an internal alkyne has carbon substitutents on all acetylenic carbons, and a terminal alkyne is therefore not an internal alkyne because at least one acetylenic carbon has a hydrogen bonded to it.

The example formula you cite is vinylacetylene, systematic name butenyne or 1-buten-3-yne (here's your IUPAC nomenclature rules favoring double bonds at work; otherwise it'd be ethynylbutene or similar). It is indeed an example of a terminal alkyne, having the exploded formula (C2H3)C2H.

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  • $\begingroup$ and an internal alkyne is in the center and an external alkyne is between the end and the center. Like for example pent-2-yne or 2-pentyne is an external alkyne but pent-3-yne is an internal alkyne $\endgroup$
    – Caters
    May 23, 2015 at 23:32

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