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We were asked in class which have the lower $\mathrm{p}K_\mathrm{a}$ values: terminal alkynes or non-terminal alkynes. The teacher told that terminal alkynes have the lowest $\mathrm{p}K_\mathrm{a}$ values without really explaining why.

Please provide some explanation on why do terminal alkynes have lower $\mathrm{p}K_\mathrm{a}$ values than non-terminal alkynes?

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    $\begingroup$ It has to do with hybridisation of the resulting anion, see chemistry.stackexchange.com/q/32341/16683 $\endgroup$ Commented Apr 17, 2021 at 15:10
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    $\begingroup$ This post may be helpful. chemistry.stackexchange.com/questions/124896/… $\endgroup$
    – user55119
    Commented Apr 17, 2021 at 15:25
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    $\begingroup$ Maybe I am not understanding your question, but non-terminal alkynes don't have a proton that can be removed so which pKa are you considering? $\endgroup$
    – S R Maiti
    Commented Apr 17, 2021 at 16:01
  • $\begingroup$ @ShoubhikRMaiti There's no such thing as proton that can't be removed. $\endgroup$
    – Mithoron
    Commented Apr 17, 2021 at 16:31
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    $\begingroup$ @Mithoron Yes of course, I was just confused because those protons would not usually be referred to as alkyne protons obviously. So the question basically boils down to why alkyne protons are more acidic than alkene and alkane protons. $\endgroup$
    – S R Maiti
    Commented Apr 17, 2021 at 22:04

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$\mathrm{p}K_\mathrm{a}$ is an indicator of acidity constant and it is defined as the negative decimal logarithm of the constant itself: $\mathrm{p}K_\mathrm{a} = -\lg K_\mathrm{a}.$ So, (non-formal) the higher is the dissociation, the lower is $\mathrm{p}K_\mathrm{a}.$

Terminal alkynes have general formula $\ce{R-C#CH},$ where $\ce{R}$ is a radical. The hydrogen atom which is attached to sp-hydridised carbon atom can easily dissociate from it: $\ce{R-C#C- + H+}.$ The resulting carbanion is stable enough to favour the dissociation, therefore the lower is the $\mathrm{p}K_\mathrm{a}.$ On the contrary, in non-terminal alkynes dissociation is not favourable due to non-stable resulting carbanion.

As a result, the $\mathrm{p}K_\mathrm{a}$ of terminal alkynes is lower in comparison with that of non-terminal alkynes.

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  • $\begingroup$ Non-terminal alkynes (aka internal alkynes) don't even have a hydrogen atom to potentially lose! What could possibly be acidic about them? $\endgroup$
    – Curt F.
    Commented Apr 21, 2021 at 17:41

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