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Here's a picture from Organic Chemistry by Solomons, Fryhle & Snyder :

enter image description here Why is it that ammonia is less acidic than terminal alkynes even though water, and even alcohols, are more acidic than them?

Water and ammonia seem to be very similar to me:

  • Electronegativity difference of oxygen and nitrogen is only 0.4 (Pauling scale)
  • Both are very similar and differ by only number of hydrogen atoms.
  • Both are $\ce{sp^3}$ hybridized
  • Oxygen and nitrogen atoms have similar size

However, the difference in acidic strength is huge. If my understanding of $\mathrm{p}K_\mathrm{a}$ values is correct, water is nearly $10^{10}$ times more acidic than terminal alkynes, whereas ammonia is less acidic than terminal alkynes by a factor of $10^{13}$!

Shouldn't the list be like: water > alcohols > ammonia > terminal alkynes > terminal alkenes > alkanes or water > ammonia > alcohols > amines > terminal alkynes > terminal alkenes > alkanes ?

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It is the alkyne that is not in line. In the alkynes, the carbon is sp hybridized and this gives it much more s character. Because of this, the electrons are closer to the carbon nucleus and the $\ce{C-H}$ bond is slightly polarized. The carbon has some δ - and the hydrogen δ +.

Therefore the terminal alkynes are more acidic and have a lower $\mathrm{p}K_\mathrm{a}$ than ammonia.

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  • $\begingroup$ Ok ... so, I googled the $pK_a$ of HF and it's $3.17$. Seems like a change of around $10$ $pK_a$ is normal when moving in the second period. So, my next question would be — how can we, theoretically, assign acidity order for terminal alkenes and alkynes? Is there any data on the electronegativity (if that's the right term) of carbon in $sp^2$ and $sp$ hybridisation? I read a question related to this on Quora which uses %s character to find the E.N. of $sp$ carbon, but the reasoning is not very convincing. $\endgroup$ – Anurag B. Jul 20 '18 at 12:02
  • $\begingroup$ Please ask this as a new question, explaining that it follows up from this one. $\endgroup$ – Oscar Lanzi Jul 20 '18 at 12:47

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