I know that ethyne is more acidic than ethane because of more s character. For water and ammonia, I used the fact that oxygen is more electronegative than nitrogen and will be able to stabilize the charge better. So water will be more acidic than ammonia.Now how do I compare ammonia and ethyne?

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    $\begingroup$ sodamide is used to deprotonate alkynes $\endgroup$ – Waylander Apr 12 '19 at 6:57

The order of acidic nature would be: water > ethyne > ammonia > ethane.

Since oxygen has highest electronegativity amongst the atoms of the compounds, so its hydrogen will be most acidic and hence will be best acid among them.

Ethyne contains $\mathrm{sp}$-carbon and as a result the electronegativity of the carbon in ethyne is more than $\mathrm{sp^3}$ due to increased $\mathrm{s}$-character (50%) in the bonds between the carbon atoms.

The electronegativity of the carbon increases to a great extent when and turns out to be 3.2, which is greater than the electronegativity of nitrogen which is 3.04. Hence the hydrogen of ethyne is more acidic than that of $\ce{NH3}.$

Ethane does not possess any acidic hydrogen hence it is least acidic.

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  • $\begingroup$ Ok thanks. And what will be the order if it is ethene instead of ethyne? $\endgroup$ – Riya Apr 12 '19 at 15:49
  • $\begingroup$ If it is ethene in place of ethane then $NH_3$ will be a better acid than ethene as in ethene the electronegativity of $sp^2$ carbon is 2.74 (33%s character)which is less than that of nitrogen $\endgroup$ – Harsh Wasnik Apr 12 '19 at 16:04

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