Order of acidity of water, an alcohol, an amine, and acetylene


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> What is the order of acidity of $\ce{H2O}$, $\ce{ROH}$, $\ce{RNH2}$, $\ce{C2H2}$?
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I began by forming the conjugate base of each. The conjugate bases are as follows $\ce{-OH}$, $\ce{-RO}$, $\ce{-RNH}$, $\ce{-C2H}$.


Between stability of $\ce{-OH}$ and $\ce{-ROH}$, $\ce{-ROH}$ is more unstable. Also, in $\ce{-RNH}$, the negative charge is on nitrogen atom making it more unstable than the other two groups.


In $\ce{-C2H}$ the pi elctron cloud will face repulsion of with the negative charge on the carbon atom.


So by this logic, decreasing order of acidity should be:


$$\ce{H2O > ROH > RNH2 > C2H2}$$


But the $\mathrm{p}K\_\mathrm{a}$ values suggest the very reverse order. What is the fault in my reasoning?




According to


$$
\begin{array}{lc}
\hline
\text{Compound} & \mathrm{p}K\_\mathrm{a} \\
\hline
\text{Water} & 15.7 \\
\text{Methanol} & 16.0 \\
\text{Methylamine} & 36.0 \\
\text{Ethyne} & 25 \\
\hline
\end{array}
$$


the order of acidity is


$$\text{water} > \text{methanol} > \text{ethyne} > \text{methylamine}$$


You logic is absolutely correct, but only there is an exception in case of electronegativities of $\mathrm{sp}$ carbon and $\mathrm{sp^3}$ nirogen.


The $\mathrm{sp}$ carbon is more electronegative than $\mathrm{sp^3}$ nitrogen and hence ethyne is more acidic here. The $\mathrm{p}K\_\mathrm{a}$ values also tell you the same story.


 1

pKa values are -logKa.


Ka is the dissociation constant of the acid


Smaller the Ka value, weaker is the acid.


Therefore, pka values will go on decreasing from strong acid to weak acid.


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