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

What is the order of acidity of $\ce{H2O}$, $\ce{ROH}$, $\ce{RNH2}$, $\ce{C2H2}$?

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?

• If you just start with being able to rationalize the difference in acidity between ammonia and water, you're more than half way to the answer.
– Zhe
Jan 18 '18 at 16:37
• @Zhe Sois my answer correct Jan 18 '18 at 16:56
• Unless I'm mistaken, acetylene should be more acidic than a primary amine. Jan 18 '18 at 23:05

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.

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.

• I think everybody knows that Jan 18 '18 at 16:40
• @starunique2016 I wouldn't assume that. But I agree that this may not answer the OP's question. It's not clear to me what they're confused about...
– Zhe
Jan 18 '18 at 16:42
• I don't know the pka values of the given compounds, and I'm assuming that the person asking the question has got his Ka and Pka mixed up, which is why I wrote the answer. Jan 18 '18 at 16:42
• Yep but certainly this is not an answer with definite reasoning. Jan 18 '18 at 16:43
• I can't post an answer with definite reasoning, because I don't know the Pka values, and so, I can't examine the trends in them. I think the OP's reasoning behind the acidity order is right, so the only confusion is in the Pka values. I can't comment due to lack of reputation, otherwise I would've done that Jan 18 '18 at 16:46