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?

  • 1
    $\begingroup$ 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. $\endgroup$
    – Zhe
    Jan 18 '18 at 16:37
  • $\begingroup$ @Zhe Sois my answer correct $\endgroup$
    – user57048
    Jan 18 '18 at 16:56
  • 1
    $\begingroup$ Unless I'm mistaken, acetylene should be more acidic than a primary amine. $\endgroup$ 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.

  • 3
    $\begingroup$ I think everybody knows that $\endgroup$
    – Pole_Star
    Jan 18 '18 at 16:40
  • 2
    $\begingroup$ @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... $\endgroup$
    – Zhe
    Jan 18 '18 at 16:42
  • $\begingroup$ 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. $\endgroup$ Jan 18 '18 at 16:42
  • $\begingroup$ Yep but certainly this is not an answer with definite reasoning. $\endgroup$
    – Pole_Star
    Jan 18 '18 at 16:43
  • $\begingroup$ 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 $\endgroup$ Jan 18 '18 at 16:46

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.