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

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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.

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Updated on January 04, 2023

Comments

  • user57048
    user57048 10 months

    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?

    • Zhe
      Zhe almost 6 years
      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.
    • user57048
      user57048 almost 6 years
      @Zhe Sois my answer correct
    • Tan Yong Boon
      Tan Yong Boon almost 6 years
      Unless I'm mistaken, acetylene should be more acidic than a primary amine.
  • Pole_Star
    Pole_Star almost 6 years
    I think everybody knows that
  • Zhe
    Zhe almost 6 years
    @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...
  • Sarvesh Sontakke
    Sarvesh Sontakke almost 6 years
    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.
  • Pole_Star
    Pole_Star almost 6 years
    Yep but certainly this is not an answer with definite reasoning.
  • Sarvesh Sontakke
    Sarvesh Sontakke almost 6 years
    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