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The decreasing order of acid strength of the following compounds:

The given compounds are
Water ($\ce{H2O}$)
Ammonia ($\ce{NH3}$)
Acetylene ($\ce{C2H2}$)
Triphenyl system ($\ce{Ph3CH}$)

a) $\ce{NH3}$ > $\ce{Ph3CH}$ > $\ce{C2H2}$ > $\ce{H2O}$
b) $\ce{H2O}$ > $\ce{C2H2}$ > $\ce{Ph3CH}$ > $\ce{NH3}$
c) $\ce{C2H2}$ > $\ce{H2O}$ > $\ce{Ph3CH}$ > $\ce{NH3}$
d) $\ce{Ph3CH}$ > $\ce{C2H2}$ > $\ce{H2O}$ > $\ce{NH3}$

I gave the answer to be $\ce{Ph3CH}$ > Acetylene > Water > $\ce{NH3}$. Here I have marked the triphenyl system as the most acidic because it has resonance stabilization involved and $\ce{NH3}$ as the least acidic because it has basic nature. Using these two facts I have marked the answer as above however it was wrong. What is the correct order and why?

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    $\begingroup$ The real, not the thought order is to be explained. To create the real order by comparing quantities, it is very useful to know them. C2H2 pKa is very different to H2O one, guess which direction ? Even more for (CH3)3CH. $\endgroup$
    – Poutnik
    Commented Apr 12, 2021 at 7:59
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    $\begingroup$ Think like this: Which is the stronger base? $\ce{NH2-}$ or $\ce{OH-}$? Thus order is opposite for their conjugate acid ($\ce{NH3}$ and $\ce{H2O}$ ) strengths. $\endgroup$
    – Mathew Mahindaratne
    Commented Apr 12, 2021 at 8:16

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Here, this was a simple question of checking the most acidic compound out of the four and this would have given you the answer. $\ce{H2O}$ will be most acidic as the negative charge will be formed on an oxygen compared to nitrogen and carbon in the other two cases.

However, the interesting question here is the part in the middle, what about $\ce{C2H2}$ and $\ce{Ph3CH}$? Which is more acidic?

If you take a look at the $\mathrm p K_\mathrm a$ values of the given compounds, you would notice that the values would come out to be:

\begin{array}{l|c} \hline \text{Compound} &\mathrm {p}K_\mathrm {a} \\ \hline \ce{H2O} & 15.7 \\ \ce{NH3} & 38\\ \ce{C2H2} & 24\\ \ce{Ph3CH} & 31.5 \\ \hline \end{array}

Now, this is the definitive set of values which states the order should be $\ce{H2O > C2H2 > Ph3CH > NH3}$.

Note: Next part is speculative as I do not have a solid reason for $\mathrm pK_\mathrm a$ values being so for acetylene and the triphenyl system.

A question that may arise at this point is why is this so? Why would a triphenyl system with $3$ phenyl rings be less acidic than acetylene? What about the three rings providing resonance?

This is because in reality, only one phenyl ring provides resonance at a time. Due to the size of phenyl groups, the structure is not fully planar. Another reasoning is that the negative charge lies on an $\mathrm{sp^3}$ carbon whereas in acetylene it is on an $\mathrm{sp}$ carbon.

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  • $\begingroup$ Also, lone pair in the carboanion is not exactly in a $\mathrm{p}$-orbital. $\endgroup$
    – Mathew Mahindaratne
    Commented Apr 12, 2021 at 13:08
  • $\begingroup$ It is in a $\mathrm{sp^3}$ orbital, I believe. So $\pi$-system of phenyl groups does bot participate. You already have edited and added that to the post. $\endgroup$
    – Mathew Mahindaratne
    Commented Apr 13, 2021 at 13:31
  • $\begingroup$ Why is this a Community Wiki? $\endgroup$
    – user1271772
    Commented Apr 18, 2021 at 3:21
  • $\begingroup$ @user1271772 I have asked a question in the process of making this question that was answered by the community. However, if you notice, I didn't make it a community wiki until I got an answer from the site itself. $\endgroup$
    – Safdar Faisal
    Commented Apr 18, 2021 at 12:38

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