Which hydrogen in 3-ethynyltoluene is the most acidic?


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> Arrange the three types of hydrogens in 1‐ethynyl‐3‐methylbenzene in the order from the most acidic to the least.
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> [![1‐ethynyl‐3‐methylbenzene](https://i.stack.imgur.com/zYnOw.png)](https://i.stack.imgur.com/zYnOw.png)
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From my understanding, the hydrogen on methyl would be the most acidic as the negative charge after removing the H would be in resonance.


The next one would be the the ethyne hydrogen because the negative charge after removing it would stay on a sp-carbon.
The least acidic H would be the one from the benzene ring as the negative charge would fall on a sp2-carbon.


But the answer says the correct order is: ethyne's H > methyl's H > benzene ring's H. Where am I going wrong?




Based on the papers I've found, the answer is correct.
According to Bouchoux,[1] ethynylbenzene – an analogue to the compound in question – the $\Delta G$ of deprotonation for terminal $\ce{H}$ of ethyne group is $\pu{1522.2 kJ mol-1}$ while that for ring $\ce{H}$s of benzene at *o*-, *m*-, *p*-positions are $1612, 1614,$ and $\pu{1614.1 kJ mol-1}$, respectively.


According to Koppel,[2] $\Delta G$ of methyl $\ce{H}$ for toluene is $\pu{373.7 kcal mol-1}$ (equivalent to $\pu{1564.23 kJ mol-1}$).


According to Hansch,[3] the Hammett constant for methyl substitution is $-0.07$ and $-0.17$ when it is at *m*- and *p*-position, respectively, which means that being substituted by a methyl of phenyl ring would almost not affect its proton acidity.


So, the sequence is ethyne's $\ce{H} \gt$ methyl's $\ce{H} \gt$ benzene ring's $\ce{H}$.




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**References:**


1. Bouchoux, G. Gas phase acidity of substituted benzenes. *Chem. Phys. Lett.* **2011**, *506*, 167-174. [DOI 10.1016/j.cplett.2011.03.032](https://doi.org/10.1016/j.cplett.2011.03.032)
2. Koppel, I. A.; Koppel, J.; Pihl, V.; Leito, I.; Mishima, M.; Vlasov, V. M; Yagupolskiid, L. M.; Taft, R. W. Comparison of Brønsted acidities of neutral CH acids in gas phase and dimethyl sulfoxide. *J. Chem. Soc., Perkin Trans. 2* **2000**, *6*, 1125-1133. [DOI: 10.1039/B001792M](https://doi.org/10.1039/B001792M)
3. Hansch, C.; Leo, A.; Taft, R. W. A survey of Hammett substituent constants and resonance and field parameters. *Chem. Rev.* **1991**, *91*, 165-195. [DOI 10.1021/cr00002a004](https://doi.org/10.1021/cr00002a004)


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