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Wouldn't the most acidic be the H connected to F? However, my professor says it is answer A, the alpha Hydrogen in carboxylic acid.

Edit: Sorry, I did not phrase my question right. What I meant to say is I thought that the H connected to the same carbon as F would be the most acidic. Also, my professor just sent the answer and did not say why A is the most acidic. I just assumed she meant the alpha H in the carboxylic acid, or is that only for ketone and aldehyde?

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    $\begingroup$ I'm having trouble seeing an H connected to F. Which molecule are you referring to? In A, it is the hydrogen connected to the oxygen that would dissociate, not the alpha hydrogen. $\endgroup$ – Karsten Theis Mar 20 '19 at 3:02
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    $\begingroup$ It would all right, but it is not there. $\endgroup$ – Ivan Neretin Mar 20 '19 at 5:21
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    $\begingroup$ "A" is a carboxylic acid. While all the other compounds can give away a proton, do not lose sight of that fact. $\endgroup$ – TAR86 Mar 20 '19 at 6:13
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You can find the list of $\mathrm{p}K_\mathrm{a}$ values of some common organic compounds at Master Organic chemistry website.

The higher the value of the dissociation constant of acid $K_\mathrm{a}$, the smaller is the $\mathrm{p}K_\mathrm{a}$ value and the more acidic the compound is. To explain the $\mathrm{p}K_\mathrm{a}$ values we check for the stability of conjugate base which is obtained by removing the most acidic hydrogen and here in this case the conjugate base of the carboxylic acid will be most stable due to resonance.

Also in option B there is no hydrogen directly attached to fluorine. Option A is carboxylic acid, B is alcohol, C is ketone, D is ester and E is alkyne. You can find in the link that out of all these the $\mathrm{p}K_\mathrm{a}$ values of carboxylic acid is the least.

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  • $\begingroup$ Even though there is nothing wrong with this answer, I suspect that OP asked about the acidity determination from the presented structures in the absence of external data sources. $\endgroup$ – andselisk Mar 20 '19 at 7:40
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The hydrogens attached to carbon atoms have the least chance to be removed as there is negligible electronegativity difference between carbon and hydrogen however the hydrogen atoms attached to more electronegative species can easily be removed due to the polar nature of the bond . Acidic nature of organic compounds can be commented on the basis of stability of the resulting conjugate base . If the conjugate base thus formed is stable then the compound is more acidic or the hydrogen is more acidic and vice versa. For option A we can see there is only one acidic hydrogen ie the hydrogen connected to the oxygen atom. It's resulting conjugate base is also stable as the minus charge generated on oxygen atom is in resonance with the pie bond thus it is the most stable conjugate base among all other conjugate bases as no other compound ,containing acidic hydrogen has resonance in it .

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