# Which molecule is the most acidic? [closed]

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?

• 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. – Karsten Theis Mar 20 '19 at 3:02
• It would all right, but it is not there. – Ivan Neretin Mar 20 '19 at 5:21
• "A" is a carboxylic acid. While all the other compounds can give away a proton, do not lose sight of that fact. – TAR86 Mar 20 '19 at 6:13

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.