[Big-Daddy]

Is an -OH group more likely to deprotonate in solution if it is next to a C=O double bond (i.e. carboxylic acid group) or if it is on its own at the end of an alkyl chain? And is it true that, overall, -OH groups are more likely to deprotonate than -SH groups?

[Hunter2]

A COO^- get better mesomer stabilized, so the hydrogen is easy to seperate.

[Babcock_Hall]

If you know the pK_a values of the compounds in question, then you have your answer already.  There are many compilations of these values in textbooks and on the web.

[Big-Daddy]

If you know the pK_a values of the compounds in question, then you have your answer already.  There are many compilations of these values in textbooks and on the web.

Well I don't. I know only the functional groups I'm considering: -COOH versus -OH versus -SH. I'd like to know the hierarchy. Hunter2 said -COOH was the best at losing its proton; would you agree?

[Dan]

Search wikipedia for acetic acid, ethanol and ethanethiol - pKa data is available for all 3.

We are not here to do your google searches for you.

[Big-Daddy]

Search wikipedia for acetic acid, ethanol and ethanethiol - pKa data is available for all 3.

We are not here to do your google searches for you.

Acetic acid: 4.76
Ethanol: 15.9
Ethanethiol: 10.6

(The numbers are Wikipedia's pKa values for the 3 compounds you suggested.)

But does that necessarily mean that the order is -COOH losing its proton most easily, followed by -SH, followed by -OH?

Also, I heard previously (from a teacher at the 16-year-old level) that if asked to find which atom in an organic molecule had its proton removed in solution, it would more likely be an N atom than an O atom (so the order becomes, from first proton to be lost to last proton in solution to be lost out of these: -N with N substituted by few C atoms; -N with N substituted by only 1 H atom; -COOH group; -SH group; -OH group). Can you confirm this? I wouldn't really know what pKa values to search for to figure this out.

[sjb]

Search wikipedia for acetic acid, ethanol and ethanethiol - pKa data is available for all 3.

We are not here to do your google searches for you.

Acetic acid: 4.76
Ethanol: 15.9
Ethanethiol: 10.6

(The numbers are Wikipedia's pKa values for the 3 compounds you suggested.)

But does that necessarily mean that the order is -COOH losing its proton most easily, followed by -SH, followed by -OH?

Also, I heard previously (from a teacher at the 16-year-old level) that if asked to find which atom in an organic molecule had its proton removed in solution, it would more likely be an N atom than an O atom (so the order becomes, from first proton to be lost to last proton in solution to be lost out of these: -N with N substituted by few C atoms; -N with N substituted by only 1 H atom; -COOH group; -SH group; -OH group). Can you confirm this? I wouldn't really know what pKa values to search for to figure this out.

What is the definition of pKa

[Babcock_Hall]

What you were apparently taught is false, if I understand it correctly.  Consider the amino acid glycine.  There are two pK_a values, 2.34 and 9.60.  The first dissociation (pK_a = 2.34) is the loss of the carboxylic acid proton, and the second dissociation is the loss of the proton on the ammonium group.