Comments on: Basicity of Amines And pKaH

By: James Ashenhurst

James Ashenhurst — Fri, 05 Aug 2022 06:28:58 +0000

In reply to MGBaquero. Fixed, thank you!

By: Paul

Paul — Fri, 15 May 2020 14:59:16 +0000

Just a word of caution, Jeffery. Pyridine actually has 5 protons that it can donate (3 of them are unique). In the gas-phase, it is known that the 4-position is the most acidic, and has about the same acidity as chlorobenzene. The 3-position is about 1 – 1.5 kcal/mol less acidic, and the 2-position is worse. The 3- and 4-position are much more acidic than benzene.

I don’t know if the pKa has actually been measured, but certainly it is possible to make the conjugate base anions through something like a Grignard reaction (the N might make it more difficult, but it can be done).

Anything that has an H on it is a Bronsted acid, because, given enough energy, you can always rip that proton off. It’s not a question of “is it an acid?” it’s “how strong of an acid is it?”

My old PhD adviser suggested the weakest Bronsted acid is a He nucleus.

He —> H+ + T-
(I think that’s the balanced reaction, where T is tritium)

I haven’t gone through all the nuclei to verify it.

By: MGBaquero

MGBaquero — Tue, 07 Jan 2020 13:52:30 +0000

Hi James,
Really good post. Thank you so much.

I just wanted to ask you about these sentences:
“If you see an anomalously high pKa value for an amine, it’s likely referring to the pKaH of the conjugate acid.
For instance, here’s a pKa table where the pKa of methylamine (CH3NH2) is listed as 10.63”.

From what I understood in the explanations above, that pKa value is telling me that is relatively easy to obtain CH3NH- at pH above 10.63, which defenitely is not, and that is giving as a clue that it might be the pKaH.
Now, isn’ t 10.63 an anomalously LOW pKa for an amine, instead of high? If NH3 is 38, and methylamine is related, 10.63 is a low value in comparison.
Hope my question make any sense.

Thank you in advance

By: James Ashenhurst

James Ashenhurst — Fri, 08 Nov 2019 18:39:46 +0000

In reply to Shishan. It's certainly a lot quicker than, "the pka of the conjugate acid".

By: Shishan

Shishan — Fri, 08 Nov 2019 14:06:44 +0000

I think that pKaH should be introduced into text books of chemistry.

By: James Ashenhurst

James Ashenhurst — Fri, 03 May 2019 22:10:28 +0000

In reply to George.

Diphenylamine itself is on the Evans table. 0.78
http://evans.rc.fas.harvard.edu/pdf/evans_pKa_table.pdf

By: James Ashenhurst

James Ashenhurst — Fri, 03 May 2019 22:09:24 +0000

In reply to Jeffery. Exactly. That's the point of this rant. Many sources quote the pKa of pyridine when they really mean the pKa of the conjugate acid of pyridine, a value I describe here as pKaH.

By: Jeffery

Jeffery — Tue, 24 Oct 2017 05:13:37 +0000

Then what’s the meaning of the PKa for pyridine? Pyridine can not donate any proton.

By: George

George — Sun, 25 Jun 2017 17:04:57 +0000

Very helpful analysis/explanation James. Are there any pKa/pKb tables for diphenylamines and diphenols?

By: James

James — Wed, 19 Apr 2017 21:07:32 +0000

In reply to Paul. Thanks Paul. Glad I'm not alone on this! Wikipedia does it a lot as well.