[NewmanProj]

One of the most common sets of tabulated values that vary from source to source are the pKa values of the simplest of amines. Let's use ammonia as an example, and confine the pKa value to water.

Evans   38
Reich    ?
Bruice  40

For practical purposes, the difference between the values of 38 and 40 for ammonia are negligible, however for accurate reporting purposes they are not.

Q1)  Is anyone aware of an internet resource or publication that provides accurate pKa values for NH3, CH3NH2, i-PrNH ?

Q2)  Is anyone aware of a mathematical means of interconverting pKa and pKb values for NH3 or CH3NH2 in water ?  Using pKa + pKb = pKw does not work.

I'm beginning to suspect the variability in numerical data for essential amines may have something to do with converting pKb to pKa.

Thanks!

[Borek]

Is anyone aware of a mathematical means of interconverting pKa and pKb values for NH3 or CH3NH2 in water ?  Using pKa + pKb = pKw does not work.

Unless I am misunderstanding your question, nothing surprising here. pKa + pKb = pKw is a property of an acid and its conjugate base, so it will work for NH₃ and NH₄⁺, but not for NH₃ and NH₃.

Besides, you can't really speak about NH₃ being an acid in water solutions, as water itself is a much stronger acid (leveling effect).

[NewmanProj]

Borek, thanks for your comment. You're correct. It's not practical to talk about NH3 as an acid in water... unless you're teaching undergraduate organic chemistry and must standardize all your pKa values to water so undergrads can learn acid-base chemistry. Example

CH3OH  +  NH2(-)  <==>  CH3O(-)  +  NH3

Use pKa values to calculate the equilibrium constant. The student here needs reliable pKa values for CH3OH and NH3, yes?

This topic has been discussed here before (2 years ago if I remember correctly), and there was no consensus on a cogent explanation regarding numerical interconversion.

[Borek]

Honestly, I don't see why we should lie to undergrads. If anything, it is better to tell them "it's complicated, but the rule of thumb is..." than to pretend we have some "real" numbers that can be used to predict "real" reality.

CH3OH  +  NH2(-)  <==>  CH3O(-)  +  NH3

Use pKa values to calculate the equilibrium constant. The student here needs reliable pKa values for CH3OH and NH3, yes?

First, this reaction doesn't call for water, so don't put it in as it only adds confusion. Second, there is orders of magnitude difference in acidities, and that's enough to predict the equilibrium lying far to one of the sides. For most practical purposes it should be enough IMHO.

[NewmanProj]

I don't lie to undergrads, however it is my responsibility to educate them properly by showing them where the data comes from. These values do help explain phenomena in organic chemistry.

What's more reactive? An acid chloride (conj acid pKa -8) or an amide (conj acid pKa 36-40). The pKa information is necessary, and standardization is necessary, and providing student with some inkling of where the data comes from is necessary. There are other arguments, however I prefer not to switch arguments when it's convenient.

It appears that crossing solvents requires some interesting mathematics: http://lisa.chem.ut.ee/~ivo/HA_UT/

And the mathematics create errors as high as 1 log unit (minimum). In the past 30 years I've seen the values of pKa data for NH3 vary as much as 5 log units! Not trivial.

The more I think about this problem, the more I want to turn it into an O-Chem lab assignment... once I understand it myself.

Thank you for your kind comments.