Most pKa values for those compounds will really be pKas of the conjugate acid, which you can convert to pKb. Just be careful and sanity check the numbers.

There are a number of good compilations of pKa values in the literature. In what respect to amidines, try with:

Janusz Oszczapowicz and Jolanta Jaroszewska-Manaj

Amidines. Part 31. pKa values of N1,N1-dialkyl-N2-phenylformamidines in water–ethanol solutions

J. Chem. Soc., Perkin Trans. 2, 1991, 1677-1680

E. Raczyńska, J. Oszczapowicz

Tetrahedron, Volume 41, Issue 22, 1985, Pages 5175–5179

Amidines-XVIII1: Tautomeric equilibria and pka values of N,N'-disubstituted amidines. Substituent effects

Some pKa values in DMSO of amidines can be found in:

http://www.chem.wisc.edu/areas/reich/pkatable/

Thank you. I have already found your suggestions. However, I am interested on pkb values and not on pka. I would like to avoid the conversion of pka to pkb. Do you know any available literature with pkb values?

The pKa is simply the pH at which the concentration of the undissociated acid (HA) is equal to the concentration of its conjugate base (A-). It is the log value for the dissociation constant Ka.

(The equation is pH = pKa + log([A-]/[HA]). When the concentrations are equal, the second term is equal to log(1), which is zero.)

Because every acid is also a base (and vice versa) it will have an association constant for the protonation of that base, Kb, which also has a log value, pKb.

The pKb is the pOH at which the concentration of the undissociated acid (HA) is equal to the concentration of its conjugate base (A-). (The pOH is the log value for the concentration of OH- ions, just as the pH is the log of the concentration of H+).

pKb is very simply calculated from pKa by the equation pKb = pKw - pKa. pKw, the log value for the self-ionisation constant of water is equal to 14. Therefore pKb = 14 - pKa

This is why the only pKa values for acids are listed in the literature. The pKb calculation is very easy.

Yes ok but what about pKas and pKbs which are not calculated in water but in an organic solvent like DMSO or MeCN? you don't deal with water anymore so the equation pKb = 14 - pKa cannot be applied on these cases no?

You are quite right. Sorry. Without water and its ions present, the acid dissociation constant equation would look like this:

pKa = log10 Ka

Ka = ( [H+] [A-] ) / [HA] )

While the base association constant equation would look like this:

Kb = ( [HA] / [H+] [A-] )

Therefore in an aprotic solvent:

Kb = 1 /(10 ^ pKa)

pKb = log10 Kb

(In a protic solvent, the solvent's own self-ionisation constant would have to be used instead of Kw and pKw.)

As Styliana has found pKa values for amidines in MeCN, she can calculate the pKb values using a spreadsheet. The presence of water makes the chemical equation more complicated, but the maths simpler.

Sorry about the rather messy equations!