At the recent Balticum Organicum Syntheticum conference Ivo Leito made a presentation titled How to make maximum use of the available pKa data in non-aqueous solvents? (Photo on the left)
The presentation started with how acid and base strengths, typically expressed as pKa values (acids) or pKaH values (bases), depend on solvation of the proton, as well as of the neutral and ionized forms of the acid/base. Every solvent has different solvation properties. Thus, the pKa values for the same acid/base in different solvents are also different (often dramatically different).
In principle, whenever using pKa values for predicting or rationalizing chemical processes, the pKa values determined in the same solvent should be used. In some solvents, e.g. water, DMSO or acetonitrile large bodies of pKa data exist, while in most solvents either very few pKa values are available or none at all. This leads to the frequent need of estimating pKa values in one solvent from the data in other solvent(s) (Picture on the right). An additional consideration is the (often problematic) quality of pKa data in the literature.
When estimating pKa values in one solvent based on the data in another solvent it is important to clearly define the aim. Is it needed to have the absolute pKa value or is it rather necessary to have the acidity/basicity differences (or acidity/basicity order) within a set of compounds? Perhaps the question is just “can base B deprotonate acid A in solvent S”? If absolute pKa value is needed then what accuracy is necessary? Depending on the aim, there are different possibilities of estimating pKa and pKaH values in a solvent on the basis of data in other solvents.
The presentation gave an overview to what extent such estimates can be usefully done, highlighting both successes and failures, as well as how to recognize clearly erroneous pKa data. The presentation also highlighted the IUPAC project Critical compilation of acid pKa values in polar aprotic solvents that is nearing completion. The critically evaluated pKa data of acids in dimethyl sulfoxide, acetonitrile, N,N-dimethylformamide, pyridine, acetone, propylene carbonate, tetrahydrofuran are available from Ivo Leito on request.
The presentation turned out to be highly interesting for the participants, receiving a large amount of questions, which extended well into the coffee break!
