ELECTROCHEMICAL DETERMINATION OF THE PKA OF WEAK ACIDS IN N,N-DIMETHYLFORMAMIDE

The electroreduction of NH-protic alpha-bromo amides in DMF generates an enolate-type base which undergoes a fast proton transfer from the parent compound (self-protonation), affording the corresponding reduced amide together with the conjugate base of the bromo amide. When an acid weaker than the bromo amide is added to the solution, a current increase in a potential region more negative than the main voltammetric reduction peak is observed under suitable conditions. The voltammetric pattern is in agreement with an unfavored protonation of the conjugate base of the starting compound by the added proton donor with regeneration of the electroactive bromo amide. The theoretical analysis of this reduction sequence has been carried out, and the voltammetric profiles have been simulated. Comparison of the experimental and simulated voltammetries led to the determination of the acidity difference, DELTA-pK(a), between the a-bromo amide and the added acid. For each alpha-bromo amide it was possible to obtain DELTA-pK(a) data ranging from 1.4 to 4.2. The use of a-bromo amides of different acidity with the same exogenous acids provided the link between the different sets of relative acidities. In this way, using six alpha-bromo amides, a relative acidity scale encompassing an overall pK(a) variation in DMF of about 10 units could be established. The relative scale was then anchored to the low pK(a) scale in DMF through both the determination of the acidity of selected acids and using a correlation between literature pK(a) data obtained in both DMF and DMSO. The application of this original electrochemical mechanism provided absolute pK(a) data in DMF ranging from about 16 to 26, i.e., a pK(a) region that is practically unexplored in this solvent.