Extension of the Self-Consistent Spectrophotometric Basicity Scale in Acetonitrile to a Full Span of 28 pK_a Units:  Unification of Different Basicity Scales

Abstract

The earlier compiled self-consistent spectrophotometric basicity scale in acetonitrile (AN) was expanded to range from 3.8 to 32.0 pK(a) units, that is 28 orders of magnitude. Altogether 54 new relative basicity measurements (DeltapK(a) measurements) were carried out and 37 new compounds were introduced to the scale (it now includes altogether 89 bases). The relative basicity of any two bases in the scale can be obtained by combining at least two independent sets of measurements. Multiple overlapping measurements make the results more reliable. The overall consistency (as defined earlier) of the measurements is s = 0.03 pK(a) units. Thorough analysis of all of our experimental data (DeltapK(a) values of this and earlier works) and experimental pK(a) data in AN available in the literature (works from the groups of Coetzee and Padmanabhan, Kolthoff and Chantooni, Jr., the Schwesinger group, Bren' et al. and some others, altogether 19 papers) was carried out. On the basis of this analysis the anchor point of the scale-pyridine-was shifted upward by 0.20 pK(a) units thereby also revising the absolute pK(a) values of all the bases on the scale. This way very good agreement between our relative data and the absolute pK(a) values of the abovementioned authors was obtained. The revised basicity scale was interconnected with the earlier published self-consistent acidity scale by DeltapK(a) measurements between acids and bases. The rms deviation between the directly measured DeltapK(a) values and the absolute pK(a) values of the compounds was 0.10 pK(a) units.

Nucleophilicities and Carbon Basicities of Pyridines

“…With pK aH values of 7.61 (Ph 3 P) and 9.96 (MePh 2 P) tertiary phosphanes are considerably weaker Brønsted bases in acetonitrile than pyridine 1 b (pK aH = 12.53) and DMAP (1 f; pK aH = 17.95). [13] These pK aH values do not rationalize the observation that DMAP (1 f) and phosphanes, but not the unsubstituted pyridine 1 b, are efficient catalysts for BaylisHillman and related reactions. [25] Our finding that the wellknown organocatalyst DMAP (1 f) and triarylphosphanes or trialkylphosphanes have comparable nucleophilicity parameters N, and even more important, comparable carbon basicities [26] (Scheme 5) suggests to employ these quantities for the systematic analysis of nucleophilic organocatalysts.…”

“…While the correlation between pyridine nucleophilicities N in water and pK aH A C H T U N G T R E N N U N G (H 2 O) [12] is only of moderate quality (R 2 = 0.901, Figure 7, top), a fairly good correlation is observed between the N parameters for pyridines in CH 2 Cl 2 and their pK aH constants in acetonitrile [13] (R 2 = 0.987, Figure 7, bottom), which can be assumed to mimic the situation in CH 2 Cl 2 better than the pK aH constants determined in water.…”

“…In previous work, we found that the rates of the reactions of carbocations with uncharged nucleophiles are only slightly affected by solvent polarity, since charges are neither cre- [12]) for the 4-X-substituted pyridines 1 a-g. Bottom: Correlation of N (in CH 2 Cl 2 ) with pK aH (in CH 3 CN, from reference [13]) for the 4-Xsubstituted pyridines 1 a-g. Figure 6. Correlation of N with s p (from reference [11]) for 4-X-substituted pyridines in CH 2 Cl 2 (N = À4.49s p + 12.8, R 2 = 0.9752) and water (N = À2.76s p + 10.9, R 2 = 0.8860).…”

Transferability of the Electrospray Ionization Efficiency Scale between Different Instruments

“…As a first stage, the individual ionization efficiency scales were compiled using the same approach as in reference [23], i.e., the logIE values were assigned to the compounds in such a way as to minimize the sum of squared differences SS between the differences of the directly measured logRIE(M i ,M j ) values and the assigned logIE(M i ) and logIE(M j ) values according to Equation 2. Tetrapropylammonium was used as temporary reference compound (M i ) at this stage.…”

Piperazine-Containing Polymer Brush Layer as Supported Base Catalyst in a Glass Microreactor

“…A second-order rate constant of 0.20±0.01/(s M) was calculated. TBD (pK a in acetonitrile=26.03) [32] is a stronger basic organocatalyst than piperazine (pK a in acetonitrile=18.69) [33]. Consequently, when the same reaction was carried out at 65°C and under ambient pressure conditions as reported for the TBD wall-coated glass microreactor [15], the reaction proceeded about ten times slower with a second-order rate constant of 1.18±0.01× 10 −2 /(s M) against 0.10±0.01/(s M), reported in the literature for the TBD-catalyzed reaction [15].…”