PKa measurement

Fixed derivatives of the two tautomeric forms of 2-hydroxypyridine 1-oxide do not give cations of similar structure which precludes the use of pKa measurements to elucidate its structure (cf. preceding... 

Therefore, a continued interest exists in the role of pKa in absorption, which often is related to its effect on lipophilicity and solubility. New methods to measure pKa values are being explored [36], e.g., using electrophoresis [37], and an instrument for high-throughput pKa measurement has recently been announced [38]. 

Most methods of pKa measurement were developed using water-soluble samples. However, many drugs are poorly soluble in water alone, and require the presence of a water-miscible co-solvent to keep them in aqueous solution. The solvent affects the pKa in two ways (i) it causes the pH scale to shift and (ii) it causes the pKa to shift. The consequence is that apparent pKa values measured in the presence of solvent are different from aqueous values. 

The semi-empirical Yasuda-Shedlovsky technique of extrapolating a series of apparent pKa values obtained in several ratios of water/solvent to obtain an aqueous value is well established [32, 33], but three or more experiments are required and this adds significantly to assay times. A method of calculating aqueous pKas for various classes of organic acids and bases from single apparent pKa values obtained in water/solvent mixtures has been reported [34], and shows promise as a means of further speeding pKa measurement. 

Indicator protonation CTAX, CHClj, H20. Apparent values of pKa measured in various mixtures. El Seoud et al., 1984... 

However, in more complicated amines, this straight correlation is violated. The bicyclic tertiary amine l-azabicyclo[4.4.4]tetradecane (22) and the acyclic tertiary amine n-Bu3N have nearly the same first IP (7.84 and 7.90 eV, respectively), but the proton affinity of the bicyclic amine is 20 kcal mol 1 lower than that of the acyclic52. On the other hand, for other bridge-head tertiary amines like l-azabicyclo[2.2.2]octane (quinuclidine, 20) and l-azabicyclo[3.3.3]undecane (manxine, 21) the expected relation between proton affinities and IP values is observed. The extraordinary properties of l-azabicyclo[4.4.4]tetradecane (22) are caused by its unusual conformation the nitrogen lone-pair is directed inward into the bicycle where protonation is not possible. In the protonated form, the strained out-conformation is adopted. This makes it the least basic known tertiary amine with purely saturated alkyl substituents. Its pKa, measured in ethanol/water, is only +0.693. Strain effects on amine basicities have been reviewed by Alder88. 

For the rapid pKa measurement with daily throughput of 96 compounds using a pressure-assisted mode with a photodiode array detector, the following modifications were required. 

In CE, temperature control is quite important (see Chapter 1), because the viscosity of the buffer employed depends on the temperature. In addition, in the case of pKa measurement, the pKa of the solute as well as the pH of the buffer is changed when the temperature is changed. In our method described earlier, the value of the applied voltage was set according to the deviation from Ohm s law, which results from the temperature rise of the solution inside the capillary (20). Gluck et al. measured the buffer temperature using Co(II), which has temperature-dependent absorbance at 495 nm... 

Figure 4 presents an example of rapid pKa measurement using a pressure-assisted system in combination with a photodiode array (PDA) detector. The migration time of DMSO (EOF marker) was measured at 220 nm, whereas the migration time of the analyte, naphazoline, was measured at 270 nm. The CE run time as well as data analysis time was drastically reduced. Consequently, this system allows the analysis of more than 96 compounds in one day. The limitation of this method is the application to drugs without UV chromophore at more than 250 nm. In some cases, it was effective to remove DMSO by evaporation under vacuum followed by the addition of methanol or acetonitrile as a neutral marker. 

UV and pKa measurements show that pyridine-3-thiol and quinoline-3-thiol occur predominantly in the zwitterion form, e.g. (133) (76AHC(S1)144). Quinolines carrying a mercapto group on the benzo ring exist in the zwitterion or the zwitterion extended quinone form to a larger extent than do the analogous hydroxy compounds (63AHC(S1)400). 

Values for Heteroatoms Evaluated from pKa Measurements of Appropriate Carboxylic Acids... 

All three arsabenzene carboxylic acids are available 119,120). pKa measurements indicate that they are all stronger acids than benzoic acid. This seems to imply that the carbon atoms of arsabenzene are slightly electron deficient relative to benzene. 

The IR spectra of 2(3H)-oxazolones exhibit amide carbonyl bands at 1780-1750 and NH absorptions at 3213 and 3124 cm-1 they therefore exist in the form (63) (56CB2825, 71CB2134). The known 2(5H)-oxazolones (64) contain two substituents at C(5) hence the position of the double bond is fixed. The thione-thiol equilibrium (65) (66) has been investigated for a series of compounds by measuring the acidity constants of the tautomers and their N- and S-alkyl derivatives (69ACS2888). It was found that the thione forms predominate to the extent of 105-107. 2-Aminooxazoles (81KGS1011) exist as such both in the solid state and in solution, as shown by pKa measurements (67BSF2040). 

The use of pure DMSO for pKa measurement has obvious advantages because of its unique properties.3 Thus its high p/Ca enables determination of acidities of very weak acids, and its large dielectric constant facilitates electrometric measurement. Nevertheless, a practical disadvantage of this system lies in the rigours associated with techniques using pure DMSO. These practical difficulties are considerably ameliorated in methods employing aqueous or alcoholic DMSO. Two such methods are available, as described below.4... 

Katritzky el al. have studied, using pKa measurements, the tautomerism of aza[10]annulenone (2-azabicyclo[4.4.1]undeca-4,6,8,10-tetraen-3-one) 68 (93H2483). Comparison of the pXa s of 68 and of model compounds 69 and 70 suggests that 68a is the dominant tautomer (pKT = -0.70).This implies a significantly diminished preference for the oxy form in compound 68 compared to 2-pyridone (pKT = 3.0). This result indicated that aza[10]-... 

It has been concluded from pKa measurements and fluorine NMR data on pentafluoro-biphenyl derivatives, for example C6F5C6H4CO2H, C6F5C6H4F etc., that the penta-fluorophenyl group inductively withdraws electrons more strongly than phenyl but much less strongly than trifluoromethyl, whilst pentafluorophenyl and pentachlorophenyl have a similar capacity for electron withdrawal [40],... 

Base-catalysed H/D exchange experiments for a series of haloforms [80] (Table 4.10) demonstrate that carbanion formation is stabilised by halogen in the order 1 > Br > Cl > F. When these results are combined with acidity measurements which show that CF3H (pKa 31) is little more acidic than methane (pKa 40) [81], we can conclude that, in these systems, fluorine attached to a carbanion centre is stabilising with respect to hydrogen but destabilising compared with the effects of other halogens. Similar conclusions can be drawn from pKa measurements of a number of halobis(trifluoromethyl) methanes [82]. 

The X-ray crystallography and EPR data, as well as pKa measurements, allow one to propose a structure in which the apical amino groups are protonated and the two coordinated amine sites are deprotonated [244]. The observed rhombic distortion results from structural disorder due to the presence of deprotonated groups in the framework. 

Compared with 1,1 -carbonyldiimidazole, COMODD is a crystalline, stable and non-hvgroscopic compound. MODD released after use of COMODD as coupling reagent is water soluble and therefore can be easily recycled. Moreover, in contrast to imidazole, MODD is acidic (pKa measured at 20°C 3.6). This characteristic can be very useful in the case of substrates sensitive to basic conditions. 

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