Group pKa values

For TPs without oxo groups pKa values have been reported (77AJC2515) they markedly differ from those of [4,3-a] isomers. N-3 was determined as the position of protonation (68JHC691) from NMR analogy with the product of methiodide reaction (Section IV,C,1). 

Deprotonation at C-2 or C-5 has not been reported. Oxadiazolin-5-ones (25a) and -5-thiones (25b) are weak acids and formally undergo deprotonation at the NH group. pKa values (aqueous DMF) in the range 6.6-7.8 have been quoted for oxadiazolinones (25a R = aryl or hetaryl) and a slightly higher pK (water) of 7.93 has been quoted for 2-methyl-A2-l,3,4-oxadiazolin-5-one. Oxadiazolinethiones (25b R = H, Me or Ph) are stronger acids, having pKa values (water) of 3.85, 4.45 and 4.27 respectively. 

The simple alkanoic acids are included as a point of reference. It is evident that all the acid series yield similar results. If polar groups are within a distance of 1 to 3 C—C bonds from an acidic functional group, pKa values are lowered by 0.1-2.0 pA units. 

Raman spectroscopy allowing the determination of global variation of the ionization of the membrane as a function of pH was studied. Experimental and theoretical Pka value was found to be 5.2 from which the average number of five carboxylic groups/graft was determined [145]. 

Other complexes of 4c, 10, and 11 show linear plots with slopes close to unity in the a pH range 6.5-8.5. These linear plots also seem to represent the ionization of hydroxyl groups, but their pKa values must be higher than 8.5. Unfortunately, it is difficult to examine higher pH s due to precipitation of the metal hydroxide. 

The following pKa values have been measured. Rxplain why a hydroxyl group in the para position decreases the acidity while a hydroxyl group in the meta position increases the acidity. 

This expression has the same form as Eqs. (6-81) and (6-84). Here, of course, the substrate is not protonated to an appreciable extent. With other suitable experiments and some luck, the steady-state situation can be distinguished from substrate titration. For example, is the pKa value deduced under the assumption of a titration reasonable for the molecule in question That is, is it reasonable for one of the functional groups of A to have a pKa near the pH of the bend Can one detect significant amounts of two species, AH+ and A, at a pH near the presumed pKal Can one modify the substrate, eliminating the site of protonation If so, and if a titration mechanism operates, then (as the reader should show) the pH profile should become linear. Obviously, were substrate titration and a steady-state intermediate situation to coexist in the same system, a more complicated but not intractable situation would result. 

Charton s at values for SOPh and S02Ph are based on the pKa values for phenylsulfinyl- and phenylsulfonyl-acetic acids137, which we have already discussed in connection with the transmission of electronic effects by SO and S02 (Section III.C). These groups have also been the subject of a detailed study by Hogeveen and Montanari172, who measured the pKa values of some 3-phenylthio-, 3-phenylsulfinyl- and... 

In this chapter it is clearly impossible to do more than sample the extensive literature on the carbon acidity of sulfinyl and sulfonyl compounds, as it illuminates the electronic effects of these groups, particularly in connection with linear free-energy relationships. There are three main areas to cover first, as already indicated, equilibrium acidities (pKa values) second, the kinetics of ionization, usually studied through hydrogen isotopic exchange and finally, the kinetics of other reactions proceeding via carbanionic intermediates. 

Interestingly, comparison of the values of pAHB and the acidity constants pKa in a series of the same family of compounds, such as carbonyl compounds, amines, pyridines and sulphoxides, shows that a good correlation exists between p/CHB and pgiving straight lines in each series of compounds with parallel slopes. This enables one to calculate the difference of the several pKa values at the same p/CHB value, and vice versa. Thus, at p= 0, p/CHB values of various functional groups were determined and are shown in Table 13. 

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