PKa , Table

A stronger acid (larger fCa) has a smaller pFCa, and a weaker acid (smaller Ka) has a larger pKa. Table 2.3 lists the p/Ca s of some common acids in order of their strength. A more comprehensive table is given in Appendix B. 

Case C, the titration of a weak acid with a weak base and vice versa, has in fact already been illustrated in Fig. 2.18 by the curves BB and B B are fully valid and for characteristic (3) the initial point is still dependent on the original concentration c however for the further main part of the curve we see a clean symmetry versus the equivalence point, which has become a true inflection point, independent of the concentration and simply determined by the mean value of pKg and pKb, i.e., (p/ia + pKh)/2 or (pifa + pifw - pKa.)/2. It also means that in the simultaneous titration of a polyvalent acid or a series of weak acids of different strength with a strong base and vice versa, (1) the stronger the acid the earlier it is titrated within the series, (2) the initial point and the final end-point of the series are still influenced by the concentration, but (3) the intermediate steps are only determined at the pH of the inflection point by the mean value of the pifas of the subsequent acids and in its steepness by the difference between these pKgs. Therefore, consultation of pKa tables provides the most suitable way of predicting the results of such simultaneous titrations. 

Values of Ka are small for weak acids and they range very widely (Table 4.1). It is common practice to quote values as the negative logarithm to the base ten, i.e. — log10 Ka, since such numbers are less cumbersome and positive when Ka < 1. The symbol for - log 10 is by convention p thus — log10 Ka becomes pKa. Table 4.1 shows some typical pKa values. 

Bruice and Schmir (3) have shown that for a series of imidazole derivatives, klm depends on the base strength of the catalyst and since pKA is an approximate measure of base strength, the value of klm should increase with increase in pKA. Table I shows that this is indeed the case. Imidazole, pKA = 7.08, has a catalytic constant eight times larger than that of benzimidazole, pKA = 5.53. Bronsted and Guggenheim (2) have obtained a linear relationship between log k/ and pKA for a series of carboxylic acids in the pKA range of 2 to 5, where kB is the carboxvlate anion basic catalytic constant for the mutarotation of glucose and Ka is the acid dissociation constant of the acid. Our results for imidazole and benzimidazole fit fairly well into the Bronsted plot. 

Problem 2.13 without reference to a pKa table, decide which compound in each pair is the stronger acid, a. NH3andH20 b. HBrandHCI c. H2S and HBr... 

Using the pKa table in Appendix A, determine whether each of the foiiowing bases is strong enough to deprotonate the three compounds listed below. Bases [1] OH [2] CHsCHg [3] NHg [4] NH3 [5] HC=C . 

Molecular height Ig/nolJ 151.17 logP [neulrall 0 45 0pH T pKa Table... 

Mdecula Foimula C13H22N403S Mdecjlar Weigh (g/molt 1314.4043 Reference logD 3 27 <3>pH l0.5 pKa Table I... 

Mdftcolar Foimula-1 C35H38CI2N804 Mdecular height (g/mol) 705.65 logP Ineulral) 4 88 pH T pKa Table... 

Bordwell pKa table Avialable at http // www.chem.wisc.edu/areas/reich/ pkatable/index.htm (accessed 11.04. 2008)... 

Web collections of data include those of H. Reich (University of Wisconsin) at http //www.chem.wisc.edu/areas/reich/pkalable/kacont.hm and D. Ripin and D. A. Evans (Harvard) at http //www2.lsdiv.harvard.edu/labs/evans/pdf/evans pKa table.pdf. 

Evans p/

Which compounds can be deprotonated by OH, so that equilibrium favors the products Refer to the pKa table in Appendix A. 

Differences in the availability of the P lone pair for a series of phosphines are likely to have profound effects on their biological chemistry. For instance, the question arises as to whether a particular phosphine is likely to become protonated in vivo. The pKa s of hydrophobic phosphines can not be measured in H2O. Consequently most of the experimental data have been obtained by titration in non-aqueous media (usually nitro-methane). Conversion to aqueous pKj values is usually carried out using a relationship between half neutralisation potentials in CH3NO2 and pKa(H20) that exists for amines. For monodentate tertiary phosphines, introduction of phenyl-substituents lowers the pKa (Table 2). There appear to have been no similar determinations of the pKa s of diphosphines. It seems likely that phenyldiphosphines (e.g. dppe) are weakly basic (pKa < 3) and therefore will not be significantly protonated at pH 7. Preliminary experimental data measured by R. Norman in our laboratory suggest that this is the case . ... 

Ripin, D. H. Evans, D. A. pK s of Inorganic and Oxo-Acids. http //ccc. chem.pitt.edu/wipl7MechOMs/evans pKa table.pdf (accessed February 16, 2015). 

What pKa value would you expect to find next to this molecule on a pKa table ... 

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