P//a values

When, in a column headed M.p., a value is given in parenthesis, it indicates that the compound is liquid at room temperature and that the value given is consequently the boiling-point. Conversely in a column headed B.p., values given in parenthesis are those of the melting-point. A blank space indicates that the compound has not apparently been recorded. 

It is presumptuous to report that a substance is not hydrated simply because there are no drifts in the readings obtained during potentio-metric measurements or because the experimentally determined p a value is not very different from the predicted value. A small amount of hydration may cause only a small difference in the ionization constant and hence other tests should be applied. A number of heterocyclic compounds which have seemingly normal pvalues may well be partially hydrated. 

The listing of p/acidity from one carboxylic acid to another. For example, trifluoroacetic acid (Ka = 0.59) is 33,000 times as strong as acetic acid (Ka = 1.75 X 10 3). How can we account for such differences ... 

Pinacolone, o-(diphenylphosphino)benzoyl-coordination chemistry, 2, 401 Ping-pong reactions copper(II) complexes, 5, 717 Piperidine, /V-hydroxy-metal complexes, 2, 797 P a values azole ligands, 2, 77 Plant roots amino acids, 2, 962 carboxylic adds, 2,962 Plants... 

As observed for the p a values of pyridinocrowns [9] in methanol (see Table 6), no peak basicity of the (log/f)rei values was found for 18-membered concave pyridines [13] in ethanol. Whether a peak basicity would exist in water cannot be ascertained due to their low solubility. 

The p a values of two systems with two intra-annular COOH groups have been measured. In Cram s study (Bell et al., 1982), the macrocyclic diacid [36] and an open-chain monomeric analogue [37] had almost identical pK values (see Table 18). In contrast, Gennari s compound [38], which contains two intra-annular COOH groups and in addition two"ethyl ester groups, has a different acidity from that of the analogues [39] and [40], as shown in Table 19 (Gennari et al., 1992). But in both systems, the difference Ap a between pK i and p a2 was comparable (ApA a [36] = 1.7, Ap/La[38] = 1.5). The increase from p ai to P a2 may occur for two... 

The p a values are listed for the sodium salt. The of the strong acidic sulfonic acid group is not listed. Shinkai (1993). Shinkai et at. (1989). 

Table 27 p a values for polyazamacrocyclic phenols [64] and comparable molecules. 

In contrast to the borylation of alkane C-H bonds, the coupling of aryl halides with amines was based on a literature precedent from another group published about a decade before our initial studies. Kosugi, Kameyama and Migita published the coupling of aryl halides with tin amides." Mechanistic studies we conducted on this process led us to the perhaps obvious realization that the reaction" could be conducted with amines and a silylamide base instead of tin amides (equation 4)." Surveys of bases with similar p a values led Janis Louie to conduct reactions with alkoxide bases. Similar studies were conducted at nearly the same time by Steve Buchwald and coworkers."... 

A group with a more powerful (electron-withdrawing) inductive effect, e.g. NOa, is found to have rather more influence. Electron-withdrawal is intensified when the nitro group is in the o- or p-position, for the interaction of the unshared pair of the amino nitrogen with the delocalised it orbital system of the benzene nucleus is then enhanced. The neutral molecule is thus stabilised even further with respect to the cation, resulting in further weakening as a base. Thus the nitro-anilines are found to have related p a values ... 

Using this value of X() and the entering solids volume fraction ([Pg.452]

The basicity of the donors, which is proportional to the p/ a values, becomes a measure of the relative donor strength. Thus, diglyme (pK.d 4) is a weak donor, while Bu3PO (pKa 0) is a strong donor. 

Table 7.2 p/

Another example of the use of transition state pKa values has been provided by Pollack (1978). From the rate constants for the decarboxylation of substituted a,a-dimethylbenzoylacetic acids ([37] — [38]) and their anions, he calculated pK for reaction of the acids (Table A6.2). The values vary significantly with the phenyl substituent (p = +1.7), much more so than the p/(a values of the substrate acids (p = +0.2). This difference is consistent with the proton being much closer to the phenyl group in the transition state than in the initial state, and it may even denote a relatively late transition state (Pollack, 1978). However, from the pKa values of the reactant acids (approximately 3.4), the transition states (approximately 4.4), and the enol product (11.8) (Pruszynski et al., 1986), the Leffler index... 

Fractionation factor acid-dissociation constants in H2O and D2O by Kreevoy (Kreevoy et al., 1977 Kresge and Chiang, 1973). This value is not as low as that for the many other systems which are known to have weaker hydrogen bonds. 

There is also a relationship between p a-values of ethanol, phenol and acetic acid on the one hand, and ethylamine, aniline and acetamide on the other (Liler, 1971c, p. 108), which may be presented as a linear free energy relationship (Figure 6) by regarding Et, Ph and CH3 CO as substituents (R). This excellent relationship leaves no doubt that the acidic group in the R—NH3 series remains unchanged, i.e., that acetylammonium ion is formed in the protonation of acetamide in aqueous acid (half-protonation in ca. 19% sulphuric acid). 

Now let us consider the amino group in a-amino acids. The p a values of the conjugate acids are found to range from about 8.8 to 10.8. We shall consider a typical group with p a 10.0. From... 

The optimum pH will be the isoelectric point as described under amino acids (see Section 4.11.3). This is the pH at which the concentrations of cationic and anionic forms of morphine are equal, and is the mean of the two p a values. Morphine has pATa (phenol) 9.9, and pATa (amine) 8.2, so that pi = 9.05. Note that the protonated amine is a stronger acid than the phenol, so that the intermediate between the two ionized forms will be the non-ionized alkaloid. 

The leaving groups most commonly employed in nature are phosphates and diphosphates. These good leaving groups are anions of the strong acids phosphoric (pATa 2.1) and diphosphoric (pATa 1.5) acids respectively. The p a values given refer to the first ionization of these polyfunctional acids (see Section 4.7). 

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