Acidity constants correlations

The pA"a values of 49 derivatives of pyrazol-3-one were measured by poten-tiometric titration and their H NMR spectra were recorded in DMSO- fe- The experimental acidity order correlates for structurally similar compounds as do substituent constants and HMO electron densities (76JPR555). 

The linear correlation of log[NO+]/[HN02] with HR yields an acidity constant pATR = -7.8 in H2S04 corresponding to equal concentrations of NO+and HN02 at about 57% H2S04. ... 

Remes et al. (1976) also investigated the kinetics of the N-azo coupling of nine a-amino acids. They are aware of earlier investigations in which the major products were pentaz-1,4-dienes, but they claim that under their reaction conditions (pH 8.00-10.25, thirty-fold excess of amino acid) only the triazenes are formed. The rates were found to be first-order with respect to diazonium ion which is consistent with their conclusion however, in the opinion of the present author the results suggest a significant (say, 10%) contribution of pentazdiene formation to the total rate process. No significant correlation was found between the rate constants and the acidity constants of the nine amino acids. 

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. 

Intrinsic Binding Constants, Correlation Functions, and Corresponding Free Energies (in kcal/mol) for Various Carboxylic Acids ... 

Estimation of Acidity Constants The Hammett Correlation Illustrative Example 8.2 Estimating Acidity Constants of Aromatic Acids and Bases Using the Hammett Equation... 

A simple case where the general a constants in Table 8.5 do not succeed in correlating acidity constants is when the acid or base function is in direct resonance with the substituent. This may occur in cases such as substituted phenols, anilines, and pyridines. For example, owing to resonance (see Fig. 8.4), a para nitro group decreases the pKa of phenol much more than would be predicted from the o para constant obtained from the dissociation of p-nitrobenzoic acid. In such resonance cases (another example would be the anilines), a special set of o values (denoted as oJpara) has been derived (Table 8.5) to try to account for both inductive and resonance... 

As with arene-amine radical ion pairs, the ion pairs formed between ketones and amines can also suffer a-deprotona-tion. When triplet benzophenone is intercepted by amino acids, the aminium cation radical can be detected at acidic pH, but only the radical formed by aminium deprotonation is detectable in base (178). In the interaction of thioxanthone with trialky lamines, the triplet quenching rate constant correlates with amine oxidation potential, implicating rate determining radical ion pair formation which can also be observed spectroscopically. That the efficiency of electron exchange controls the overall reaction efficiency is consistent with the absence of an appreciable isotope effect when t-butylamine is used as an electron donor (179). 

The nucleophilicity parameters for carbanions of nitronates and malonic acid derivatives have been investigated.143 The nucleophilic reactivities do not correlate with the acidity constants of the conjugate CH acids, and from the poor correlation of the reactivities of the substituted a-nitrobenzyl anions with Hammett s ex-constants it can be inferred that the nucleophilic reactivities are strongly controlled by solvation. 

A good linear correlation exists between the acidity constants and the charge densities on the deprotonation centers in the respective electronic states. 

Beside the effect of pH, the difference between ULM-3 and ULM-4 showed that the volume of the amine also plays a role on the nature of the observed solids. Consequently, a careful analysis of the structural evolution in the system using different amines was required. Two parameters are important the shape of the amine (either spherical or linear) and its correlated ability to be protonated. Indeed, there is a relation between their shape and their acidobasic characteristics. Their acidity constants are very different and, if the pKa of aliphatic diamines lies in the 7-10 range, for spherical amines, it is only in the range 4-6. That means that both the volume, the anisotropy and the charge will influence the structure of the solid and therefore the shape of the cavities. 

On CNDO/2 calculations with pK (as base) = 2.30 for triazole, the basicities of H and AH tautomers are estimated to be equal, and maximum separation of protonated nitrogens i.e. N-1 and N-4 rather than N-1 and N-2) is predicted to afford the most stable cation (68TL3727). Acidity constants of 1,2,4-triazoles correlate with total and rr-electron densities but not with the lone pair character of the pyridine-type N in MO calculations (70JCS(B)i692, 70BCJ3344). 

Influence of Structural Moieties on Acidity Constants the Hammet Correlation... 

Wagner claims that this structure con-esponds to HNg, but no consideration was given to the effect of the H atom. He carried out similar calculations on various pseudohalide anions and was able to show that the pA of the corresponding acids varies linearly with the difference in the 7r-energy between the ions and their acids. Linear correlations were also found betw een (a) calculated bond orders and stretching force constants, (b) calculated charge densities and n.m.r. chemical shifts. 

For the ANI radical cation, an acidity constant pKa = 6.4 was obtained182 however, no experimental BDE of this ion has been reported. A linear correlation of the oxidation potentials of anilines versus the acidities of the corresponding radical cations was observed. Recent ab initio calculations198 derived a value of BDE(N+—H) = 418 10 kl mol-1 for the aniline radical cation, relative to the Ph—N—H+ cation in its singlet ground state. Removal of an electron reinforces the strength of the N—H bond, due to the electron delocalization. 

The log P s in these equations are for the calculated partition coefficients of the sodium salts (where log P ait = log P 4.90). These can also be analyzed in terms of distribution coefficients to obtain eq 30. The log D0 obtained should apply at any pH in contrast to log P0. Because the a-hydroxy acids all have the same pKg there is no way to assess the effect of acidity (or correlated properties) on bacteriostatic activity. Any electronic factor is buried in the constant. 

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