PATa value

The ligand pATa values and transition metal chelate stability constants of arylisoxazoles were detected photometrically and the stability of the complexes studied (79JlCi25i). 

The UV spectra of 3,6-di-ter/-butyl-substituted 2H-, 3H- and 4//-azepines in various ethanol solutions have been recorded.30 On the basis of pH-dependent spectra, pATa values of 7.02, 4.54 and 6.81, respectively, have been calculated for the conjugate acids of these azepines. The increase in basic strength (2H > 4H > 3//) can be reconciled with mesomeric stabilization of the respective azepinium cations. 

Detailed IR and UV spectroscopic data on 5//-dibenzfc,c]azepine5 and its 7-methoxy- and 7-methylsulfanyl derivatives7 have been reported. IR, UV and HNMR spectroscopic data for 2-amino-3//-l-benzazepin-3-one, 2-amino-l//-3-benzazepin-l-one, and 6-amino-7H-dibenz[7>,(/]azepin-7-one have been gathered,27 and confirm their oxo amidinc structures. Spec-trophotometrically determined pATa values of 4.46 and 5.20 (in water at 21 C) have been obtained for 1 l//-dibenz[(>,e]azepine and its 6-methyl derivative, respectively.28... 

This diazotization is typical of many aminoazoles the diazonium ions formed are relatively strong acids. The pATa values of five di-, tri-, and tetrazolediazonium ions are reported to be between 3 and 4, i. e., about 10 units lower (more acidic) than those of the respective unsubstituted heterocycles (Vilarrasa et al., 1974). Therefore, deprotonation of the diazonium ion is easy and, depending on reaction conditions, yields either the diazonium salt or its conjugate base, the diazo compound. The electrophilic reactivity of the P nitrogen atom in the diazo group of the base is lower than the reactivity of the diazonio group of the cation (Diener and Zollinger, 1986 see Sec. 12.2). 

For arene- and heteroarenediazonium ions with substituents that are subject to their own acid-base equilibria the situation is more complex. For example, the hydroxy group of the 4-hydroxybenzenediazonium ion has a pAfa value of 3.40 (Lewis and Johnson, 1959) whereas the 2-hydroxy-5-sulfo-benzenediazonium zwit-terion has a pATa value of only —0.04 (Jermini et al., 1970). The 0 group of the conjugate base greatly reduces the acidity of the diazonio group, as indicated by the mesomeric quinonediazide structure in Scheme 5-13. 

Macrocycles with intra-annular carboxylic acid groups [33] have been synthesized by Cram and Reinhoudt. As for the analogous pyridinocrowns [9], the pATa values of the acids were measured in water at 22°C and at 25°C (Newcomb et al., 1977b Aarts et al., 1986). 

As for the pyridines [9] (see p. 73), the acidity of the acids [33] also depended on the ring size. The 18-membered ring [33b] had the largest pK but the pATa values of the 15-, 21- and 24-membered rings were also quite... 

In the polyazamacrocyclic phenols [64], an acidic function, the phenol group, has an intra-annular orientation while the basic units, the nitrogen atoms of the macrocycles, have no defined orientation. In water at 25°C, the pATa values of the phenols were measured and compared with those of other macrocyclic and non-macrocyclic phenols (Kimura et al., 1987a,b). Because the nitrogen atoms and the phenol function both possess acid-base properties, more than one pXg value could be measured. By the use of UV measurements, the values of the phenol group could be distinguished from those of the amines. 

Table 29 pATa values for 2,6-substituted pyridines [70] in 50% ethanol/water. ... 

The energies of protonation of the complete series of methyl and ethyl phosphines have been calculated. pATa values for the hydroxyphenyl-... 

The accuracy of PHMD methods and their feasibility for studying pH-dependent conformational phenomena of proteins can be assessed by pKa calculations. In this case, PHMD simulations are performed with several pH values. The resulting occupancy values for deprotonated states (.S dc prot) are plotted against pH (Figure 10-3). A titration curve and pATa values (Figure 10-3) can be obtained by fitting the data to the generalized HH equation (Eq. 10-5). 

The hydroxyethyl quaternary ammonium ion in (20) is a weak acid the pATa-value of the non-micellizing model compound, choline, is 12.8 (Haber-field and Pessin, 1982). Reactions of the alkoxide zwitterion are therefore followed at relatively high pH, and from the variation of rate constant with... 

Example separation of 3-chloro and 4-ethylbenzoic acids. The log P values of these compounds are similar, and their pATa values are 3.96 and 4.35, respectively. As seen by their chromatographic behaviour in Figure 4.5, the separation is difficult at low pH due to the similar log P values giving similar log k0 values. The separation can be easily performed in an eluent with a pH of about 4.2, due to the difference in pKa values and different degrees of ionization. 

In the case of protonated pyrroles, the p Ta value lies in the range between 4 and —4, whereas the pATa value of acetonitrile is about —10. Therefore, the oligomerization of pyrrole in pure acetonitrile may already stop at the level of a-intermediates of hi- or more likely of tetrapyrrole. Acetonitrile is a weaker base than the a-intermediates. Consequently, a stronger base must be used to initiate the elimination of protons. Water fulfills this condition. Pyrrole can be polymerized in acetonitrile in the presence of 1% water [6, 37]. A similar effect results from the application of a sterically hindered base such as 2,6-di-tert-butylpyridine [38]. However, the concentration should be kept low because, at high concentrations proton, abstraction from the monomeric radical cation may occur, thus forming a neutral radical [28d]. The base effect can be also observed in the case of thiophenes. 

Plotting ixbase VS. pH gives a sigmoidal curve, whose inflection point reflects the apparent base-pAi, which may be corrected for ionic strength, I, using Equation 6.11 in order to obtain the thermodynamic pATa value in the respective solvent composition. Parameters A and B are Debye-Hiickel parameters, which are functions of temperature (T) and dielectric constant (e) of the solvent medium. For the buffers used, z = 1 for all ions ao expresses the distance of closest approach of the ions, that is, the sum of their effective radii in solution (solvated radii). Examples of the plots are shown in Figure 6.12. 

Therefore, the efficiency of an activator depends on its acidity which decreases pulp pH, and its bonding ability with cations which forms soluble stable complexes with cations at the p5nite surface. In other words, a better activator should have a lower pATa value(stronger acidity) and a greater Ag value (stronger affinity). A combination of these factors defines the activity index Ai as follows ... 

As we use pATa values, we shall find that, in most cases, relative, rather than specific, values are all we need to consider to help us predict chemical behaviour and reactivity. Thus, from pATa values, we can see that acetic acid (pATa 4.75) is a weaker acid than hydrochloric acid (pATa — 7). 

The pATa values for the following acids illustrate that, as the carbon atom adjacent to the carboxylic acid group changes from sp to sp to sp hybridization, the acidity increases, in accord with the electronegativity explanation above. Note that benzoic acid (xp hybridization) has a similar pATa to acrylic acid (propenoic acid), which also has sp hybridization. 

When substituents can also be involved in the resonance effects, changes in acidity become more marked. Consider hydroxy- and methoxy-benzoic acid derivatives. The pATa values are found to be 3.0, 4.1, and 4.6 for the ortho, meta, and para hydroxy derivatives respectively, and 4.1, 4.1, and 4.5 respectively for the corresponding methoxy derivatives. 

The pATa values for the amines ammonia, methy-lamine, dimethylamine, and trimethylamine are 9.2, 10.6, 10.7, and 9.8 respectively. The electron-donating effect of the methyl substituents increases the basic strength of methylamine over ammonia by about 1.4 pATa units, i.e. by a factor of over 25 (10 " = 25.1). However, the introduction of a second methyl substituent has a relatively small effect, and the introduction of a third methyl group, as in trimethylamine, actually reduces the basic strength to nearer that of methylamine. 

When pATa values are measured in the gas phase, where there are no hydrogen bonding effects, they are found to follow the predictions based solely on electron-donating effects. In water, mono-, di-, and tri-alkylated amines all tend to have rather similar pATa values, typically in the range 10-11. 

This is precisely opposite to what would be expected from the inductive effects of alkyl groups, and the observations are likely to be the result primarily of solvation (hydrogen bonding) effects. Note, the cations shown all have negative pATa values. In other words, they are very strong acids and will lose a proton readily. Conversely, the non-protonated compounds are weak bases. 

It is important to avoid confusion in such cases, and this requires an appreciation of typical pATa values for simple acids and bases. There is no way we would encourage memorizing of pATa values, but two easily remembered figures can be valuable for comparisons. These are pATa around 5 for a typical aliphatic carboxylic acid, and pATa around 10 for a typical aliphatic amine. These then allow us to consider whether the compound in question is more acidic, more basic, etc. 

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