Dissociation constant ortho-effect

Because of the scarcity of electronic paramagnetic resonance data, and because of the frequent unreliability of the data from paramagnetism, boiling point elevation, spectrophotometry, and ortho-para hydrogen conversion, most published radical dissociation constants can be accepted only with reservations. An error of 50 % is not at all improbable in many cases. We are therefore not yet in a position to explain, or rather to test our explanations of, small differences in dissociation constants. Table I shows the values of K corresponding to various hexaarylethanes in benzene at 25°. Because of the order of magnitude differences in Table I, however, it is likely that some of the expected large effects, such as steric and resonance effects, exist. 

A convenient system for studying substituent effects is the equilibrium between meta- and />ara-substituted benzoic acids and their corresponding anions (reaction 3.5). The acids are straightforward to synthesize, and the acidities in water at 25 °C are readily determined the pH of a solution containing equal molar quantities of the acid and its fully ionized sodium salt will be equal to — log A, where KA is the dissociation constant of the acid. Ortho substituents are not considered because of complications caused by steric effects. 

Dickey PA, Collins AG, Fajardo I (1972) Chemical composition of deep formation waters in southwestern Louisiana. Am Assoc Pet Geol Bull 56 1530-1533 Dippy JFJ, Lewis RH (1937) Studies of the ortho-effect. Part II. The dissociation constants of some o-substituted acids. J Chem Soc 1937 1426-1429 Domalski ES (1972) Selected values of heats of combustion and heats of formation of organic compounds containing the elements C H N O P, and S. J Phys Chem Ref Data 1 221-277... 

Spectral properties and dissociation constants of a series of phenols carrying in their ortho- or para-position a heterocyclic substituent (including the benzothiazolyl group) have been determined. The linear relation of dissociation constants with Hammett o-p constants was used to calculate substitution constants for the heterocyclic residues. Owing to their electron-withdrawing effect, the hetero-substituents invariably enhance the acidity of the phenolic hydroxy-group. 

Dissociation constants of 2 have been reported to be higher for dimers with electron donating substituents in the para positions of the phenyl rings [ 15]. We found that these groups in ortho positions resulted qualitatively in very different properties. Any ortho substituent in Ar decreased dissociation. Thus, a benzene solution of the stable dimer of 2-(2-chlorophenyl)-4,5-diphenylimidazolyl radical 2d exhibited no radical absorption spectrum below 80°, whereas the 4-chloro compound 2e was partially dissociated in solution at room temperature. Ortho substituents in Ar and Ar" affected the equilibrium quite modestly but appeared to have the opposite effect, that is, increased dissociation. Qualitatively, meta substituents had little effect on the equilibrium of dissociation. 

We have stated earlier that because of proximity effects, no generally applicable aj values may be derived for ortho substitution. Nevertheless, one can determine a set of apparent 0)ortho values for a specific type of reaction, as for example, for the dissociation of substituted phenols. Table 8.7 gives such apparent O)ortho constants for estimating pKa values of substituted phenols and anilines. Of course, in cases of multiple substitution, substituents may interact with one another, thereby resulting in larger deviations of experimental from predicted pKa values. Some example calculations using the Hammett equation are given in Illustrative Example 8.2. 

Clearly, the retention times of the anions investigated decrease with an increasing amount of carbonate at a constant bicarbonate content (pH 8.35-10.23). However, when bicarbonate is added at constant carbonate concentrations (pH range 10.98-10.28), the retention times hardly decrease. This effect is imderstandable as bicarbonate exhibits only a small elution power. The decisive factor for the retention behavior of multivalent ions is the pH value resulting from the concentration ratio of both eluent components. As can be seen from Figure 3.135, bromide and nitrate are superimposed by orthophosphate within a very narrow pH range between 9.6 and 10.0. Interferences also occur at pH>10.8. In both cases, this may be attributed to changes in the dissociation equilibria of ortho-phosphoric acid. 

The kinetics of complex formation between Ni(II) and ortho-phosphate, ribose monophosphate and cytidine monophosphate (CMP) in water have been reported.The results are consistent with an/ mechanism involving protonated (HL ) and unprotonated (L ) ligands, and rate constants for reaction with L and HL" are about 1.5 x 10 dm mol s and 2.3 x 10" dm mol" s respectively, for all three systems. The cytidine ring appears to exert no effect on the binding of Ni(II) to CMP. The rate parameters have also been reported for the reaction of nickel(II) with 4-phenylpyridine and isoquinoline in water-r-butanol mixtures. Rate parameters for the dissociation of the isoquinoline and thiocyanate complexes of Ni(II) in 1-propanol, and of the former in ethanol and in water, are accommodated within an Id mechanism, and Tanaka has commented further on the relationships between the activation enthalpies for the dissociation of the same two complexes and the Gutmann donor number of the solvent. 

The substituent constants (known as Hammett constants) are measured for the add-dissociation eqrrUibrirrm of benzoic add and its para- or mcM-substituted derivatives, for which the reaction constant p is set to unity. Positive values of the Hammett constant indicate that the substituent is a better electron acceptor than H and can thus enhance addity by helping to stabilize the carboxylate ion negative values indicate that the substituent is a poorer electron acceptor than H (i.e., an electron donor instead). When the substituent R is in the para position, the resulting Cp values are taken as a measure of the polar effea of the substituent based on its combined resonance and inductive properties. When the substituent R is in the meta position, the resulting values are taken as a measme of the polar effea of the substituent based largely on its inductive properties. When a substituent is in the ortho position, it is assumed that the substituent effects are influenced by steric properties also. 

---