Acidity constant variation with solvent

The inversion, with or without invertase or acid, has also been useful in studying the chemical effect of the deuterium ion, of high pressure, of sonic and supersonic energy of ultraviolet light " of high-frequency electric energy and of variation in the dielectric constant of the solvent. ... 

E. Bosch, C. Rafols, and M. Roses, Variation of acidity constants and pH values of some organic acids in water-2-propanol mixtures with solvent composition. Effect of preferential solvation, A a/. Chim. Acta 302 (1995), 109-119. 

Solvent effects are essentially free-energy correlations [57] and we shall omit in this chapter those of strongly acidic media as these are clearly not pertinent to biochemistry. The effect of variation of solvent on a rate or equilibrium constant may be treated in the same way as a substituent effect. We may apply the same criteria to elucidate mechanistic or rate-determining step changes to free-energy relationships with solvent effects. Let us consider the hydrolysis of 5-phenyl-oxazoline-2-ones... 

This changeover molar ratio appears to be ind endent of the concentration of nitric acid and also of the orgcinic solvent used and is surprisingly constant in the range 0.2 to 0.3. Variation with reactivity of the aromatic substrate is certainly only slight. 

Useful exploratory studies of acid-base behaviour in solvents of low dielectric constant have been made by conductance " and potentio-metric " titrations. Association constants are usually obtained from spectrophotometric measurements. The strengths of various bases can be compared by means of their association with an indicator acid like 2,4-dinitrophenol. If both acid and base are colourless, a competition for the base can be established between the acid and an indicator acid like bromophthalein magenta In solvents like benzene, other reactions than simple 1 1 association between B and RX may occur. Self-association of the acid or base is one such auxiliary reaction. A classic example is the dimerisation of carboxylic acids in benzene. If allowance is not made for this, constant values of the quotient [BRX]j[B][RX] will not be obtained. (Variations in the quotient cannot be attributed to interionic forces or other nonideal behaviour BRX is scarcely dissociated into ions at all and in spectrophotometric work very low concentrations of B and RX can be used.) Evidence for association ratios other than 1 1 can be obtained from indicator studies. The method developed by Kolthoff and Bruckenstein for studying reactions in anhydrous acetic acid fails for reactions in benzene and similar solvents because more than one acid molecule reacts with the indicator to give complexes of the form/w J r"(HX)yi. In such studies it is generally a good approximation... 

In most of the investigations mentioned so far in this section involving solvent mixtures it is likely that the primary solvation shell does not vary with solvent composition. However, there are occasions when variation of reactivity with solvent mixture composition is attributed to changes in primary solvation shell composition, as in the case of reaction of Ni + with ammonia in aqueous methanol. In the reaction of Be + with sulphate in aqueous DMSO, there is strong n.m.r. evidence for variation in primary solvation shell composition. In the reaction of Co with tetraphenylporphine in acetic acid-water the kinetics also reflect the presence of varying amounts of mixed solvates. By way of contrast, in reactions of copper(ii) with polythiaethers in methanol-water mixtures only Cuaq + reacts mixed solvates are claimed to be of negligible reactivity. In the reaction of chromium(iii) with edta in methanol- and ethanol-water mixtures, variation in pK for the Cr + has an effect on the formation rates, but the individual rate constants for the reactions of Cr + and of CrOH + with the ligand seem to be practically independent of solvent composition. ... 

Different Types of Proton Transfers. Molecular Ions. The Electrostatic Energy. The ZwiUertons of Amino Acids. Aviopro-tolysis of the Solvent. The Dissociation Constant of a Weak Acid. Variation of the Equilibrium Constant with Temperature. Proton Transfers of Class I. Proton Transfers of Classes II, III, and IV. The Temperature at Which In Kx Passes through Its Maximum. Comparison between Theory and Experiment. A Chart of Occupied and Vacant Proton Levels. 

The catalytic constants measured in 95% aqueous dioxan have been compared with piT-values in water. The twenty-four acids referred to in Table 3 are mainly carboxylic acids, but also include nitric acid, o-chloro-phenol and water. Two oximes show large positive deviations, and saccharin has considerably less catalytic activity than anticipated these substances have not been included in the correlation. A number of strong acids gave closely similar catalytic constants— HCl (3-05), HBr (2-30), CoHb.SOsH (2-30), MeSOsH (2-15), HCIO (1-25)—and the minor variations within this series are not in the expected order of acid strengths HCIO4 > HBr > HCl > ObHb. SO3H > MeSOsH. Presumably all these acids are converted in solution to the hydronium ion, the catalytic power of which is somewhat modified by ion-pairing with different anions in the solvent of low dielectric constant. The catalytic constants observed are consistent with the conventional value pJT = —1-74 for H36+. 

Variations in dielectric constant should alter the relative strength of acids of different charge types, since the amount of electrical work involved in a proton-transfer reaction must vary with the dielectric constant of the medium. Since a lowering in dielectric constant increases the work required to separate the ions (for example, to ionize an uncharged acid one must create an anion and a cation), any addition of organic solvent should lead to an increase of pXa values. 

The rest of this chapter is a variation on a theme the use of equilibrium constants to calculate the equilibrium composition of solutions of acids and bases. We begin with solutions of acids, bases, and salts, explore the contribution of the autoprotolysis of the solvent to the pH, which is significant in very dilute solutions, and see how to handle the complications of acids that can donate more than one proton. Although the applications are varied, the techniques are all very similar and are based on the material in Chapter 9. 

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