Predicting Acid Strength in Solution

The kinetics of reactions cataly2ed by very strong acids are often compHcated. The exact nature of the proton donor species is often not known, and typically the rate of the catalytic reaction does not have a simple dependence on the total concentration of the acid. However, sometimes there is a simple dependence of the catalytic reaction rate on some empirical measure of the acid strength of the solution, such as the Hammett acidity function Hq, which is a measure of the tendency of the solution to donate a proton to a neutral base. Sometimes the rate is proportional to (—log/ig)- Such a dependence may be expected when the slow step in the catalytic cycle is the donation of a proton by the solution to a neutral reactant, ie, base but it is not easy to predict when such a dependence may be found. 

The dependence of J on w predicted by Eq. (51) is illustrated by Fig. 17 for 2-naphthoic acid dissolving in an aqueous solution having an ionic strength of... 

Any consideration of sovent effects on rates or equilibria must start from solvent activity coefficients, VI for reactants, transition states and products (Wiberg, 1964 Laidler, 1950 Parker, 1966). Once solvent activity coefficients are available, or can be predicted, it is highly probable, as indicated at the end of this article, that an enormous amoimt of information on the kinetics of reactions in solution and on equilibrium properties such as solubility, acid-base strength, ion-association, complexing, redox potentials and kinetics of reactions in different solvents (Parker, 1962, 1965a, 1966) can be reduced to a relatively small number of constants which can then be used in appropriate linear free energy relationships. 

The cyanide ion is an ambident base. The carbon end is seen to be much softer than the nitrogen end, as would be predicted. The greater strength of the H-CN bond, compared with H-NC, means that the carbon end is much more basic than the nitrogen end. This would be true even in solution, since the ions are the same for both acids. As a result, binding to carbon will be more common than binding to nitrogen, even for hard acids. 

Just as most acid-base reactions in solution reach a state of equilibrium, so most aqueous redox reactions reach equilibrium. Just as the favored side of an acid-base equilibrium can be predicted from acid-base strength, so the favored side of a redox equilibrium can be predicted from oxidizing agent-reducing agent strength. 

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