Predicting the Outcome of Acid-Base Reactions

In a proton transfer reaction, the stronger acid reacts with the stronger base to form the weaker acid and the weaker base. 

A proton transfer reaction represents an equilibrium. Because an acid donates a proton to a base, thus forming a conjugate acid and conjugate base, there are always two acids and two bases in the reaction mixture. Which pair of acids and bases is favored at equilibrium The position of the equilibrium depends on the relative strengths of the acids and bases. 

Because a strong acid readily donates a proton and a strong base readily accepts one, these two species react to form a weaker conjugate acid and base that do not donate or accept a proton as readily. Comparing pA a values allows us to determine the position of equilibrium, as illustrated in Sample Problem 2.4. 

Determine the direction of equilibrium when acetylene (HC=CH) reacts with NH2 in a proton transfer reaction. 

Follow three steps to determine the position of equilibrium  

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HOWTO PREDICT THE OUTCOME OF ACID-BASE REACTIONS... 

Formation of the weaker acid and base is an important general principle for predicting the outcome of acid-base reactions. 

With a knowledge of pATa values, or a rough idea of relative values, one can predict the outcome of acid-base interactions. This may form an essential preliminary to many reactions, or provide us with an understanding of whether a compound is ionized... 

One of the most useful tools for predicting the outcome of chemical reactions is the principle of hard and soft acids and bases (HSAB), formulated by Pearson in 1963 [13-15]. This prindple states that hard acids will react preferentially with hard bases, and soft acids with soft bases, hard and soft referring to sparsely or highly polarizable reactants. A selection of hard and soft Lewis acids and bases is given in Table 1.1. 

Using base strength to predict the outcome of substitution reactions of carboxyiic acid derivatives... 

If the world were made of pure substances, our development of the thermodynamic model would now be complete. We have developed a method, based on measurements of heat flow, that enables predictions to be made about which way reactions will go in given circumstances. But one of the reasons that the world is so complex is that pure substances are relatively rare, and strictly speaking they are nonexistent (even pure substances contain impurities in trace quantities). Most natural substances are composed of several components, and the result is called a solution. Therefore, we need to develop a way to deal with components in solution in the same way that we can now deal with pure substances - we have to be able to get numerical values for the Gibbs energies, enthalpies, and entropies of components in solutions. We will then be able to predict the outcome of reactions that take place entirely in solution, such as the ionization of acids and bases, and reactions that involve soUds and gases as well as dissolved components, such as whether minerals will dissolve or precipitate. Our thermodynamic model will then be complete. 

This concept allows strengths of different acids and bases to be compared to predict the outcome of a reaction. As an example, consider the reaction of perchloric acid, HClO., and water. 

The electrodeposition of tellurium and silver has been investigated in dilute aqueous solutions of tellurous acid and Ag " ions (concentrations in the order of 10 to 10 " M) in 0.1 M HCIO4 [164], In particular, cyclic voltammetry experiments were conducted with rotating glassy carbon disk electrodes in baths with various concentration ratios of Ag(I) and Te(IV) precursors, and their outcome was discussed in terms of the voltammetric features. For a Ag(I)/Te(IV) ratio close to 0.8, formation of quasi pure silver telluride, Ag2Te, was reported. The authors, based on their measurements and on account of thermodynamic predictions, assumed that silver is deposited first on the electrode (Ag" + e Ag), and then Te(IV) is reduced on the previous silver deposit with formation of Ag2Te according to the reaction... 

Recently, Simon and Goodman have been able to develop a model, based on DFT calculations, that is able to predict the stereochemical outcome of more than 40 imine reactions catalyzed by binol-derived phosphoric acids (Figure 2.36) [156b]. The majority of nucleophilic additions to imines take place via a Type I transition state only if the R2 substituent of the imine is larger than R3 and if the nucleophilic atom is not on line with the OH bond (cf. additions of indoles or of ene carbamates to imines)... 

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