Abstract
A general scale of solute hydrogen-bond basicity has been constructed using a vast body of 1:1 hydrogen-bond equilibrium constants taken from literature. The data is assembled as a series (or set) of log K values for bases measured against a given hydrogen-bond acid in dilute tetrachloromethane solution. 34 such sets have been collected and a computational method regresses the log K data into 34 optimized equations of the form; log K[(set1)] = LA[i]. log K[H][B] + De[i] (1) For each set, the constants (LA and De) characterize the particular reference acid and the log K HB values characterize (and thereby scale) the bases over all sets. Certain acid-base combinations must, however, be excluded from equation 1 and this provides evidence that the log K HB scale is not entirely general towards all reference acids. In the regression procedure, use has been made of the novel observation that the lines represented by equation 1 intersect at a point where log K = log K HB = -1. 1. It is further shown that this condition enables the estimation of 1: 1 hydrogen-bond log K values in tetrachloromethane via equation 2; log K = 0.3421. (log K HA + 1. 1)(log HB + 1. 1) - 1.094 (2) where log K HA is a scale of solute hydrogen-bond acidity. The log K HB scale has been analysed in terms of the two principle factors of basicity according to the method devised by Maria and Gal et al. This analysis is shown to be informative in establishing and interpreting the relationships of log K HB with other basicity-dependent properties. For the most part, the experimental studies undertaken are concerned with a critical examination of the existing methods of determining G and H for 1:1 hydrogen-bond formation. The customary assumption of an exclusive 1:1 model of acid-base association in dilute solution has been tested with respect to the interaction of cyclohexanone with 3,5-dichlorophenol in cyclohexane. Using infrared spectroscopy, evidence for higher complex formation is presented and a method has been developed for determining simultaneously the formation constants of both the 1: 1 (AB) and 2: 1 (A2B) complexes (K1 and K2, respectively). 2:1 complex formation is also shown to interfere significantly with the K and H values obtained from a classical calorimetric titration procedure, limiting its application to hydrogen-bonding studies. For this reason, an improved calorimetric method for the independent determination of 1:1 hydrogen-bond enthalpies has been developed. Application of the devised methods to the interaction of 3,5-dichlorophenol with series of aliphatic ketones and aliphatic ethers in cyclohexane has yielded the values of K1 and K2, together with the thermodynamic parameters (G1, H1 and S1) of 1:1 hydrogen-bond formation. Within each series, the small differences in K1 and H1 resulting from the very subtle structural variations in the base component are discussed in terms of electronic and steric effects. The values of K1 and K2 are found to be consistent with a theoretically derived equation predicting a linear relationship between log K1 and log K2, and the values of H1 are used to critically appraise the validity of the Badger-Bauer relationship.