Hydrogen bond acceptor solvents

Murray, J. S., S. Ranganathan, and P. Politzer. 1991. Correlations Between the Solvent Hydrogen Bond Acceptor Parameter (3 and the Calculated Molecular Electrostatic Potential. J. Org. Chem. 56, 3734. 

Hydrogen bond donor solvents are simply those containing a hydrogen atom bound to an electronegative atom. These are often referred to as protic solvents, and the class includes water, carboxylic acids, alcohols and amines. For chemical reactions that involve the use of easily hydrolysed or solvolysed compounds, such as AICI3, it is important to avoid protic solvents. Hydrogen bond acceptors are solvents that have a lone pair available for donation, and include acetonitrile, pyridine and acetone. Kamlet-Taft a and ft parameters are solvatochromic measurements of the HBD and HBA properties of solvents, i.e. acidity and basicity, respectively [24], These measurements use the solvatochromic probe molecules V, V-die lliy I -4-n i in tan iline, which acts as a HBA, and 4-nitroaniline, which is a HBA and a HBD (Figure 1.17). 

Kamlet MJ, Taft RW (1976) The solvatochromic comparison method. 1. The /6-scale of solvent hydrogen-bond acceptor (HBA) basicities. J Am Chem Soc 98 377-383. 

The value of kd was obtained from the determination of triplet lifetimes by measuring the decay of phosphorescence and found to be insensitive to changes in solvent polarity. The k2 values derived from Eqs. 10 and 11 were correlated with solvent parameters using the linear solvation energy relationship described by Abraham, Kamlet and Taft and co-workers [18] (Eq. 12), which relates rate constants (k) to four different solvation parameters (1) or the square of the Hildebrand solubility parameter (solvent cohesive energy density), (2) n or solvent dipolarity or polarizability, (3) a, or solvent hydrogen bond donor acidity (solvent electrophilic assistance), and (4) or solvent hydrogen bond acceptor basicity (solvent nucleophilic assistance). 

P empirical parameter of solvent hydrogen-bond acceptor basicity (Taft and Kamlet) ... 

Murray, J.S., Ranganathan, S. and Politzer, P. (1991) Correlations between the solvent hydrogen bond acceptor parameter p and the calculated molecular electrostatic potential./. Org. Chcm., 56,3734—3737. 

To investigate the possibility of variable ESA, we have used the solvatochromic method of Kamlet and co-workers (7-10). According to this approach, a solvent-dependent phenomenon (in this case log solvolysis rate k) is a function of four solvent properties solvent hydrogen-bond donor ability or electrophilicity, a solvent hydrogen-bond acceptor ability or nu-... 

For the analysis of SN1 solvolyses, Abraham et al. (9) have proposed an equation (equation 3) based on sensitivities toward solvatochromatic properties. In equation 3, tr is a measure of solvent dipolarity-polarization, a is a measure of solvent hydrogen bond donor acidity, and P is a measure of solvent hydrogen bond acceptor basicity. More recently, a term governing cavity effects has been added, and this term is considered to represent an important contribution (10, 11). The cavity term can be directly related to the square of the Hildebrand solubility parameter (10-12). A similar analysis by Koppel and Palm (13, 14) involves terms governed by solvent polarity, solvent polarizability, electrophilic solvation ability, and nucleophilic solvation ability. Recently, a cavity term has also been added to this analysis (12). 

Average hydration number of a cation in a water-saturated water-immiscible solvent Hydrogen-bond acceptor... 

The following treatment is based on the use of three different scales [i.e., (S), g2(S), and g3(S)] which have been determined empirically the polarity scale 7T, the a scale of solvent hydrogen bond donor (HBD) acidities (71), and the /3 scale of solvent hydrogen bond acceptor (HBA) basicities (72). To avoid possible pitfalls resulting from experimental errors or from specific solvent effects, the solvatochromic parameters have been arrived at by averaging multiple, s determined for each solvent with a variety of different indicators. Quite generally, the purpose of this study is the systematic correlation of solvent effects on diverse properties and reactivity parameters, XYZ, by means of expressions of the type. 

M. J. Kamlet and R. W. Taft,/. Am. Chem. Soc., 98,377 (1976). Solvatochromic Comparison Method. 1. Beta-Scale of Solvent Hydrogen-Bond Acceptor (HBA) Basicities. 

Kamlet MJ, Taft RW. The solvatochromic comparison method. I. The.beta.-scale of solvent hydrogen-bond acceptor (HBA) basicities. J Am Chem Soc. 1976 98 377-383. Taft RW, Kamlet MJ. The solvatochromic comparison method. 2. The.alpha.-scale of solvent hydrogen-bond donor (HBD) acidities. J Am Chem Soc. 1976 98 2886-2894. Yokoyama T, Taft RW, Kamlet MJ. The solvatochromic comparison method. 3. Hydrogen bonding by some 2-nitroaniline derivatives. J Am Chem Soc. 1976 98 3233-3237. 

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