Solvent hydrogen-bond basicity

The p scale was proposed to measure solvent hydrogen bond basicity, i.e. the ability of a bulk solvent to act as hydrogen bond acceptor. This scale was derived by systematic application of the solvatochromic comparison method the final p values were calculated by averaging 13 p parameters for each solvent obtained with different solutes and different physicochemical properties [Kamlet et al, 1981a Kamlet et al, 1983]. 

The Koppel-Paju B scale was proposed to measure solvent hydrogen bond basicity, based on solvent shifts of the IR stretching frequencies of the free and hydrogen bonded OH group of phenol in CCI4 [Koppel and Paju, 1974]. 

They clearly indicate that the pf is not the same as solvent hydrogen bond basicity, /ii, because the pf value treats the solvent as a solute in the chemical interactions, and that the pf and fli scales are relatively collinear but not interchangeable . (The latter scale is based on the comparison of the indicators p-nitroaniline and p-nitro-Af, N-dimethylaniline.) Neither is the value connected to solute proton-transfer basicity . They have established that this pf value is relatively constant for homologous series of solvents, and that substituents on the parent structure of the solvent do not overly influence the pf value in terms of inductive or polar effects, unless the substituent is halogenated, in which case the will decrease. Chain branching of the parent also has little effect on the pf value. This makes it possible to predict average P2 values for solutes whose ATg values are not known. Correlations of kinetic data with pf are not always accurate because the pf parameter does not take into consideration solvent size, which can lead to steric hindrance of hydrogen bond formation . ... 

Electron Transfer. Neta and coworkers have worked extensively with halogen-substituted methyl peroxyl radicals (X H COO , where X = Cl, Br or F) in aqueous and non-aqueous media, using combinations of solvents in different ratios to change the polarity of the mixture. They describe the mechanism for the reaction of the water-soluble antioxidant Trolox with their peroxyl radicals as H-mediated electron transfer , having determined that the rate of the reaction increases with an increase in solvent polarity. They examined solvent polarity in terms of the dielectric constant of the solvent, e, and solvent basicity, reported as either the coordinate covalency parameter, f, which is a measure of solvent proton-transfer basicity, or the value, which is a measure of solvent hydrogen bond basicity . 

Single electron transfer (SET), as antioxidant mechanism 844, 896, 897 Size exclusion chromatography 953 Slash pine bark, phenolic compounds in 944 Smiles rearrangement 466-470, 759 S Ar reactions 673 Soil samples, phenolic compounds in, analysis of 932, 965, 972, 985 field screening of 938 Sol-gel technique 1082 Solid acid catalysis 612-621 Solid-phase extraction (SPE) 930-933, 936, 942, 944-950, 955, 958, 960, 962-964, 969, 972, 985, 986, 995, 1354 Solvation energy 500, 992 Solvation free energy 5 Solvatochromic comparison method, for solvent hydrogen-bond basicity 591 Solvent effects,... 

Abraham and co-workers concluded that at least four solvent parameters are necessary to adequately correlate solvolysis of t-butyl chloride with solvent properties in the most general case. These four properties are solvent dipolarity, solvent hydrogen bond acidity, solvent hydrogen bond basicity, and the cohesive energy density of the solvent. They note, however, that a smaller number of parameters may be adequate for studies in which solvent properties vary in more restricted ways. Abraham, M. C. Doherty, R. M. Kamlet, M. J. Harris, J. M. Taft, R. W. /. Chem. Soc. Perkin Trans. 2 1987, 913, 1097. 

Where FCl is the solute gas-liquid partition coefficient, r is the tendency of the solvent to interact through k- and n-electron pairs (Lewis basicity), s the contribution from dipole-dipole and dipole-induced dipole interactions (in molecular solvents), a is the hydrogen bond basicity of the solvent, b is its hydrogen bond acidity and I is how well the solvent will separate members of a homologous series, with contributions from solvent cavity formation and dispersion interactions. 

The correlation of Snyder s solvent strength e° with molecular dipolarity and polarizability (7t ) and the hydrogen-bond acidity (a) and the hydrogen-bond basicity ((3) solvatochromic parameters for adsorption chromatography can be achieved, although most papers on solvatochromic parameters deal with reversed-phase systems [18]. 

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). 

Attempts to evaluate the effective solute hydrogen bond acidity and basicity40 produce quantitative scales of solute hydrogen-bonding, which may be of interest for application also to biochemical processes41. The indicator pair 4-nitroanilinc/4-nitro-/V,/V-dimethylaminoaniline are used to state the equivalence42 of solute and solvent scales of hydrogen bond basicity. 

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. 

Since solvatochromic parameters are derived from direct measurements of the energy resulting from intermolecular interaction, they can be used to predict solubility, which is determined by solute-solute, solvent-solvent, and solute-solvent interaction energies. For nonself-associated liquid aliphatic compounds with a weak or nonhydrogen-bond donor (Taft etal., 1985 Kamlet etal., 1986), the solubility in water at 29S was related to molar volunWjf, hydrogen-bond basicity j and polarity/polarizability (jf) by a linear solvation energy relationship (LSER) as in Equation 3.55 ... 

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