HBD acidity

Taft RW, Kamlet MJ (1976) The solvatochromic comparison method. 2. The a-scale of solvent hydrogen-bond donor (HBD) acidities. J Am Chem Soc 98 2886-2894. 

The physical meaning of Eq. [42] is tied to the interpretation of some of the terms, some being convoluted. The A term can be associated with dispersion interactions an increase in surface area suggests an increase in dispersion interactions (attractions) and, thus, increased solubility in octanol that in turn results in enhanced Pq/w values. A similar interpretation holds if one associates p /V with dipolarity/polarizability effects. The positive sign on the HBA term ( Umin) fot the solute suggests that the HBD acidity of water is less important than the HBA basicity of water for those molecules partitioning between phases. This implies that increased solute HBD acidity would increase the solute-water interaction. 

The visible n n absorption band is shifted hypsochromically by A1 = 53 nm on changing the medium from n-hexane to water. Based on the negative solvatochrom-ism of this aminyloxide radical, a spectroscopic solvent polarity scale, called the scale of Eewis acidity has been proposed [336]. Because of its pronounced negative sol-vatochromism, 4-nitropyridine-l-oxide has been recommended as an empirical indicator of the HBD acidity of solvents [330] cf. Section 7.4. The solvatochromic range of this A-oxide, measured in 48 different solvents, amounts to Av = +2840 cm (A1 = —31 nm) for the solvent change n-hexane water. 

Solution calorimetry of the molecular probes pyrrole, A-methylpyrrole, benzene, and toluene in 35 solvents has been used by Catalan el al. to determine a solvent HBA basicity scale, ranging from the gas phase to HMPT [31a]. Analogously, a solvent HBD acidity scale was derived calorimetrically using A-methylimidazole and A-methylpyrrole as probe molecules in 3 6 solvents, ranging from the gas phase to 2,2,2-trifluoroethanol [31b]. 

In going from tetrahydrofuran to ethanol, these Av values increase from —39 to 1300 cm In order to obtain a normalized scale of near-zero HBD acidity for non-HBD solvents and a value of roughly unity for HBD solvents, the SA scale was defined as follows [296, 337a] ... 

The direct determination of SA values for more acidic solvents was not possible because in such solvents the indicator dyes (52) and (53) are protonated at the carbonyl oxygen atom. For such acidic solvents, 3,6-diethyl-l,2,4,5-tetrazine (DETZ) was introduced as a third probe dye [337b]. This aromatic tetrazine exhibits a solvent-dependent n n absorption in the visible region a change from methylcyclohexane = 550 nm) to hexafluoro-2-propanol (Imax = 517 nm) leads to a hypsochromic band shift of A2 = —33 nm (Av = +1160 cm ), mainly due to the HBD acidity of the... 

The general SPP scale of solvent dipolarity/polarizability and the specific SB and SA scales of solvent HBA basicity and HBD acidity, respectively, are orthogonal to one another and they can be used in the correlation analysis of solvent effects in single- or, in combination with the others, in two- or three-parameter correlation equations, depending on the solvent-influenced process under consideration see also Section 7.7. Examples of the correlation analysis of a variety of other solvent-dependent processes by means of SPP, SB, and SA values, including those used for the introduction of other solvent polarity parameters, can be found in references [335-337, 340-342]. In particular, comparisons with Kamlet and Taft s n scale [340] and Winstein and Grunwald s Y scale [341] have been made. 

TT is the dipolarity/polarizability parameter, a is the solvent HBD (acidity) parameter. 

The most important scales for solvent HBD acidity are reported here. 

For solute HBD acidity different scales were proposed mainly based on complexa-tion constants and enthalpies of complexation. TTie most important are reported here. 

The a scale was proposed for solute HBD acidity based on log f values for 1 1 complexation of series of acids against a given base in dilute solution of CCI4 [Abraham et al., 1989b]. Forty-five Unear equations have been solved for each considered base by a series of acids ... 

Taft, R.W. and Kamlet, MJ. (1976). TTie Solvatochromic Comparison Method. 2. The a-Scale of Solvent Hydrogen-Bond Donor (HBD) Acidities. J.Am.Chem.Soc., 98, 2886-2894. 

With respect to ILs as cosolvents, the interest was centred on two types of ILs aUcylammonium and 1,3-dialkylimidazolium-based ionic liquids. For the first type, we selected ethylammonium nitrate, considering that it is a protic ionic liquid (PIL) and that it can act as a potential acid catalyst. With respect to the second type of ILs, we selected those based on l-methyl-3-n-butylimidazolium cation, characterised by a slighter HBD acidity than that of the alkylammonium type (Figs. 13.2, 13.3). These ILs exhibit a wide spectrum of physicochemical properties. It was demonstrated that their water content, density, viscosity, surface tension, melting point and thermal stability are affected by the length of the alkyl chain and the nature of the anion. Several anions were incorporated in this class of ILs. 

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