Solvents HBD acidity

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]. 

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

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

Again we have found, however, that like the earlier examples cited, A/ values of nitroxides in protic solvents reflect combined effects of solvent polarity and solvent HBD acidity. Thus, the total solvatochromic equation for the nitrogen hyperflne splitting constant of di-tert-butyl nitroxide (66) in 17 aliphatic solvents (1,7,9,13,18,25,29,32,50,61,101,102,103,104,105,107,111) is... 

Pyridine N-NMR Shifts. A property that is highly sensitive to solvent HBD acidity, and which might in the future prove to be quite useful for the determination of new a values is the N-nmr spectrum of pyridine (67). Shifts (relative to external 1.0 M H NOa) in five aliphatic solvents (2,29,105,111,113), as reported by Duthaler and Roberts (173) show excellent correlation with ir ... 

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. 

Streitwieser et al. [160] and BordweU et al. [161] used the lyate ions of organic solvents such as cyclohexylamine and dimethyl sulfoxide in the determination of the C—H acidity of weak organic carbon acids. Using super base systems such as alkali metal salts of cyclohexylamine [i.e. lithium and cesium cyclohexylamides) [160] and dimethyl sulfoxide (sodium dimsyl) [161] in an excess of these non-HBD solvents, relative acidity scales for weak carbon acids have been established. In this way, values for the ionization of over a thousand Bronsted acids in dimethyl sulfoxide have... 

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. 

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. 

Another important treatment of multiple interacting solvent effects, in principle analogous to Eq. (7-50) but more precisely elaborated and more generally applicable, has been proposed by Kamlet, Abboud, and Taft (KAT) [84a, 224, 226], Theirs and Koppel and Palm s approaches have much in common, i.e. that it is necessary to consider non-specific and specific solute/solvent interactions separately, and that the latter should be subdivided into solvent Lewis-acidity interactions (HBA solute/HBD solvent) and solvent Lewis-basicity interactions (HBD solute/HBA solvent). Using the solvato-chromic solvent parameters a, and n, which have already been introduced in Section 7.4 cf. Table 7-4), the multiparameter equation (7-53) has been proposed for use in so-called linear solvation energy relationships (LSER). 

A solvent HBD parameter a (acidity) is determined by a solvatochromic comparison methodbased on these conditions ... 

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. 

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