Hydrogen bond donor acidity

Attempts have also been made to separate non-specific effects of the local electrical field from hydrogen-bonding effects for a small group of ionic liquids through the use of the k scale of dipolarity/polarizability, the a scale of hydrogen bond donor acidity, and the (i scale of hydrogen bond basicity (see Table 3.5-1) [13, 16]. 

A new acidity scale has been developed based on calorimetric measurement of A-methylimidazole and A-methylpyrrole in bulk solvents. A revised version of this method was shown to give better results in some cases. Another scale of solvent acidities was developed based on the hydrogen-bond donor acidities in aqueous DMSO. ... 

The a scale of hydrogen bond donor acidities measures the ability of a solvent to donate a proton in a solvent-solute hydrogen bond,... 

In 1976, Kamlet and Taft introduced their solvatochromic comparison method [25, 26], The hydrogen-bond donor acidity a and basicity /3 together with the solvent polarity and polarizability jv were employed to correlate the solvent effects on reaction rates, equilibria, and spectroscopic properties XYZ according to equations of the form... 

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). 

For other, more recent quantitative scales of the hydrogen-bond donor acidities and hydrogen-bond acceptor basicities of solvents, see the end of Section 2.2.5 (and references [329-334] cited therein). 

Results for solvolysis of CFsSOs" indicated that solvent basicity is as important as polarity and hydrogen-bond donor acidity, consistent with some association of solvent as a base with the metal ion in the transition state. 

Fan, W., El Tayar, N., Testa, B. and Kier, L.B. (1990). Water-Dragging Effect A New Experimental Hydration Parameter Related to Hydrogen-Bond-Donor Acidity. J.Phys.Chem., 94, 4764-4766. 

Taft, R.W. and Kamlet, M.X (1979). Linear Solvation Energy Relationships. Part 4. Correlations with and Limitations of the a Scale of Solvent Hydrogen Bond Donor Acidities. J.Chem.Soc. Perkin Trans.2,1723-1729. 

Abstract This chapter presents the design and analysis of the microscopic features of binary solvent systems formed by ionic liquids, particularly room temperature ionic liqnids with molecular solvents. Protic ionic liquids, ethylammonium nitrate and l-n-butyl-3-methylmidazohum (bmim)-based ILs, were selected considering the differences in their hydrogen-bond donor acidity. The molecular solvents chosen were aprotic polar (acetonitrile, dimethylsulphoxide and MA(-dimethylformide) and protic (different alcohols). The empirical solvatochromic parameters n, a and P were employed in order to analyse the behaviour of each binary solvent system. The study focuses on the identification of solvent mixtures of relevant solvating properties to propose them as new solvents . Kinetic study of aromatic nucleophilic substitution reactions carried out in this type of solvent systems is also presented. On the other hand, this is considered as a new approach on protic ionic liquids. Ethylammonium nitrate can act as both Bronsted acid and/or nucleophile. Two reactions (aromatic nucleophilic substitution and nncleophilic addition to aromatic aldehydes) were considered as model reactions. 

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). 

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