Basicity of solvents

The chemistry of Lewis acid-base adducts (electron-pair donor-acceptor complexes) has stimulated the development of measures of the Lewis basicity of solvents. Jensen and Persson have reviewed these. Gutmann defined the donor number (DN) as the negative of the enthalpy change (in kcal moL ) for the interaction of an electron-pair donor with SbCls in a dilute solution in dichloroethane. DN has been widely used to correlate complexing data, but side reactions can lead to inaccurate DN values for some solvents. Maria and Gal measured the enthalpy change of this reaction... 

In ion solvation, the solvent molecules approach a cation with their negative charge and approach an anion with their positive charge (Fig. 2.1). Therefore, cation solvation is closely related to the electron pair donor capacity or Lewis basicity of solvents and tends to become stronger with the increase in donor number (DN). On the other hand, the anion solvation is closely related to the electron pair acceptability or Lewis acidity of solvents and tends to become stronger with the increase in acceptor number (AN). 

Physical organic chemists have tended to examine parameters based on shifts in the absorption peaks in the spectra of various dyes or indicator molecules. The a and P scales of Taft and Kamlet, the ET(30) scale of Dimroth and Reichardt, the 7t scale of Taft and co-workers and the Z value of Kosower are all examples of this type of parameter. The definitions and measurement means for these parameters, as well as important references, are shown in Table 5. An alternative definition of the Dimroth-Reichardt parameter is the dimensionless, ETN, which is now preferred by some organic chemists (for a discussion see Ref. 15). The Z value is important in that it led to the scale of Dimroth and Reichardt, which overcomes many of the limitations of the earlier scale. Several workers have shown that relationships exist, with good correlation coefficients, between similar parameters. Thus, DN is linearly related to p, both parameters being designed to measure the donor properties (or Lewis basicity) of solvent molecules. Also, Lr(30) is related to a as well as to AN all three parameters purport to measure the electron acceptor properties (or Lewis acidity) of solvent molecules. It has been found that different solvent types have different coefficients in linear relationships between n and the dipole moment. The Taft and Dimroth-Reichardt parameters, in particular, have been found to correlate with free energies and... 

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). 

Solvent effects in heterogeneous catalysis are examined in terms of physical or chemical modifications to control the chemo-, regio- and stereoselectivity of a reaction. The main factors affecting selectivity are reactant solubility, polarity, reactivity or acido-basicity of solvents and competitive chemisorption of products and solvents In the special case of molecular sieves, selectivity control of a reaction by competitive adsorption, diffusion or shape selectivity and confinement catalysis are also examined. 

LNA, which is an acid, can adsorb on alumina via acid-base interaction or hydrogen bonding. The structural formula of LNA and those of other organic dispersants are given in Fig. 2. The interaction between the solvent and LNA cannot be responsible for the viscosity curves because the most basic solvent, tetrahydrofuran (THF) [11], would interact with LNA the most. The acid-base interaction is based on Drago s work on Lewis acid-base interaction energies [12]. Fowkes [2,13,14] expanded the concept to ceramic processing. The relative acidity and basicity of solvents used are listed in Table 1. The ceramic powders used are listed in Table 2. 

Reduction of 3-trimethy1s11yl-2-propyn-l-ol exemplifies the problem of stereoselectivity in hydride reduction of acetylenic alcohols to E-allyl alcohols, Early reports that lithium aluminum hydride stereoselectively reduced acetylenic alcohols gave way to closer scrutiny which revealed a striking solvent dependence of the stereochemistry, Specifically, the percentage of trans reduction is seen to increase with increasing Lewis basicity of solvent. Similarly, the addition of less Lewis acidic cations to the reducing mixture leads to improved trans/cis ratios, Sodium b1s(2-... 

We shall consider the problems of acidity and basicity of solvents, medium effects, acidity scales, homo- and hetero-conjugation, ionisation and dissociation. Experimental investigations of the most important solvent systems are covered to mid-1970. The selection of a few papers for discussion from the voluminous literature of the subject necessarily reveals the interests and prejudices of the writer. The reader is referred elsewhere for the analytical applications. " The annual Fundamental Reviews published in Analytical Chemistry give full coverage to current developments with extensive references to both fundamental and applied work. 

In the following sections we examine the acidity and basicity of solvents, and medium effects. Thereafter, we take up the principal solvents one by one according to the classification scheme that has just been presented. 

Were we able to measure it directly, nYB would give us a way of expressing the basicity of solvents relative to some common standard. Water was chosen as the standard in our development of the concept, but if its peculiarities are judged to make it unsuitable, another solvent such as methanol or dimelliylformamide could be substituted. 

Their base strengths towards the proton could be regarded as a very good characteristic of the donor strengths of solvents. Unfortunately, the determination of an unambiguous scale of the exact basicities of the various solvent molecules towards the proton is difficult. Benoit and Domain [Be 74] consider the heat of solvation of the proton in the gaseous state to be the most suitable parameter for the characterization of the basicities of solvent molecules. In principle this is undoubtedly correct, but its practical determination is possible only by indirect means. 

The donor number is frequently used in various fields of polymer chemistry (see Chapter 10). Another classification based on acidity/basicity of solvents allows the division of solvents into six groups containing protic-neutral protogenic protophilic aprotic-protophilic aprotic-protophobic and aprotic-inert. ... 

Nicolet, P. and Laurence, C. (1986) Polarity and basicity of solvents. Part 1. A thermosolva-tochromic comparison method. J. Chem. Soc., Perkin Trans. 2,1071-1079. 

The DN scale has historical merits. It was among the first attempts to establish Lewis base strength quantitatively. It was widely used in the field of solution, coordination and organic chemistry to describe the Lewis basicity of solvents. However, after 40 years of use, convergent critical analyses have pointed out the serious limitations of DN. They concern (i) the choice of SbCls as a reference Lewis acid, (ii) the sample of Lewis bases studied, (iii) the quality of calorimetric measurements and (iv) the domain of validity of the scale. 

Koppel, I. and Paju, A. (1974) Parameters of general basicity of solvents. Reakts. Sposobn. Org. Soedin., 11,121-136. 

Finally, the total red shift of 5380 cm of the solvatochromic band of 4-nitrophenol, on going from the gas phase to acetonitrile solution, has been resolved into a 785 cm hydrogen-bond shift (15% of the total shift) and a 4595 cm shift caused by nonspecific (van der Waals) interactions. With triethylamine as solvent, the hydrogen-bond shift amounts to 42% of the total shift. These examples show the importance of quantifying the Lewis basicity of solvents for a quantitative description of solvent effects in chemistry. 

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