Solvents, classification donors

Note Solvent classification into groups based on solvent polarity selectivity parameters proton acceptor, proton donor, x dipole interactors) and solvent strength on alumina nd on silica gel 0. Physical constants viscosity (t)), surface tension (y), dielectric constant (8). Solvatochromic polarity parameters 7, j.(30) and Ej. ... 

A classification of the solvents and empirical solvent strength (donor and acceptor strength) scales are given, based on various experimental parameters, together with various correlations empirically describing the solvent effect the scope of their use and limitations is discussed. Methods for the purification of solvents and ways of checking their purity are also presented. 

The donor and acceptor solvent classification is not entirely unambiguous either. There are solvents which may act both as donors and as acceptors [Au 53, Gu 68, Ja 49]. Examples are water and several of the alcohols. Their donor function is ensured by the electron pairs of their oxygen atoms, and the acceptor functions by their hydrogen atoms, which establish connections with the electron-pair donor anions by hydrogen bonding. 

Xi will represent a selectivity parameter for measuring the proton donor character i = d), the proton acceptor character i = e), and the strong dipole character (i = n). The sum of the three parameters is thus normalized to 1. Polarity index and selectivity parameters for some solvents are listed in Table 2. Using this approach, solvents that are chemically similar yield similar selectivity parameters, but solvents that are chemically different may appear in a single selectivity group. Some discrepancies can occur between the solvent classification according to the solubility parameters and to the polarity... 

The values of Ej(30) and are known for several hundred solvents. Based on this large data set, molecular solvents can be classified into three groups hydrogen-bond donor (HBD) or protic solvents (Ej from 0.5 to 1.0), dipolar non-hydrogen-bond donor (non-HBD) or dipolar aprotic solvents (Ej from 0.3 to 0.5), and apolar non-HBD solvents (Ej from 0.0 to 0.3), confirming the earlier qualitative solvent classification of Parker. ... 

Table 3. Classification of Donors, Acceptors, and Solvents Based on Polarizability...

On the Brpnsted theory (p. 51), solutions with concentrations of H3O+ greater than that in pure water are acids (proton donors), and solutions rich in OH are bases (proton acceptors). The same classifications follow from the solvent-system theory of acids and bases... 

The solvent triangle classification method of Snyder Is the most cosDBon approach to solvent characterization used by chromatographers (510,517). The solvent polarity index, P, and solvent selectivity factors, X), which characterize the relative importemce of orientation and proton donor/acceptor interactions to the total polarity, were based on Rohrscbneider s compilation of experimental gas-liquid distribution constants for a number of test solutes in 75 common, volatile solvents. Snyder chose the solutes nitromethane, ethanol and dloxane as probes for a solvent s capacity for orientation, proton acceptor and proton donor capacity, respectively. The influence of solute molecular size, solute/solvent dispersion interactions, and solute/solvent induction interactions as a result of solvent polarizability were subtracted from the experimental distribution constants first multiplying the experimental distribution constant by the solvent molar volume and thm referencing this quantity to the value calculated for a hypothetical n-alkane with a molar volume identical to the test solute. Each value was then corrected empirically to give a value of zero for the polar distribution constant of the test solutes for saturated hydrocarbon solvents. These residual, values were supposed to arise from inductive and... 

Protic character. The protic character of the solvent is an important consideration because electrochemical intermediates (particularly radical anions) frequently react rapidly with protons. The classification of solvents into protic or aptotic solvents is somewhat arbitrary. A simple classification1 is that protic solvents (such as hydrogen fluoride, water, methanol, formamide, and ammonia) are strong hydrogen-bond donors, exchange protons rapidly, and in-... 

A classification of solvents can be developed on the basis of the stability of the radial anion produced by reduction of aromatic hydrocarbons, such as naphthalene and anthracene. The solvent reactions of such anions have been widely studied2 and have generally been found to go by a sequence of reactions in either a protic solvent or in the presence of a proton donor in an aprotic solvent 3... 

This classification has been broadened39,40 by replacing the Brpnsted acid (proton donor) with a Lewis acid (an electron acceptor) and the Brpnsted base with a Lewis base (an electron donor). (A Brpnsted acid is a Lewis acid but not necessarily vice versa.) Solvent-proton interactions are therefore included as one subdivision of this classification, but many solvation reactions of cations with solvents also will be included as reactions of Lewis acid-base systems. This approach still does not solve the problem of fitting specific solvation interactions into the classification scheme. For example, acetonitrile behaves as a good Lewis base toward silver ion, but a poor one toward hydronium ion. The broader scheme also does not specifically take into account hydrogenbonding effects in hydroxylic and other solvents, which affect both the dielectric... 

The technique used to develop the four-solvent systems was based on procedures elucidated by Lehrer (6), Rohrschneider (7), and Glajch (8). After trials with individual solvents chosen from the comers of the Snyder solvent-selectivity triangle—a system of classification of solvents by the degree to which they function as proton donors, proton acceptors, or dipole interactors—an ideal solvent system was calculated. Ethanol, acetonitrile, and tetrahydrofuran were the reverse-phase solvents used, and water was the carrier solvent. Once the ideal solvent strength of one solvent-water combination was empirically determined, that of the other combinations could be estimated by use of the following equation (9) ... 

Solvents can be classified as EPD or EPA according to their chemical constitution and reaction partners [65]. However, not all solvents come under this classification since e.g. aliphatic hydrocarbons possess neither EPD nor EPA properties. An EPD solvent preferably solvates electron-pair acceptor molecules or ions. The reverse is true for EPA solvents. In this respect, most solute/solvent interactions can be classified as generalized Lewis acid/base reactions. A dipolar solvent molecule will always have an electron-rich or basic site, and an electron-poor or acidic site. Gutmann introduced so-called donor numbers, DN, and acceptor numbers, AN, as quantitative measures of the donor and acceptor strengths [65] cf. Section 2.2.6 and Tables 2-3 and 2-4. Due to their coordinating ability, electron-pair donor and acceptor solvents are, in general, good ionizers cf. Section 2.6. 

A m or challenge to completing a practical model and description of mobile-phase effects in LSC is the further elucidation of hydrogenbonding effects. This will involve a more fundamental classification of solutes and solvents in terms of their proton-donor and proton-acceptor properties, so that values of can be estimated as a function of the molecular structures of solute X and solvent C. It will also require a more precise description of the adsorbate-surface bonding that occurs in the adsorbed monolayer, so that values of can likewise be rationalized and predicted. 

Most chiral HPLC analyses are performed on CSPs. General classification of CSPs and rules for which columns may be most appropriate for a given separation, based on solute structure, have been described in detail elsewhere. Nominally, CSPs fall into four primary categories (there are additional lesser used approaches) donor-acceptor (Pirkle) type, polymer-based carbohydrates, inclusion complexation type, and protein based. Examples of each CSP type, along with the proposed chiral recognition mechanism, analyte requirement(s), and mode of operation, are given in Table 3. Normal-phase operation indicates that solute elution is promoted by the addition of polar solvent, whereas in reversed-phase operation elution is promoted by a decrease in mobile-phase polarity. 

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. ... 

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