Dielectric constants, of common solvents

The Structure of Ice and Water. It has not yet been necessary to consider in detail the properties of particular solvents. In Table 1 we gave a list of values for the dielectric constants of various solvents but apart from this we have not yet paid attention to the observed properties of solvents or of the ions which are commonly dissolved in them. Before continuing the discussion which was in progress in Sec. 23, it will be useful to review in some detail the state of our knowledge of the liquids which are used as solvents, and of the species of ions which are most often studied in solution. Although non-aqueous solutions are of great interest for the sake of comparison, nevertheless aqueous solutions are still of paramount importance, and we shall pay most of our attention to H20 and D20 and to ions dissolved in these liquids. 

Aryl and, more so, chlorine substituents on silicon enhance thermal stability of silacyclobutanes. The rate of the first-order thermal decomposition of silacyclobutanes varies inversely with the dielectric constant of the solvent used. Radical initiators have no effect on the thermal decomposition and a polar mechanism was suggested. Thermal polymerization of cyclo-[Ph2SiCH212 has been reported to occur at 180-200°C. The product was a crystalline white powder which was insoluble in benzene and other common organic solvents [19]. 

As a rule, the presence of a high dielectric constant medium leads to the rise of the dipole moment of the compound. However, the relative increase of the dipole moment varies substantially from one structure to another. For instance, the calculated dipole moment of 2(H)-3-pyrazolone (7) increases by 72% when transferred from the medium with e = 1 to the medium with s = 80, whereas the corresponding change in the dipole moment of pyrazole (3) is only 15%. Therefore, it is questionable to assume constant dipole moment values for a series of congeneric compounds in different dielectric media and derive correlations of the chemical reactivity or physical properties of such series of compounds with some fixed function of the dielectric constant of the solvent (a common approach in the linear free energy relationship studies of solvent effects, cf. [65-67]). 

Solute solubility in the solvent chosen for extraction can be enhanced by various manipulations of which the more commonly employed are Variation of the dielectric constant of the solvent, salting-in, hydrogen bonding and pH adjustment. These manipulations are illustrated below with special reference to biochemically important extractions wherein their role is crucial. 

Perhaps the most common way to calculate solvent effects is to use a continuum solvent model. Here, the molecular structure of the solvent is ignored and the solvent is modeled as a continuous dielectric of infinite extent that surrounds a cavity containing the solute molecule M. (A dielectric is a nonconductor of electricity.) The continuous dielectric is characterized by its dielectric constant (also called relative permittivity) e whose value is the experimental dielectric constant of the solvent at the temperature and pressure of the solution. The solute molecule can be treated classically as a collection of charges that interacts with the dielectric or it can be treated quantum mechanically. In a quantum-mechanical treatment, the interaction between a solute... 

For the development of edible and biodegradable bioplastics, it is required solvents and a pH regulating agent, when necessary, in addition to the plasticizer and polymer. The pH adjustment in the case of proteins is necessary to control the solubility of the polymer. Some regulators of pH found in the literature [13] acetic acid and sodium hydroxide. The solvents commonly used to prepare these bioplastics are water, ethanol or a combination of both [14]. A crucial aspect in the preparation of films is the solubility of proteins and the ability to interact with the same solvent used, since the total solubility of the protein is required for films formation [15]. The dispersion of the protein molecule in water is possible due to the large number of amino acid residues that interact with the polar solvents. These interactions can be improved depending on the dielectric constant of the solvent, since this constant is inversely proportional to the strength of intermolecular attraction. Films can be simple, made with one type of macromolecule or composed by two or more types of macromolecules, and can be formed with two or... 

A common solvent effect for the three reactions is obtained as far as die dielectric constant of the solvent is augmented, the energy difference between PI and P2... 

Common solvents can be divided into two groups protic and aprotic. Furthermore, solvents are classified as polar and nonpolar based on their dielectric constant. The greater the value of the dielectric constant of a solvent, the better it solvates and thus the smaller the interaction between ions of opposite charge dissolved in it. We say that a solvent is a polar solvent if it has a dielectric constant of 15 or greater. A solvent is a nonpolar solvent if it has a dielectric constant of less than 5. Solvents with a dielectric constant between 5 and 15 are borderline. 

The alkaline oxidations of etamsylate (ETM) and salbutamol (SBL) , and the acidic oxidations of phenylephrine (PHE) and butacaine sulfate (BCS) " with N-chlorobenzenesulfonamide (CAB) in MeOH have some common features. The rates in each case increase with decreasing dielectric constant of the solvent. The reactions are of fractional order in ETM, SBL, PHE, and BCS. A fractional order in OH and H" " ions is noted in the reaction of ETM and PHE, respectively. However, a negative fractional order in OH ions and rate retardation with benzenesulfonamide (BSE) is observed in the oxidation of SBL. The observed Michaelis-Menten kinetics provide the basis of the proposed mechanism in each case. The Ir(IIl)-catalysed A-chlorobenzenesulfonamide (CAB) oxidation of DMSO in acidic solution has an order less than one in Ir(III) and the rate decreased with increase in H" " ions. A suitable mechanism involving formation of an intermediate is proposed. ... 

Both entropy and enthalpy change have to be considered when dissolving a salt in any solvent Dissolution can lead to either a positive or negative overall entropy change. In polymer electrolytes, a negative entropy of dissolution is common and can be an important consideration at higher temperatures. This effect arises because the dielectric constant of the solvent polymer (solid or liquid) is usually... 

A distance-dependent dielectric constant is commonly used to mimic the effect of solvent in molecular mechanics calculations, in the absence of explicit water molecules. 

Many other measures of solvent polarity have been developed. One of the most useful is based on shifts in the absorption spectrum of a reference dye. The positions of absorption bands are, in general, sensitive to solvent polarity because the electronic distribution, and therefore the polarity, of the excited state is different from that of the ground state. The shift in the absorption maximum reflects the effect of solvent on the energy gap between the ground-state and excited-state molecules. An empirical solvent polarity measure called y(30) is based on this concept. Some values of this measure for common solvents are given in Table 4.12 along with the dielectric constants for the solvents. It can be seen that there is a rather different order of polarity given by these two quantities. 

Water is the most common solvent used to dissolve ionic compounds. Principally, the reasons for dissolution of ionic crystals in water are two. Not stated in any order of sequence of importance, the first one maybe mentioned as the weakening of the electrostatic forces of attraction in an ionic crystal known, and the effect may be alternatively be expressed as the consequence of the presence of highly polar water molecules. The high dielectric constant of water implies that the attractive forces between the cations and anions in an ionic salt come down by a factor of 80 when water happens to be the leaching medium. The second responsible factor is the tendency of the ionic crystals to hydrate. 

Table 6.5 Dielectric Constants of Some Common Solvents... 

A common technique for measuring the values has been to employ species that produce anions with useful ultraviolet (UV) or visible (vis) absorbances and then determine the concentrations of these species spectropho tome trie ally. Alternatively, NMR measurements could be employed, but generally they require higher concentrations than the spectrophotometric methods. A hidden assumption in Eq. 5 is that the carbanion is fully dissociated in solution to give a free anion. Of course, most simple salts do fully dissociate in aqueous solution, but this is not necessarily true in the less polar solvents that are typical employed with carbanion salts. For example, dissociation is commonly observed for potassium salts of carbanions in DMSO because the solvent has an exceptionally large dielectric constant (s = 46.7) and solvates cations very well, whereas dissociation occurs to a small extent in common solvents such as DME and THE (dielectric constants of 7.2 and 7.6, respectively). In these situations, the counterion, M+, plays a role in the measurements because it is the relative stability of the ion pairs that determines the position of the equilibrium constant (Eq. 6). 

Whereas the polarity effect is ascribed to the dielectric constant, the hydrophobic effect is a consequence of the high cohesive energy density (c.e.d.) of water, resulting from a unique hydrogen-bonding network (Lubineau et al., 1994). Given table 6.5, which compares the cohesive energy density and the dielectric constant of selection of common solvents at 25°C, there is no correlation between the structuralization and the polarity of the solvents. 

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