Dielectric constant common solvents

Solvents which are poor donors are commonly used in glycoside synthesis, for instance dichlorometh-ane, cyclohexane or petroleum ether. These solvents favor SN2-type reactions. Solvents which are better donors, for instance ethers (diethyl ether, THF, etc.), acetonitrile, pyridine, nitromethane etc., each result in a typical change in the reaction course due to their different participation in the stabilization of the reaction intermediates. With ethers, acetonitrile and pyridine participation leads to onium-type intermediates (Scheme 5 8 and 9), which eventually provide, via fast equilibration, mainly the -anomer (8), due to their higher thermodynamic stability, based on the inverse anomeric effect .Thus a-product formation is often favored in these solvents (see Section 1.2.3.2.5). Solvents with even higher dielectric constants commonly result in lower diastereocontrol in glycoside synthesis. 

Another common approach is to do a calculation with the solvent included in some approximate manner. The simplest way to do this is to include the solvent as a continuum with a given dielectric constant. There are quite a few variations on this technique, only the most popular of which are included in the following sections. 

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. 

Table 1. Solubility of Aminophenols in Common Solvents Arranged in Order of Increasing Polarity (Dielectric Constant)...

Solvent for Displacement Reactions. As the most polar of the common aprotic solvents, DMSO is a favored solvent for displacement reactions because of its high dielectric constant and because anions are less solvated in it (87). Rates for these reactions are sometimes a thousand times faster in DMSO than in alcohols. Suitable nucleophiles include acetyUde ion, alkoxide ion, hydroxide ion, azide ion, carbanions, carboxylate ions, cyanide ion, hahde ions, mercaptide ions, phenoxide ions, nitrite ions, and thiocyanate ions (31). Rates of displacement by amides or amines are also greater in DMSO than in alcohol or aqueous solutions. Dimethyl sulfoxide is used as the reaction solvent in the manufacture of high performance, polyaryl ether polymers by reaction of bis(4,4 -chlorophenyl) sulfone with the disodium salts of dihydroxyphenols, eg, bisphenol A or 4,4 -sulfonylbisphenol (88). These and related reactions are made more economical by efficient recycling of DMSO (89). Nucleophilic displacement of activated aromatic nitro groups with aryloxy anion in DMSO is a versatile and useful reaction for the synthesis of aromatic ethers and polyethers (90). 

Most organic reactions are done in solution, and it is therefore important to recognize some of the ways in which solvent can affect the course and rates of reactions. Some of the more common solvents can be roughly classified as in Table 4.10 on the basis of their structure and dielectric constant. There are important differences between protic solvents—solvents fliat contain relatively mobile protons such as those bonded to oxygen, nitrogen, or sulfur—and aprotic solvents, in which all hydrogens are bound to carbon. Similarly, polar solvents, those fliat have high dielectric constants, have effects on reaction rates that are different from those of nonpolar solvent media. 

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. 

Table 8-2 lists several physical properties pertinent to our concern with the effects of solvents on rates for 40 common solvents. The dielectric constant e is a measure of the ability of the solvent to separate charges it is defined as the ratio of the electric permittivity of the solvent to the permittivity of the vacuum. (Because physicists use the symbol e for permittivity, some authors use D for dielectric constant.) Evidently e is dimensionless. The dielectric constant is the property most often associated with the polarity of a solvent in Table 8-2 the solvents are listed in order of increasing dielectric constant, and it is evident that, with a few exceptions, this ranking accords fairly well with chemical intuition. The dielectric constant is a bulk property. 

Until one develops a feel for recrystallization, the best procedure for known compounds is to duplicate a selection in the literature. For new compounds, a literature citation of a solvent for an analogous structure is often a good beginning point. To assist in the search, Table A3.4 lists several of the common recrystallizing solvents with useful data. The dielectric constant can be taken to be a rough measure of solvent polarity. 

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. 

An interface between two immiscible electrolyte solutions (ITIES) is formed between two liqnid solvents of a low mutual miscibility (typically, <1% by weight), each containing an electrolyte. One of these solvents is usually water and the other one is a polar organic solvent of a moderate or high relative dielectric constant (permittivity). The latter requirement is a condition for at least partial dissociation of dissolved electrolyte(s) into ions, which thus can ensure the electric conductivity of the liquid phase. A list of the solvents commonly used in electrochemical measurements at ITIES is given in Table 32.1. 

The physical constants (viscosity T], surface tension y, and dielectric constant e) [26] for some common solvents used in chromatography are given in Table 4.2. 

Water, however, is a wonderful solvent for ionic-bonded substances such as salt. The secret to its success lies in the electric dipoles created by the polar covalent bonds between the hydrogen and oxygen atoms. In water, the polar bonds are asymmetric. The hydrogen side is positive the oxygen side is negative. One measure of the amount of charge separation in a molecule is its dielectric constant. Water has a dielectric constant that is considerably higher than that of any other common liquid. 

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. 

Three types of methods are used to study solvation in molecular solvents. These are primarily the methods commonly used in studying the structures of molecules. However, optical spectroscopy (IR and Raman) yields results that are difficult to interpret from the point of view of solvation and are thus not often used to measure solvation numbers. NMR is more successful, as the chemical shifts are chiefly affected by solvation. Measurement of solvation-dependent kinetic quantities is often used (<electrolytic mobility, diffusion coefficients, etc). These methods supply data on the region in the immediate vicinity of the ion, i.e. the primary solvation sphere, closely connected to the ion and moving together with it. By means of the third type of methods some static quantities entropy and compressibility as well as some non-thermodynamic quantities such as the dielectric constant) are measured. These methods also pertain to the secondary solvation-sphere, in which the solvent structure is affected by the presence of ions, but the... 

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]. 

These YA values are found not to run in parallel with the dielectric constant values for the solvents concerned. Obviously the dielectric constant value for the solvent must be involved in some way in YA, as separation of opposite charges is a crucial feature of the rate-limiting step in an SN1 reaction formation of the T.S. leading to the ion-pair intermediate (47). But specific solvation of the separating charges must also be involved and YA will reflect those, and quite possibly other properties of the solvent as well. It is common to describe YA as representing a measure of the ionising power of the solvent A. 

Table 6.5 Dielectric Constants of Some Common Solvents... 

In this work we have demonstrated that a new class of heavily fluorinated acrylic and methacrylic resins can be efficiently synthesized and then cured to solid form with radical initiator at elevated temperatures. These cured resins were found to have low dielectric constants, which are close to the minimum known values for Teflon and Teflon AF. In contrast to tetrafluoroethylene, our monomers are processable owing to the fact that they are liquids or low-melting solids, and moreover are soluble in common organic solvents. Lower dielectric constants are obtained as fluorine contents on the polymer backbone or side chain increase, when acrylate is replaced by methacrylate, when ether linkages are present in the fluorocarbon, and when aromatic structure is symmetrically meta-substituted. 

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