Solvents, properties

Solvent Properties.—Dielectric Constant. The dependence of rate constants, for substitution and for redox reactions, on solvent dielectric constant (Z ) has long been a popular subject of investigation and source of mechanistic assignment and speculation. The linearity of plots of logarithms of rate constants (log k) against 1/D or against D - 1)/(2Z + 1) has been used to support the attribution of a dissociative mechanism for a variety of reactions of cobalt(iii) complexes. Thus linear 

In the case of reactions of ra//5-ICo(dmgH)2X(tu)] (X = Cl, Br, I, or NCS ) with thiourea, linearity of plots of log k vs. (D — 1)/(2D + 1) is said to indicate D mechanisms, which this time is consistent with strong irrmsAabiliTaXion by the thiourea. For aquation of /raKj-[Rh(dmgH)2Cl(tu)] in aqueous ethanol, propan-2-ol, or acetone, log k vs. (D — 1)/(2D + 1) is linear for the reverse anation reaction it is log k vs. IID which is linear. This time, in contrast to assignments above, the former reaction is assigned an Id mechanism, the latter a D mechanism.  

The variation of rate constant with solvent dielectric constant for the formation reactions of nickel(ii) with ammonia in aqueous alcohols and of iron(m) with chloride in aqueous methanol is in both cases consistent with dissociative activation. 

Plots of log A vs. IID are smooth, gentle curves for two reactions thought to proceed by associative mechanisms, base hydrolysis of [PPh4]Cl in aqueous acetone or dioxan and aquation of [Cr(ox)3] in aqueous dioxan.  

Linear log k vs. IID plots have also been reported for some redox reactions which have, or may have, rate-determining substitution steps previous to electron transfer. Examples include reactions of [Mn(ox)2(OH2)2l in aqueous DMF or DMSO and of iodide with bromate in aqueous ethanol.  

Solvent Properties.—Dielectric Constant. The examination of the dependence of rate on dielectric constant remains a popular exercise, for substitution as well as for redox reactions. The small increase in the rate of hydrolysis of 2,4-dichlorophenyl phosphate on changing the solvent from 10% to 60% dioxan, i.e. on decreasing the dielectric constant markedly, is claimed to demonstrate dispersion of charge in transition-state formation. Other reactions of compounds of the ip-block elements where the effect of dielectric constant variation on rate is discussed include the semi-organic reactions of butyl-lithium with t-butyl peroxide and of formate with 

Determined trends of reactivity with solvent dielectric constant have been used, alongside other information, as evidence to favour dissociative mechanisms for the aquation of [Co(dmgH)2(S03)2] (ref. 17) and [Co(dioxH)2(S03)(amine)] (ref. 18) anions. A linear dependence of logarithms of rate constants against reciprocal dielectric constant was found for the aquation of one member of the latter series, the anion with dioxHa=methylglyoxime and amine=pyridine, in water-rich aqueous ethanol. A similarly tidy relation between rate constant and medium dielectric constant was not found for the reactions comprising the equilibrium of equation (4), in aqueous 

The dependence of rate constant on dielectric constant has also been considered in ring closure reactions of [PtX(gly)(amine)] complexes. Logarithms of rates of dissociation of [Ni(NH3)] + correlate linearly with reciprocal dielectric constant for acetone-water mixtures at several temperatures, and for series of methanol-. 

The normal practice in this area is to try to correlate logarithms of rate constants with reciprocal dielectric constants, as illustrated by several examples in the preceding paragraphs. However a recent article argues that dielectric constants themselves should be used. That the latter are preferable in at least some cases is proved by the sets of published results cited.  

A side effect of lowering the dielectric constant of a medium is to encourage ion-pair formation. It is this effect which determines trends in reaction rates for solvolysis of cis- and of rra 5-[Pt(N02)2L2], L=ammonia, pyridine, or triethylphosphine, in acetic acid-water mixtures containing sulphuric acid. An increase in ion-pairing increases the rate of solvolysis since the anion in the ion pair facilitates the loss of the leaving group, which is the nitrosonium cation.  

Nondestructive solvent extraction of coal is the extraction of soluble constituents from coal under conditions where thermal decomposition does not occur. On the other hand, solvolysis (destructive solvent extraction) refers to the action of solvents on coal at temperatures at which the coal substance decomposes and in practice relates in particular to extraction at temperatures between 300 and 400°C (572 and 752°F). In the present context (i.e., the solvents extraction of coal), the solvent power of the extracting liquid appears to be determined solely by the ability of the solvent to alter the coal physically (by swelling). In this respect, the most effective solvents are aromatics, phenol derivatives, naphthol derivatives, anthracene, and phenanthrene. 

For the purposes of this chapter, the solvent extraction of coal is limited to those investigations and test methods that are separate form the high-temperature treatment of coal with solvents in which the production of liquid products (liquefaction) is the goal. 

There have been many attempts to define solvent behavior in terms of one or more physical properties of the solvent, and not without some degree of success. However, it is essential to note that the properties of the coal also play an important role in defining behavior of a solvent, and it has been reported that the relative solvent powers of two solvents may be reversed from one coal type to another. Thus, two properties that have found some relevance in defining solvent behavior with coal (as well as with other complex carbonaceous materials, such as petroleum asphaltenes) are the surface tension and the internal pressure (Speight, 1994, p. 201). However, the solvent power of primary aliphatic amines (and similar compounds) for the lower-rank coals has been attributed to the presence of an unshared pair of electrons (on the nitrogen atom). 

Early work on the solvent extraction of coal was focused on an attempt to separate from coal a coking principle (i.e., the constituents believed to be responsible for coking and/or caking properties). But solvent extraction has actually been used to demonstrate the presence in coal of material that either differed from the bulk of the coal substance or was presumed to be similar to the bulk material. 

An example of the difference of the solvent extracts from the bulk material comes from a series of studies on the exhaustive extraction of coal by boiling pyridine and fractionation of the regenerated soluble solids by sequential selective extraction schemes (Berkowitz, 1979). Subsequent analyses showed that the petroleum ether-soluble material was mostly composed of hydrocarbons (e.g., paraffins, naphthenes, and terpenes), while the ether-soluble, acetone-soluble, and acetone-insoluble fractions were resinlike substances with 80 to 89% carbon and 8 to 10% hydrogen. Indeed, this and later work (Vahrman, 1970) led to the concept that coal is a two-component or two-phase system (Derbyshire et al., 1991 Yun et al., 1991). 

the numbers in brackets represent the number of observations available for each solvent. 

1 Modelling diffusion coefficients The Wilke-Chang model is 

The following model, suggested by analogy, is the initial starting point in our new analysis  

Taking logs of this expression gives the following linear model  

This model can now be fitted and the relevant coefficients estimated using least squares regression in SAS [11]. 

Marcus [155] showed that the gas solubilities in molten salts can be expressed according to a general solubifity expression in terms of the cohesive energy density of the salt (the square of its Hildebrand solubility parameter) as  

In earlier attempts of the rationalization of gas solubilities in molten salts Blander et al. [292] used the Uhlig model, according to which the solubility depends on the work required for the production of the cavity in the molten salt in which the gas molecule is to reside. This, in turn, depends on the surface tension of the salt melt and the van der Waals surface area of the gas molecules. This approach, however, under-estimates the actual solubilities [289, 290]. 

Lide D (ed) (2001-2002) Handbook of chemistry and physics, 82nd edn. CRC Press, Baton Rouge 

Barin I, Knacke O (1973) Thermochemical properties of inorganic substances. Springer, Berlin 

Janz GJ (1967) Molten salts handbook. Academic, New York 

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The powerful solvent properties of dimethylsulfoxide (DMSO), for example, are used to dissolve selectively the polynuclear aromatics found in oils and paraffins. The procedure is shown in Figure 2.5. 

Dearomatized or not, lamp oils correspond to petroleum cuts between Cio and C14. Their distillation curves (less than 90% at 210°C, 65% or more at 250°C, 80% or more at 285°C) give them relatively heavy solvent properties. They are used particularly for lighting or for emergency signal lamps. These materials are similar to kerosene solvents , whose distillation curves are between 160 and 300°C and which include solvents for printing inks. 

F. M. Fowkes, in Solvent Properties of Surfactant Solutions, K. Shinoda, ed., Marcel Dekker, New York, 1967. 

Homologous mono-alkyl ethers of ethylene glycol, such as monoethyl glycol (or 2-ethoxyethanol), HOC2H4OC2H5, form excellent solvents as they combine to a large extent the solvent properties of alcohols and ethers. The monoethyl and the monomethyl members have the technical names of ethyl cellosolve and methyl cellosolve respectively. Dioxan... 

HC0N(CH3)2, also possess exceptional solvent properties. The alkyl-glycols, dioxan and dimethyl-formamide should be used with caution, however, as their hot vapours are poisonous. 

The most widely used cleansing agent is the chromic acid cleaning mixture. It is essentially a mixture of chromic acid (CrOj) and concentrated sulphuric acid, and possesses powerful oxidising and solvent properties. Two methods of preparation are available —... 

Solvents exert their influence on organic reactions through a complicated mixture of all possible types of noncovalent interactions. Chemists have tried to unravel this entanglement and, ideally, want to assess the relative importance of all interactions separately. In a typical approach, a property of a reaction (e.g. its rate or selectivity) is measured in a laige number of different solvents. All these solvents have unique characteristics, quantified by their physical properties (i.e. refractive index, dielectric constant) or empirical parameters (e.g. ET(30)-value, AN). Linear correlations between a reaction property and one or more of these solvent properties (Linear Free Energy Relationships - LFER) reveal which noncovalent interactions are of major importance. The major drawback of this approach lies in the fact that the solvent parameters are often not independent. Alternatively, theoretical models and computer simulations can provide valuable information. Both methods have been applied successfully in studies of the solvent effects on Diels-Alder reactions. 

The solvents listed in Table 2.1 were chosen to cover a broad range in solvent properties. In fact hexane was initially also among them, but unfortunately the rate of the reaction in this solvent is extremely low. It turned out that in this solvent spontaneous decomposition of 2.4a competes with the Diels-Alder reaction. 

At one time benzene was widely used as a solvent This use virtually disappeared when statistical studies revealed an increased incidence of leukemia among workers exposed to atmospheric levels of benzene as low as 1 ppm Toluene has replaced benzene as an inexpensive organic solvent because it has similar solvent properties but has not been determined to be carcinogenic m the cell systems and at the dose levels that benzene is... 

Solvent molecules Solvent Orange 60 Solvent Orange 63 Solvent preparation Solvent process Solvent properties... 

The extremely nonpolar character of PFCs and very low forces of attraction between PFC molecules account for their special properties. Perfluorocarbons bod only slightly higher than noble gases of similar molecular weight, and their solvent properties are much more like those of argon and krypton than hydrocarbons (2). The physical properties of some PFCs are Hsted in Table 1. 

The combined pharmaceutical appHcations account for an estimated 25% of DMF consumption. In the pharmaceutical industry, DMF is used in many processes as a reaction and crystallizing solvent because of its remarkable solvent properties. For example, hydrocortisone acetate [50-03-3] dihydrostreptomycin sulfate [5490-27-7] and amphotericin A [1405-32-9] are pharmaceutical products whose crystallization is faciHtated by the use of DMF. Itis also a good solvent for the fungicide griseofulvin/72%(97-< 7 and is used in its production. 

ROOC—COOH, are not. The dialkyl esters are characterized by good solvent properties and serve as starting materials in the synthesis of many organic compounds, such as pharmaceuticals, agrochemicals, and fine chemicals (qv). Among the diesters, dimethyl, diethyl, and di- -butyl oxalates are industrially important. Their physical properties are given in Table 7. 

Solvent. The solvent properties of water and steam are a consequence of the dielectric constant. At 25°C, the dielectric constant of water is 78.4, which enables ready dissolution of salts. As the temperature increases, the dielectric constant decreases. At the critical point, the dielectric constant is only 2, which is similar to the dielectric constants of many organic compounds at 25°C. The solubiUty of many salts declines at high temperatures. As a consequence, steam is a poor solvent for salts. However, at the critical point and above, water is a good solvent for organic molecules. 

Sulfolane was first described ia the chemical Hterature ia 1916. It has been noted for its exceptional chemical and thermal stabiUty and unusual solvent properties. The search for a commercial process to produce sulfolane began about 1940. Market development quantities became available ia 1959. Siace then, both the use of and the appHcations for sulfolane have iacreased dramatically. 

Reactions. Supercritical fluids are attractive as media for chemical reactions. Solvent properties such as solvent strength, viscosity, diffusivity, and dielectric constant may be adjusted over the continuum of gas-like to Hquid-like densities by varying pressure and temperature. Subsequently, these changes can be used to affect reaction conditions. A review encompassing the majority of studies and apphcations of reactions in supercritical fluids is available (96). 

Distillation By-Products. Of the CTO distiHation by-products, ie, pitch, heads, and DistiHed TaH Oil (DTO), only the last, a unique mixture of rosin and fatty acids, has significant commercial value. Pitch and heads are used as fuel the former has a fuel value of 41,800 kj/kg. TaH oil heads have outstanding solvent properties, but also have a bad odor, which is hard to remove. They contain a relatively high fraction of palmitic acid which can be recovered by crystallization. 

Salts and Derivatives. Generally the vitamers are high melting crystalline soHds that are very soluble in water and insoluble in most other solvents. Properties of the common forms are Hsted in Table 1. The only commercially important form of vitamin B is pytidoxine hydrochloride (7). This odorless crystalline soHd is composed of colorless platelets melting at 204—206°C (with decomposition). In bulk, it appears white and has a density of - 0.4 kg/L. It is very soluble in water (ca 0.22 kg/L at 20°C), soluble in propylene glycol, slightly soluble in acetone and alcohol (ca 0.014 kg/L), and insoluble in most lipophilic solvents. A 10% water solution shows a pH of 3.2. Both the hydrochloride and corresponding free base sublime without decomposition (16). 

The NF and reagent grades are employed in the pharmaceutical industry which makes use of benzyl alcohol s local anesthetic, antiseptic, and solvent properties (17—20). It also finds use in cough symps and drops ophthalmic solutions bum, dental (21), and insect repeUant solutions and ointments and dermatological aerosol sprays. It is used in nail lacquers and as a color developer in hair dyes by the cosmetics industry (22), and in acne treatment preparations (23). 

Tetrachloroethylene [127-18-4] perchloroethylene, CCl2=CCl2, is commonly referred to as "perc" and sold under a variety of trade names. It is the most stable of the chloriaated ethylenes and ethanes, having no flash poiat and requiring only minor amounts of stabilizers. These two properties combiaed with its excellent solvent properties account for its dominant use ia the dry-cleaning iadustry as well as its appHcation ia metal cleaning and vapor degreasiag. 

Chlorotoluene [106-43-4] (l-chloto-4-methylbenzene, PCT) and y -chlorotoluene [108-41-8] (l-chloto-3-methylbenzene, MCT) ate mobile, colorless Hquids with solvent properties similar to those of the ortho isomer. 

K. Shiaoda, ed.. Solvent Properties of Suf actant Solutions, Suf actant Science Series, Vol. 2, Marcel Dekker, Inc., New York, 1967. 

In addition to its ability to make the separation feasible or easier, the ideal solvent is inexpensive, readily available, nontoxic, noncorrosive, thermally stable, and nonreactive with and easily separated from the other components in the mixture. In reaUty some compromise in solvent properties is almost always required. 

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