Solvation ability

Solubility can often be decreased by using a nonaqueous solvent. A precipitate s solubility is generally greater in aqueous solutions because of the ability of water molecules to stabilize ions through solvation. The poorer solvating ability of nonaqueous solvents, even those that are polar, leads to a smaller solubility product. For example, PbS04 has a Ks of 1.6 X 10 in H2O, whereas in a 50 50 mixture of H20/ethanol the Ks at 2.6 X 10 is four orders of magnitude smaller. 

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

Poor solvating ability of supercritical fluids High investment and operating costs... 

In mixed solvent systems the difference in the solvating abilities of solvent molecules S and S2 causes a selective solvation of cations and anions [119,120],... 

Changing the solvent in which a reaction is carried out often exerts a profound effect on its rate and may, indeed, even result in a change in its mechanistic pathway. Thus for a halide that undergoes hydrolysis by the SN1 mode, increase in the polarity of the solvent (i.e. increase in e, the dielectric constant) and/or its ion-solvating ability is found to result in a very marked increase in reaction rate. Thus the rate of solvolysis of the tertiary halide, Me3CBr, is found to be 3 x 104 times faster in 50% aqueous ethanol than in ethanol alone. This occurs because, in the S,vl mode, charge is developed and concentrated in... 

So far as actual changes of mechanistic pathway with change of solvent are concerned, increase in solvent polarity and ion-solvating ability may (but not necessarily will) change the reaction mode from SN2— SN1. Transfer from hydroxylic to polar, non-protic solvents (e.g. DMSO) can, and often do, change the reaction mode from SN1 — Sn2 by enormously increasing the effectiveness of the nucleophile in the system. 

The possible formation of a delocalised benzyl type carbocation (16) results in much lower (70%) ANTI stereoselectivity than with trans 2-butene (5 =100% ANTI stereoselectivity, p. 180), where no such delocalisation is possible. It is also found that increasing the polarity, and ion-solvating ability, of the solvent also stabilises the carbocation, relative to the bromium ion, intermediate with consequent decrease in ANTI stereoselectivity. Thus addition of bromine to 1,2-diphenylethene (stilbene) was found to proceed 90-100% ANTI in solvents of low dielectric constant, but =50% ANTI only in a solvent with e = 35. 

The degree of stereoselectivity may be influenced to some extent by the polarity and ion-solvating ability of the solvent. 

Radical cations of the most popular spin traps PBN and DMPO have been generated by the methods of ionizing radiolysis and laser flash-photolysis in solid matrices (435-437). As a polar solvent with high solvating ability for... 

The choice of an appropriate solvent is determined by the ionic character of organoxenonium salts, [RXe]Y, and their reactivity. One of the most suitable solvents is aHF because it possesses a high solvating ability and chemical stability towards oxidizing compounds. The salts [RXe]Y exist in aHF solution... 

Recent attempts to unify the polarity scales of solvents (for non-specific interactions) are of great interest in rationalizing the medium effects166. Generally, the spectroscopic properties of appropriate substances are used to check the solvating ability of solvents. 4-Nitroaniline is a useful indicator for estimating solvent polarity because it is an electron acceptor molecule which presents incomplete complexation with weak donor solvents167. 

The recent introduction of non-aqueous media extends the applicability of CE. Different selectivity, enhanced efficiency, reduced analysis time, lower Joule heating, and better solubility or stability of some compounds in organic solvent than in water are the main reasons for the success of non-aqueous capillary electrophoresis (NACE). Several solvent properties must be considered in selecting the appropriate separation medium (see Chapter 2) dielectric constant, viscosity, dissociation constant, polarity, autoprotolysis constant, electrical conductivity, volatility, and solvation ability. Commonly used solvents in NACE separations include acetonitrile (ACN) short-chain alcohols such as methanol (MeOH), ethanol (EtOH), isopropanol (i-PrOH) amides [formamide (FA), N-methylformamide (NMF), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA)] and dimethylsulfoxide (DMSO). Since NACE—UV may present a lack of sensitivity due to the strong UV absorbance of some solvents at low wavelengths (e.g., formamides), the on-line coupling of NACE... 

Another approach to isolate the catalyst from the products is the application of perfluorinated catalytic systems, dissolved in fluorinated media [63], which are not non-miscible with the products and some commonly used solvents for catalysis like THE or toluene at ambient temperature. Typical fluorinated media include perfluorinated alkanes, trialkylamines and dialkylethers. These systems are able to switch their solubility properties for organic and organometallic compounds based on changes of the solvation ability of the solvent by moving to higher temperatures. This behavior is similar to the above-mentioned thermomorphic multiphasic PEG-modified systems [65-67]. 

In addition to linear carbonates, PC was also considered as a cosolvent that could help to improve the low-temperature performance of the electrolytes, mainly due to its wide liquid range and solvation ability to lithium salts. This latter property seems to be a merit relative to the linear carbonates, whose dielectric constants are generally below 10 and whose displacement of EC usually causes the solubility of lithium salts to decrease in such mixed solvents, especially at low temperatures. 

The structure and properties of water soluble dendrimers, such as 46, is, in itself, a very promising area of research due to their similarity with natural micellar systems. As can be seen from the two-dimensional representation of 46 the structure contains a hydrophobic inner core surrounded by a hydrophilic layer of carboxylate groups (Fig. 12). However these dendritic micelles differ from traditional micelles in that they are static, covalently bound structures instead of dynamic associations of individual molecules. A number of studies have exploited this unique feature of dendritic micelles in the design of novel recyclable solubilization and extraction systems that may find great application in the recovery of organic materials from aqueous solutions [84,86-88]. These studies have also shown that dendritic micelles can solubilize hydrophobic molecules in aqueous solution to the same, if not greater, extent than traditional SDS micelles. The advantages of these dendritic micelles are that they do not suffer from a critical micelle concentration and therefore display solvation ability at nanomolar... 

The differences in the solvation abilities of ions by various solvents are seen, in principle, when the corresponding values of As ivG° of the ions are compared. However, such differences are brought out better by a consideration of the standard molar Gibbs energies of transfer, AtG° of the ions from a reference solvent into the solvents in question (see further section 2.6.1). In view of the extensive information shown in Table 2.4, it is natural that water is selected as the reference solvent. The TATB reference electrolyte is again employed to split experimental values of AtG° of electrolytes into the values for individual ions. Tables of such values have been published [5-7], but are outside the scope of this text. The notion of the standard molar Gibbs energy of transfer is not limited to electrolytes or ions and can be applied to other kinds of solutes as well. This is further discussed in connection with solubilities in section 2.7. 

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