Other Solvent Scales

Many other scales have been developed to measure the polar nature of solvents and other specific properties (Table 3.1). These scales make for handy reference when choosing a solvent for a particular purpose. Most of the other scales are based upon the solvatochro-mism of the solvent. Solvatochromism is the change in shape, intensity, and/or position of the UV / vis or emission spectrum of a chromophore or fluorophore induced by the solvent. The most extensively used scales are the Z scale and the (30) scale. 

The t(30) scale is based upon the spectrum of the pyridinium betaine shown in Eq. 3.4, which upon excitation leads to a less polar excited state due to a charge redistribution. Again, more polar solvents lead to a higher energy excitation (lower A ax)- One limitation is 

A scale known as Ji is based upon several different dyes, not just one as with the Z and Er(30) scales, and gives a good measure of the extent to which the solvent stabilizes ionic or polar species. The scale is best viewed as a measure of non-specific electrostatic solvation. Once again water wins, but formamide, DMSO, and DMF all run a close second. 

The various solvent scales can be used to determine which property of a solvent has the greatest influence on reactivity or any other physical / chemical phenomena. An example of their use in a common reaction is given in the following Connections highlight, and we will also showcase their use in a Connections highlight concerned with the hydrophobic effect in the next chapter. 

The Use of Solvent Scales to Direct Diels-Alder Reactions 

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The SPP general solvent seale, and the SA and SB speeifie solvent scales, are orthogonal to one another, as can be inferred from the small correlation eoefficients obtained in mutual fittings involving the 200 solvents listed in Table 10.3.1 [r (SPPvs. SA) = 0.13, r (SPP vs. SB) = 0.10 and r (SA vs. SB) = 0.01]. These results support the use of these seales for the multi-parameter analysis of other solvent scales or data sets sensitive to the solvent effeet on the basis of the following equation ... 

Solvent variation can gready affect the acidity of hydantoins. Although two different standard states are employed for the piC scale and therefore care must be exercised when comparing absolute acidity constants measured in water and other solvents like dimethyl sulfoxide (DMSO), the huge difference in piC values, eg, 9.0 in water and 15.0 in DMSO (12) in the case of hydantoin itself, indicates that water provides a better stabilization for the hydantoin anion and hence an increased acidity when compared to DMSO. 

Scales for bases that are too weak to study in aqueous solution employ other solvents but are related to the equilibrium in aqueous solution. These equilibrium constants provide a measure of thermodynamic basicity, but we also need to have some concept of kinetic basicity. For the reactions in Scheme 5.4, for example, it is important to be able to make generalizations about the rates of competing reactions. 

This procedure, in contrast to previous methods, comprises only one step and is readily adapted to large-scale preparative work. Furthermore dibromination is very slow in methanol and hence the crude reaction products contain only traces of dibromo ketones. This contrasts with the behavior in other solvents such as ether or carbon tetrachloride, where larger amounts of dibromo ketones are always present, even when one equivalent of bromine is used. Methanol is thus recommended as a brominating solvent even when no orientation problem is involved. It should be noted that a-bromomethyl ketals are formed along with x-bromoketones and must be hydrolyzed during the workup (Note 8).7... 

The parameter Ho reflects the ability of the solvent system to donate protons, but it can be applied only to acidic solutions of high dielectric constant, mostly mixtures of water with acids such as nitric, sulfuric, perchloric, and so on. It is apparent that the Ho treatment is valid only when/i //hi+ is independent of the nature of the base (the indicator). Since this is so only when the bases are structurally similar, the treatment is limited. Even when similar bases are compared, many deviations are found. Other acidity scales have been set up, among them // for bases with a... 

The question arises, whether and to what extent the dicarboxylic acid 1 is capable of binding other solvents besides ethanol (starting observation, cf. Sect. 1) in the crystal lattice. For this purpose, to begin with, crystallization experiments using further alcohols (straight-chain, branched, univalent and polyvalent) were carried out. It was found that 1 is apt to form crystal inclusions on a large scale, i.e. with alcohols of various constitutions. A list of different examples is given in Table 1 (Entries 1-16). 

If Z9b(ai) can be equated with P calculated from the entries in Table 2.5, then Z9b(a2) in any other solvent Ab can be estimated from Eq. (2.62). Equation (2.62) is actually a combination of four expressions of the form of Eq. (2.8) (see section 2.2.2), two for water and solvent Ai and two for water and solvent A2, presuming them to be immiscible pairs of liquids. It employs concentrations on the mole fraction scale, and assumes that the systems behave as regular solutions (which they hardly do). This eliminates the use of the solubility parameter 8 of water, which is a troublesome quantity (see Table 2.1). Solvent Ai need not, of course, be 1-octanol for Eq. (2.62) to be employed, and it suggests the general trends encountered if different solvents are used in solvent extraction. 

According to Coimbra et solvents play a central role in the majority of chemical and pharmaceutical industrial processes. The most used method to obtain artemisinin (1) from A. annua is through the use of organic solvents such as toluene, hexane, cyclohexane, ethanol, chloroform and petroleum ether. Rodrigues et al described a low-cost and industrial scaled procedure that enables artemisinin (1) enhanced yields by using inexpensive and easy steps. Serial extraction techniques allowed a reduction of 65% in solvent consumption. Moreover, the use of ethanol for compound extraction is safer when compared to other solvents. Flash column pre-purification employing silicon dioxide (Zeosil ) as stationary phase provided an enriched artemisinin (1) fraction that precipitated in hexane/ethyl acetate (85/15, v/v) solution. These results indicate the feasibility of producing artemisinin (1) at final cost lowered by almost threefold when compared to classical procedures. 

Many of these attachment reactions are also diffusion-controlled in other solvents of low electron mobility like, for example, n-hexane. It has been suggested that this is the case for all solvents for which 1 cm /Vs [118]. For this to be true, the rate constant k should scale as the mobility. For hexane, the rate constants for attachment to solutes like biphenyl, naphthalene, and difluorobenzene are close to 1 x 10 sec or one-third the value in cyclohexane. The mobility in -hexane is approximately one-third that in cyclohexane [2] thus k scales with fijj for these two solvents. 

Many other solvent parameters have been defined in an attempt to model as thoroughly as possible solvent effects on the rate constants for solvolysis. These include (a) Several scales of solvent ionizing power Tx developed for different substrates R—X that are thought to undergo limiting stepwise solvolysis. (b) Several different scales of solvent nucleophilicity developed for substrates of different charge type that undergo concerted bimolecular substitution by solvent. (c) An... 

Figure 4.8 shows the potential windows obtained at a bright platinum electrode, based on the Fc+/Fc (solvent-independent) potential scale. Because of the overpotentials, the window in water is 3.9 V, which is much wider than the thermodynamic value (2.06 V). The windows for other solvents also contain some overpotentials for the reduction and the oxidation of solvents. However, the general tendency is that the negative potential limit expands to more negative values with the decrease in solvent acidity, while the positive potential limit expands to more positive values with the decrease in solvent basicity. This means that solvents of weak acidity are difficult to reduce, while those of weak basicity are difficult to oxidize. This is in accordance with the fact that the LUMO and HOMO of solvent molecules are linearly related with the AN and DN, respectively, of solvents [8]. 

Very little is known about the influence of the use of other solvents on the yield, although it is expected that other aprotic solvents would be as efficient as benzene. Toluene and CH2CI2 are interesting alternatives to the use of carcinogenic benzene, which have been proved to be efficient in this oxidation. e It can be advisable to cool the reaction flask on an ice-water bath during the initial mixture of components on multigram scale oxidations when exotherms can be expected. As the DMSO freezes at 18°C, operations at low temperature must be done in the presence of a co-solvent, like benzene. f Normally, it takes between 1 h and 1 day. 

FREE ENERGIES OF TRANSFER, ON THE MOLE FRACTION SCALE, FROM METHANOL TO OTHER SOLVENTS OF TETRAALKYLTINS, IODINE, AND THE TETR A A LK Y LTIN/lODI NE... 

The difficulty in dealing with solvent influences on reaction rates is that the free energy of activation, AG, depends not only on the free energy of the transition state but also on the free energy of the initial state. It is therefore of considerable interest to dissect solvent influences on AG into initial-state and transition-state contributions. As far as electrophilic substitution at saturated carbon is concerned, the only cases for which such a dissection has been carried out are (a) for the substitution of tetraalkyltins by mercuric chloride in the methanol-water solvent system (see page 79), and (b) for the iododemetallation of tetraalkylleads in a number of solvents (see p. 173). Data on the latter reaction (6) are more useful from the point of view of the correlation of transition-state effects with solvent properties, and in Table 13 are listed values of AG (Tr), the free energy of transfer (on the mole fraction scale) of the tetraalkyllead/iodine transition states from methanol to other solvents. 

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