Methanol solvent properties

The solvent dependence of the reaction rate is also consistent with this mechanistic scheme. Comparison of the rate constants for isomerizations of PCMT in chloroform and in nitrobenzene shows a small (ca. 40%) rate enhancement in the latter solvent. Simple electrostatic theory predicts that nucleophilic substitutions in which neutral reactants are converted to ionic products should be accelerated in polar solvents (23), so that a rate increase in nitrobenzene is to be expected. In fact, this effect is often very small (24). For example, Parker and co-workers (25) report that the S 2 reaction of methyl bromide and dimethyl sulfide is accelerated by only 50% on changing the solvent from 88% (w/w) methanol-water to N,N-dimethylacetamide (DMAc) at low ionic strength this is a far greater change in solvent properties than that investigated in the present work. Thus a small, positive dependence of reaction rate on solvent polarity is implicit in the sulfonium ion mechanism. 

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... 

Fio. 16. Dependence of solvent properties pertinent to RPC on composition of water-methanol mixture at 25 C. SuifisiM tension y data were obtained from Hmmemums (/i ) the viscosity and dielectric constant data c were taken from Carr and Riddick (135) and Akeriof (136), respectively. Reprinted from Horvdth and Melander (/29). J. Chroma-togr. Sci., with permission from Reston Publications. 

A mixture of benzene and methanol (19 to 1) was used for spreading the alkyl phosphonates. To minimize the influence of benzene on the film properties, the concentrations of the spreading solutions were > 1.5 X 10 3 gram per ml., and the experiments were performed at tt > 4 dynes per cm. (25). Moreover, higher proportions of methanol in the spreading solution did not alter the film properties under study for selected monolayers. For the sulfates, a mixed solvent containing water-benzene-2-propanol (1 10 10) was used because with the benzene-methanol solutions the properties of the films depended on the age of solution from which the films were prepared. Stearic and palmitic acids were spread from either hexane or the benzene-methanol solvent used for the phosphonates. Identical desorption results were obtained with the two solvents. 

Since nonpolar lipids (e.g., triacylglycerols) are generally not readily soluble in methanol, a solvent that can dissolve both neutral and polar lipids is necessary to facilitate the reaction in methanol. Toluene is chosen for this purpose because of its good solvent properties and relatively low toxicity. 

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. 

Cyclobutyl, substituted cyclobutyl, and related carbocations were reviewed.23 A study of the photophysical properties and photochemistry of 1-adamantyl aryl ethers in methanol solvent showed that, although the majority of the ethers underwent photolysis by homolytic pathways, irradiation of the 4-cyanophenol ether resulted, in part, in the methyl ether, implicating an ionic mechanism with the 1-adamantyl cation as an intermediate.24 Labelling experiments demonstrated that the fragmentation of 7-norbomyloxychlorocarbene to 7-norbornyl chloride proceeds with both retention and inversion.25 While an ion pair [Cl CO R+] is a possible intermediate, computational studies suggest that the fragmentations proceed via transition states that lead to either retention or inversion. 

Table 5.2 Comparison of solvent properties of methanol, ethanol and glycerol.

Supercritical water exhibits better solvent properties for apolar organic compounds than water itself and was applied by Jerome and Parsons [79] as well as Dinjus and co-workers [80] as the solvent for the Co-mediated cyclotrimeriza-tion of monosubstituted acetylenes to benzene derivatives. Eaton et al. published the cyclotrimerization of acetylenes bearing functional groups in a water/methanol (80 20) mixture using an R-Cp cobalt cod complex as the catalyst. The water solubility of the Co complex was achieved by the special substituent R=C0(CH2)2CH20H on the Cp ligand [81]. 

Using ethylammonium nitrate (EAN) as PIL ( . , =0.95, ji = 1.12, a=1.10, P = 0.46), following types of binary mixture models were selected for the analysis and quantification of the microscopic solvent properties (a) [molecular aprotic solvent with HBA ability + PIL cosolvent], (b) [molecular aprotic solvent with both HBD and HBA ability + PIL cosolvent] and (c) [molecular protic solvent + PIL cosolvent] [31]. The molecular solvents included in this analysis were dimethylsul-phoxide (DMSO) ( ., =0.44, 7r = 1.00, a=0.02 and p=0.76) as a polar aprotic HBA solvent, acetonitrile (AN) ( .,. =0.46, 7t =0.75, a=0.19, p=0.40) as polar aprotic HBA/HBD solvent and methanol ( .,. =0.76, 7t =0.60, a=0.98, p=0.66) as a protic solvent. EAN is a N-H-bond donor. In all cases, the pure component part of the mixtures was capable of forming associated species through hydrogen-bonding interactions. For the explored solvent mixtures, empirical parameters . n, a and P were calculated from the wave numbers of the absorbance maxima of the corresponding chemical probes at 25°C. 

One of the problems with using a liquid as the extraction solvent is its removal when the extraction is finished. The most recent way to eliminate this problem is to use a supercritical gas, COj being the gas of choice at the moment. A gas in the supercritical state has solvent properties comparable to a liquid but it is less viscous, so it can penetrate the sample faster. When the extraction is complete, the pressure is released, and the gas evaporates away from the extracted components. CO2 is nonpolar so more polar compounds such as methanol are sometimes added in small amounts. This exceWeni supercritical fluid extraction (SEE) technique is described in Chapter 13. 

Because of their crucial role in the ionization step, solvents have a profound effect on the rates of El reactions. These rates for a number of tertiary halides have been determined in a variety of solvents. For r-butyl chloride there are huge differences in the rates in water (log k = -. 54), ethanol (log k = -7.07), and diethyl ether (log k = — 2.1A)P Similarly, the rates of the El reaction of 1-methylcyclopentyl bromide range from 1 x 10 s in methanol to 2 x 10 s in hexane. Polar aprotic solvents such as DMSO (k = 2x lO s ) and acetonitrile (k = 9x 10 s ) are also conducive for ionization. The solvent properties that are most important are polarity and the ability to assist leaving group ionization. These, of course, are the same features that favor reactions, as we discussed in Section 3.8. 

Cosolvents, entrainers, or modifiers are added to enhance solubilities and solute selectivities. They are normally used at low concentrations, i.e., 1—5 mole %. Methanol is a good example. For nonpolar solutes containing no functional groups, cosolvent-induced solubility enhancement is quite similar for all cosolvents and depends only on cosolvent concentration. This kind of enhancement results from alteration of the solvent properties rather than specific interactions. In the case of polar or heterocyclic solutes, the nature of the cosolvent becomes an important limit on the magnitude of the enhancement factor, e.g., hydrogen bonding or dipole-dipole interactions with the solute. It is these types of specific interactions that make it possible to tailor a solvent/cosolvent mixture to enhance the solubility of a particular solute (18). 

Wilke and Chang, 1955 (32) studied the diffusivity of both iodine and toluene in alkanes and included in their analysis other systems from the literature. They investigated the influence of solvent properties, such as, viscosity, molar volume, molecular weight and heat of vap>orization and found a linear relationship between Log (Dpg/T) and Logp with a slope of (0.5). They examined also the influence of solute properties, by collecting diffusion data for a variety of solutes in the solvents, water, methanol, ethanol, hexane, toluene and carbon tetrachloride and they observed a linear relationship between Log (Dpg/T) and log V, the slope being (-0.6). They proposed the following equation... 

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