Dimethyl sulphoxide, solvent variation

Variation of the fluorine-19 chemical shift on association has been used to determine solvent effects and basicities28. Muller29 has examined the chemical shifts of 6,6,6-trifluoro-l-hexanol (TFH) in mixtures of water and organic liquids such as acetone, dioxane, THF, ethylene glycol, 2-methoxyethane, 1,2-dimethoxyethane, methanol, r-butanol, dimethyl-formamide and dimethyl sulphoxide. Typical plots are shown in Figures 2 and 3. Solvents... 

Fig, 4. Variation of the primary kinetic isotope effect in the elimination reaction of 2-phenylethyl-dimethylsulphonium bromide with hydroxide ion as the solvent composition is changed from water to 84% dimethyl sulphoxide. (A similar profile has recently been obtained for the elimination reaction of 2-phenylethyltrimethylammonium ion under the above reaction conditions " ). (Reproduced with permission from Cockerin ".)... 

In cases where the more established kinetic approaches become insensitive, mixed solvent studies on rate of elimination may prove enlightening. Although p and knlko remain unchanged in the elimination of 2-arylethyl bromides with t-butoxide in t-butyl alcohol on the addition of dimethyl sulphoxide (0-2.23 M), a rate increase of 120 times is noted" . Smaller increases in rate are observed for similar variations in the concentration of added dipolar aprotic solvent in elimination from 1-phenylethyl bromide and 9-bromo-9,9 -bifluorenyP , but a steeper rise in rate is observed for 2,2-diphenylethyl tosylate. A cor-... 

Many other examples could be cited which show a smooth variation of k /kP with the strength of the base, and in a few cases some indication of a maximum isotope effect when ApK (and hence the standard free energy change of the reaction) is close to zero. Such a maximum may well be concealed in the results for sodium propan-2-one-l-sulphonate in Table 25, since kP/k has almost the same value for reaction with 2,6-lutidine and hydroxide, for which ApK is -h7.4 and —2.0 respectively. One way of changing the value of Api is to modify the nature of the solvent, and in particular the addition of dimethyl sulphoxide to aqueous solutions containing hydroxide ions will displace the equilibrium SH-hOH"... 

Solvent Structure. There has been some discussion of the importance of solvent structure in kinetics, for example in connection with aquation of cobalt(m) complexes in binary aqueous mixtures. There are difficulties in squaring the kinetic parameters for dimethylformamide and for dimethyl sulphoxide exchange at iron(u) with Bennetto and Caldin s model of solvent structural effects, but this model proved useful in discussion of the aquation of [Co(NH3)s(DMSO)] + in binary aqueous mixtures. Bulky hydrophobic groups in solvent molecules have an effect on solvent structure which is reflected in the kinetics of complex formation. - For dissociative solvent exchange at some M + ions, activation enthalpies appear to be determined by the solvation enthalpy of the metal ion and the solvent structure as manifested in its enthalpy of vaporization. In the reaction of Ni + with malonate, the range of solvent variation of activation parameters is comparable with their likely errors, preventing the authors from discussing their results in terms of Bennetto and Caldin s theories. ... 

Some time ago it was reported that rates of dimethyl sulphoxide exchange with nickel(ii) in dimethyl sulphoxide-dichloromethane and dimethyl sulphoxide-nitromethane solvent mixtures were almost independent of solvent composition. Recently, rates of dimethylformamide exchange at nickel(n) in dimethylformamide-nitromethane solvent mixtures have also been found to be remarkably insensitive to solvent-composition variation. Whereas in the dimethyl sulphoxide-dichloromethane solvent mixtures nickel(n) exists as [Ni(DMSO)e] +, in dimethylformamide-nitromethane mixtures the composition of the nickel(n) co-ordination shell varies with solvent composition, with preferential solvation by the nitromethane rather than by the dimethylformamide. These observations are difficult to explain in terms of an /d... 

The variations of /uc- h d 7ih-ih acetone and dimethyl sul-phoxide in various solvents show an almost linear relationship with the carbonyl-and chemical shifts/ " Hydrogen bonding with the solvent, or the presence of large solvent dipoles, may increase the relative importance of polar resonance structures of the carbonyl or sulphoxide group (33), and the electron withdrawl experienced by the methyl group will result in increased values of C—H couplings. 

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