Solvent protic, aprotic

Obviously, this shift implies the self-association of DMSO. Further frequency shifts to even lower wave numbers (1050-1000 cm " ) are observed in both aprotic polar and protic solvents. In aprotic solvents such as acetonitrile and nitromethane, the association probably takes place between the S—O bond of DMSO and the —C=N or the —NOz group in the molecules by dipole-dipole interaction as shown in Scheme 331,32. Moreover, the stretching frequency for the S—O bond shifts to 1051 cm 1 in CHC13 and to 1010-1000 cm -1 in the presence of phenol in benzene or in aqueous solution33. These large frequency shifts are explained by the formation of hydrogen bonds between the oxygen atom in the S—O bond and the proton in the solvents. Thus, it has been... 

Solvent, protic and dipolar aprotic, rates of bimolecular substitution-reactions in, 5,173 Solvent-induced changes in the selectivity of solvolyses in aqueous alcohols and related mixtures,... 

Short-lived organic radicals, electron spin resonance studies of, 5, 53 Small-ring hydrocarbons, gas-phase pyrolysis of, 4, 147 Solid state, tautomerism in the, 32, 129 Solid-state chemistry, topochemical phenomena in, 15, 63 Solids, organic, electrical conduction in, 16, 159 Solutions, reactions in, entropies of activation and mechanisms, 1, 1 Solvation and protonation in strong aqueous acids, 13, 83 Solvent effects, reaction coordinates, and reorganization energies on nucleophilic substitution reactions in aqueous solution, 38, 161 Solvent, protic and dipolar aprotic, rates of bimolecular substitution-reactions in,... 

Since the advantage of using nonaqueous systems in electrochemistry lies in their wide electrochemical windows and low reactivity toward active electrodes, it is crucial to minimize atmospheric contaminants such as 02, H20, N2, C02, as well as possible protic contaminants such as alcoholic and acidic precursors of these solvents. In aprotic media, these contaminants may be electrochemically active on electrode surfaces, even at the ppm level. In particular, when the electrolytes comprise metallic cations (e.g., Li, Mg, Na), the reduction of all the above-mentioned atmospheric contaminants at low potentials may form surface films as the insoluble products precipitate on the electrode surfaces. In such cases, the metal-solution interface becomes much more complicated than their original design. Electron transfer, for instance, takes place through electrode-solution rate limiting interphase. Hence, the commonly distributed solvents and salts for usual R D in chemistry, even in an analytical grade, may not be sufficient for use as received in electrochemical systems. 

Generally, in mixtures of protic solvents with aprotic solvents and especially in mixtures of two aprotic solvents the dependence of AG° j on composition is fixed by the transfer free energy of ion i between both pure solvent components. For cations quite often the sign of d (AG° +)/dx2 is opposite to that of AG +(x2 = 1) 46-49) ag +(x2 = 1) < 0. That means, AG + decreases on addition of component 2, because the cation is preferentially solvated >y solvent component 2. If, on the other hand, AG + (x2 = 1) > 0, the solvent 2 changes AG + only drastically when the mixture is composed mainly out of solvent component 2 that men as, d (AG +)/ dx2 > 0 In the case of anions, these relations only hold sometimes X... 

Figure 2.8 Specific conductivities of protic vs. aprotic ionic liquids, showing matching of concentrated lithium chloride solution conductivity by solvent-free aprotic liquids. Note that at low temperature, the conductivity of protic nitrate in excess nitric acid is higher than that of the aqueous Lid case with the same excess solvent. (From Xu and Angell [17] by permission)...

In hydrogen-bond donor solvents such as alcohols and trichloromethane, the tautomeric equilibrium is shifted in favor of the colourless form (12a) more than in other solvents. This is obviously due to the formation of hydrogen bonds between / 12a) and these protic solvents. In aprotic solvents, A//° is negative and the reaction is exothermic. Since, however, all AG° values are positive, the negative value of KH° must be over-compensated by a positive entropy change cf. Eq. (4-4). 

Solvents may be classified according to their polarity into three groups apolar aprotic solvents, dipolar aprotic solvents, and (polar) protic solvents. Examples of these three classifications for some common laboratory solvents are listed in Table 16.1, in order of increasing polarity (indicated by dielectric constant), together with some other solvent properties. For information on the hazards and toxicity of solvents, see Chapter 11. 

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