Carbon acids, ionization, solvent

A study of acid-catalysed enolization and carbon-acid ionization of isobutyrophenone has combined the solvent isotope effect k /kv = 0.56 and substrate isotope effect kH/kD = 6.2 determined for the enolization in H2O and D2O with literature information in order to estimate the solvent isotope effect on the enolization equilibrium, A e(H20)/A e(D20) = 0.92, and on the CH ionization of butyrophenone, kf (R20)/kK(D20) = 5.4.130 This is the first report of an isotope effect on AY forketo-enol equilibrium of a simple aldehyde or ketone. 

A study of acid-catalysed enolization and carbon-acid ionization of isobutyrophenone has combined the solvent isotope effect — 0.56 and substrate isotope effect... 

Rate constants for ionization of carbon acids (chloroform-r and acetophenone-r) in alkaline aqueous sulfolane have been determined and their dependence on solvent composition and temperature has been interpreted for this highly basic medium. °... 

Aluminas, silicas, clays, zeolites or graphite can be selected as acidic, basic or neutral supports, depending on the type of reactions. Alumina alone can act as a base towards a acid compounds but if a strong base is necessary, KF on alumina can ionize carbon acids up to pKa = 35. On the other hand, montmorillonites (clays) such as K10 or KSF offer acidities very comparable to strong mineral acids (e.g., nitric and sulfuric acids, respectively). Reactions are simply performed by mixing reagents and a mineral support in the absence of any organic solvent followed by microwave irradiation [52]. 

Streitwieser et al. [160] and BordweU et al. [161] used the lyate ions of organic solvents such as cyclohexylamine and dimethyl sulfoxide in the determination of the C—H acidity of weak organic carbon acids. Using super base systems such as alkali metal salts of cyclohexylamine [i.e. lithium and cesium cyclohexylamides) [160] and dimethyl sulfoxide (sodium dimsyl) [161] in an excess of these non-HBD solvents, relative acidity scales for weak carbon acids have been established. In this way, values for the ionization of over a thousand Bronsted acids in dimethyl sulfoxide have... 

It should be mentioned that a solvent change affects not only the reaction rate, but also the reaction mechanism (see Section 5.5.7). The reaction mechanism for some haloalkanes changes from SnI to Sn2 when the solvent is changed from aqueous ethanol to acetone. On the other hand, reactions of halomethanes, which proceed in aqueous ethanol by an Sn2 mechanism, can become Sn 1 in more strongly ionizing solvents such as formic acid. For a comparison of solvent effects on nucleophilic substitution reactions at primary, secondary, and tertiary carbon atoms, see references [72, 784]. 

Many reactions become possible only in such superbasic solutions, while others can be carried out under much milder conditions. Only some examples of preparative interest (which depend on the ionization of a C—H or N—H bond) will be mentioned here. The subsequent reaction of the resulting carbanion may involve electrophilic substitution, isomerization, elimination, or condensation [321, 322]. Systematic studies of solvent effects on intrinsic rate constants of proton-transfer reactions between carbon acids and carboxylate ions as well as amines as bases in various dimethyl sulfoxide/ water mixtures have been carried out by Bernasconi et al. [769]. 

The problems involved in the measurement of acidities or relative acidities of weak acids are illustrated by the widely different estimates which have been given for the acidity of substituted acetylenes. Two different approaches have been used for measuring the equilibrium acidity of carbon acids which do not ionize in the pH range. In one approach, the ionization of a carbon acid is studied in mixed solvents containing base. Some of these solutions are more basic than aqueous solutions and by varying the solvent mixture the ionization of acids with pK values in the range 12—25 can be studied. Values at the low end of the pK range are directly compared with aqueous p/iC values. It is assumed that ratios of the activity coefficients (f) for the ionized (S-) and unionized acids (SH) are the same for all the acids studied and an acidity function (86)... 

Salts of weak bases or weak acids are hydrolyzed in aqueous solution (see Chap. XI) and they behave as if they contained excess of strong acid and strong base, respectively. According to the law of mass action the presence of one acid represses the ionization of a weaker one, so that the effective conductance of the water, which is due mainly to carbonic acid, is diminished. The solvent correction in the case of a salt of a weak base and a strong acid should thus be somew hat less than the total conductance of the w ater. For solutions of salts of a w eak acid and a strong base, vhich react alkaline, the correction is uncertain, but methods of calculating it have been described they are based on the assumption... 

Acidity constants for ionization of weak carbon acids in water caimot be determined by direct measurement when the strongly basic carbanion is too unstable to exist in detectable concentrations in this acidic solvent. Substituting dimethyl-sulfoxide (DMSO) for water causes a large decrease in the solvent acidity because, in contrast with water, the aprotic cosolvent DMSO does not provide hydrogenbonding stabilization of hydroxide ion, the conjugate base of water. This allows the determination of the pfC s of a wide range of weak carbon acids in mixed DMSO/water solvents by direct measurement of the relative concentrations of the carbon acid and the carbanion at chemical equilibrium [3, 4]. The pfC s determined for weak carbon acids in this mixed solvent can be used to estimate pfC s in water. 

In contrast to carbocations, the ionization of carbon acids to generate carbanions has been somewhat more difficult to demonstrate. This appears to be due largely to the difficulty of finding solvents of suitable dielectric constant to support ionization but yet not too acidic (allowing proton transfer) or otherwise too reactive. Thus, while diethyl ether (ether [(CH3CH2)20]) and other similar solvents are, in principle. 

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