Acetonitrile acid-base equilibria

Sections 3.3.1 and 4.2.1 dealt with Bronsted acid/base equilibria in which the solvent itself is involved in the chemical reaction as either an acid or a base. This Section describes some examples of solvent effects on proton-transfer (PT) reactions in which the solvent does not intervene directly as a reaction partner. New interest in the investigation of such acid/base equilibria in non-aqueous solvents has been generated by the pioneering work of Barrow et al. [164]. He studied the acid/base reactions between carboxylic acids and amines in tetra- and trichloromethane. A more recent compilation of Bronsted acid/base equilibrium constants, determined in up to twelve dipolar aprotic solvents, demonstrates the appreciable solvent influence on acid ionization constants [264]. For example, the p.Ka value of benzoic acid varies from 4.2 in water, 11.0 in dimethyl sulfoxide, 12.3 in A,A-dimethylformamide, up to 20.7 in acetonitrile, that is by about 16 powers of ten [264]. 

A mechanistic study of acetophenone keto-enol tautomerism has been reported, and intramolecular and external factors determining the enol-enol equilibria in the cw-enol forms of 1,3-dicarbonyl compounds have been analysed. The effects of substituents, solvents, concentration, and temperature on the tautomerization of ethyl 3-oxobutyrate and its 2-alkyl derivatives have been studied, and the keto-enol tautomerism of mono-substituted phenylpyruvic acids has been investigated. Equilibrium constants have been measured for the keto-enol tautomers of 2-, 3- and 4-phenylacetylpyridines in aqueous solution. A procedure has been developed for the acylation of phosphoryl- and thiophosphoryl-acetonitriles under phase-transfer catalysis conditions, and the keto-enol tautomerism of the resulting phosphoryl(thiophosphoryl)-substituted acylacetonitriles has been studied. The equilibrium (388) (389) has been catalysed by acid, base and by iron(III). Whereas... 

The pA of acetonitrile, CH3CN, is 25. Calculate the equilibrium constant for the acid-base reaction of acetonitrile with LDA. The pA of isopropylamide is 40. 

An unsuccessful attempt has been made to determine the separate electronic and steric effects of alkyl groups on the acidities of hydrocarbons, acetophenone derivatives, and acetone derivatives CH3COCHR1R2 (at either site) by multivariational analyses of experimental and theoretical acidities for each set.15 A thermodynamic cycle has been used to estimate the aqueous phase p/C, = 22.7 1.0 for the methyl group of acetic acid and p/C, = 3.3 1.0 for the corresponding enol.16 Equilibrium acidities have been determined for several nitroaryl substituted nitroalkanes and cyanomethanes, 2,4,6-TNT, and 9-cyanofluorene17 in acetonitrile the influence of common cation BH+ on the electronic spectra of the anions obtained in the presence of strong guanidine bases (B) has been attributed to formation of two types of ion pair.18... 

The isolation and full characterization of a number of large hydrido clusters have in many cases been hampered by their extremely high acidity. The clusters [Ni3gPt8(CO)48H,6- )]" ( = 3-6), for instance, exist as an equilibrium mixture of anions a (n = 3), b (n = 4), c ( = 5), and d (n = 6) in acetonitrile solution. This mixture has been found to be easily converted into one of its components by controlled addition of acid or base [Eq. (26)] (373). 

Organic solvents influence the ionization constants of weak acids or bases in several ways (note that they influence the analytes and the buffer as well). Concerning ionization equilibria, an important solvent property is the basicity (in comparison to water), which reflects the interaction with the proton. From the most common solvents, the lower alcohols and acetonitrile are less basic than water. Dimethyl sulfoxide is clearly more basic. However, stabilization of all particles involved in the acido-basic equilibrium is decisive for the pKa shift as well. For neutral acids of type HA, the particles are the free, molecular acid, and the anion, A . In the equilibrium of bases, B, stabilization of B and its conjugated acid, HB, takes place. As most solvents have a lower stabilization ability toward anions (compared to water), they shift the pK values of adds of type HA to higher values in general. No such clear direction of the change is found for the pK values of bases however, they undergo less pronounced shifts. 

The first hypothesis of an in-situ reduction of acetonitrile and propionitrile in the presence of 1 to 4 and 5 suggested that one might be able to use this approach as a new synthetic route to this class of heterocyclics. Hexahydropyrimidines are conventionally prepared by condensation of aldehydes or ketones with 1,3-diamines (4). Water is a by-product in these reactions and must be removed either to favor the imine or enamine equilibrium or for product purification. Generally, the condensation is acid or base catalyzed and run in solvents (6). In some cases... 

In competitive deprotonation experiments in KO-t-Bu/DMSO, the relative proton acidities of cations la, lb, and Ig were estimated from P-NMR equilibrium measurements to be between 27 and 30 [72]. More recently, the pKa values of 2b, 2d, 2e, 2f, and 10a were measured in acetonitrile and they are listed in Table 1 along with values for other non-ionic bases for comparison. Although bases 1 are about ten orders of magnitude weaker in basicity than P4-t-Bu or Pj-t-Bu in acetonitrile, they are ten to twenty orders of magnitude stronger than nitrogen bases in this solvent. It should also be recalled that, unlike phosphazene bases, pro-azaphosphatranes protonate at the phosphorus rather than at one of the two types of nitrogens in these molecules. 

Similarly, the addition of acid to an acetonitrile solution of the cluster [Ni38Pt6(CO)4s] leads to a mixture of the mono, di and trihydrides in an equilibrium that may be easily governed by controlled addition of acid or base. 

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