Acid-Base Reactions in Non-Aqueous Solvents

Potentiometry is often used in determining dissociation constants, pKa, and homoconjugation constants, Kf(HA2 , for HA-type acids in aprotic solvents. Therefore, to begin with, we describe the method for obtaining the values of pKa and K HA ) in an aprotic solvent in which no data on acid-base equilibria is yet available. Procedures 1 to 4 seem to be appropriate in such a case [20, 21]. 

Procedure 1 We select several HA-type acids, for which pKa values of 7.5 are expected, and determine their pKa and K HAJ) accurately by method(s) other than potentiometry. If the selected acid is a nitro-substituted phenol that has no tendency to homoconjugate (p. 71), we dissolve various amounts of it in the solvent and measure the UV/vis spectrophotometric absorption for the phenolate anion formed by dissociation. For the conductimetric determination of pKa and K HA])), see Section 7.3.2. 

Procedure 2 We construct cell (V) using the above acids and their tetraalkylam-monium salts, R4NA, and measure the emfs of the cell by varying the ratio Ca/Cs in wide ranges  

If the emf-pH relation is linear and has a slope of 59.2 mV/pH (at 25 °C), it means that the pH response of the glass electrode is Nemstian. Although the actual confirmation of the Nernstian response is in a limited pH region, we temporarily assume that the response is Nemstian in all pH regions.9  

Procedure 3 We construct cell (V) using the HA type acid under study and its conjugate base and measure the emfs by varying the ratio Ca/Cs. The procedure to vary the ratio Cs/Ca can be replaced by the titration of the acid HA with a strong base (BiuNOH in MeOH, toluene-MeOH, or 2-PrOH), but the alcohol introduced with the base may have some influence on the results. Because the glass electrode has been calibrated in Procedure 2, the emf values can be converted to pH values. 

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Acid-base reactions in non-aqueous solvents have been extensively studied. Many books and reviews are available concerning acid-base equilibria and acid-base titrations in non-aqueous solvents. References [1-3] are particularly useful. 

The Bronsted-Lowry theory of acids and bases referred to in Section 10.7 can be applied equally well to reactions occurring during acid-base titrations in non-aqueous solvents. This is because their approach considers an acid as any substance which will tend to donate a proton, and a base as a substance which will accept a proton. Substances which give poor end points due to being weak acids or bases in aqueous solution will frequently give far more satisfactory end points when titrations are carried out in non-aqueous media. An additional advantage is that many substances which are insoluble in water are sufficiently soluble in organic solvents to permit their titration in these non-aqueous media. 

This chapter discusses acid-base reactions in lion-aqueous solvents, with particular emphasis on how they differ from those in aqueous solutions. The problem of pH in non-aqueous solutions is also discussed. The Bmisted acid-base concept is adopted, i.e. an acid is a proton donor and a base is a proton acceptor. The relation between an acid A and its conjugate base B is expressed by ... 

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]. 

C. G. Swain and J. F. Brown (1952), Concerted displacement reactions. VII. The mechanism of acid base catalysis in non-aqueous solvents. J. Amer. Chem. Soc. 74, 2534-2537. 

The polymerization of N-carboxy anhydrides (NCA s) is a complicated process that Is difficult to study. The sensitivity of NCA s to moisture and other Impurities, the limited solubility of the products of NCA polymerizations In most solvents that are suitable for anionic polymerizations, the tendency of NCA s to associate with polypeptides, leading to enhanced monomer concentrations In the vicinity of growing polypeptide chains, the general cosplexlty of Ionic reactions in non-aqueous solvents and the diversity of possible mechanisms for amide bond formation or destruction. Including catalysis by COj, acids, bases, etc., collectively make It difficult to establish mechanisms for NCA polymerizations. 

SchifT s bases A -Arylimides, Ar-N = CR2, prepared by reaction of aromatic amines with aliphatic or aromatic aldehydes and ketones. They are crystalline, weakly basic compounds which give hydrochlorides in non-aqueous solvents. With dilute aqueous acids the parent amine and carbonyl compounds are regenerated. Reduction with sodium and alcohol gives... 

The single largest use of ammonia is its direct apphcation as fertdizer, and in the manufacture of ammonium fertilizers that have increased world food production dramatically. Such ammonia-based fertilizers are now the primary source of nitrogen in farm soils. Ammonia also is used in the manufacture of nitric acid, synthetic fibers, plastics, explosives and miscellaneous ammonium salts. Liquid ammonia is used as a solvent for many inorganic reactions in non-aqueous phase. Other apphcations include synthesis of amines and imines as a fluid for supercritical fluid extraction and chromatography and as a reference standard in i N-NMR. 

The choice of base is critical for the Suzuki reaction and in some cases can have a rate accelerating effect on this reaction. In particular, for sterically hindered boronic acids like 139 reacting with halopyridines 140 to afford 141 in non-aqueous solvent [49], it was found that as one increased the basicity not only did the yield of the reaction improve, but also the time required to complete the reaction was dramatically reduced. 

The importance of water as a medium for inorganic reactions stems not only from the fact that it is far more readily available than any other solvent, but also because of the abundance of accurate physicochemical data for aqueous solutions compared with the relative scarcity of such data for solutions in non-aqueous solvents. This chapter is concerned mainly with equilibria and we begin by reviewing calculations involving acid-base equilibrium constants. 

The Arrhenius theory views all bases as substances that produce OH (aq) ions. However, acid-base type reactions can occur in non-aqueous solvents, in which OH (aq) cannot be present because there are no oxygen atoms in the system. For example, HCl reacts with pure liquid ammonia... 

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