Acids in non-aqueous solvents

Some complexes of nickel(0) with phosphines have been found to react with several Bronsted acids in non-aqueous solvents and under an inert atmosphere to give hydrido complexes of nickel(I) and nickel(II) (equations 85-87).264,268,269... 

PROTON TRANSFER FROM WEAK CARBON ACIDS IN NON-AQUEOUS SOLVENTS... 

In eqn [13], pK corresponds to the titrant. The most common titrant solution for the titration of acids in non-aqueous solvents is tetrabutylammonium hydroxide, and stable commercial solutions use methyl or isopropyl alcohol as solvents. The addition of isopropyl alcohol to 7-butyl alcohol can be neglected because their relative permittivity values are not very different. Methanol addition causes larger variations in pKg, values. Thus, the use of titrants without or with low contents of methanol is strongly recommended. For application of eqn [13], pKt values of tetrabutylammonium hydroxide are 2.58 in isopropyl alcohol and 4.91 in 7-butyl alcohol. 

Extensive studies have been made of the reaction of diphenyldiazo-methane with carboxylic acids in non-aqueous solvents other than ethanol. As would be expected, in a series of alcohols ROH the ratio of ether to ester formed depends upon the nature of R, but the velocity does not depend in any simple way upon the properties of the solvent, and specific solvation of the reactants or the transition state is probably involved. In non-hydroxylic solvents only one product, the ester, is formed, but there are often complications owing to the self-association of carboxylic acids. 

Acids can also exist in non-aqueous solvents. Since ammonia can also solvate a proton to give the ammonium ion. substances... 

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... 

D. Rosenthal and P. Zuman, Acid-base equilibria, buffers and titrations in water. Chap. 18 in I. M. Kolthoff and P. J. Elving (eds.). Treatise on Analytical Chemistry, 2nd edn., Vol. 2, Part 1, 1979, pp. 157-236. Succeeding chapters (pp. 237-440) deal with acid-base equilibria and titrations in non-aqueous solvents. 

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. 

Determinations in non-aqueous solvents are of importance for substances which may give poor end points in normal aqueous titrations and for substances which are not soluble in water. They are also of particular value for determining the proportions of individual components in mixtures of either acids or of bases. These differential titrations are carried out in solvents which do not exert a levelling effect. 

As indicated in Section 2.4 the strength of an acid (and of a base) is dependent upon the solvent in which it has been dissolved, and in Sections 10.19-10.21 it has been shown how this modification of strength can be used to carry out titrations in non-aqueous solvents which are impossible to perform in aqueous solution. Potentiometric methods can be used to determine the end point of such non-aqueous titrations, which are mainly of the acid-base type and offer very valuable methods for the determination of many organic compounds. 

Titanium, D. of as oxide, via tannic acid and phenazone complexes, (g) 470 by hydrogen peroxide, (s) 696 Titan yellow 692 Titrand 257 Titrant 257 Titration 257 classification of, 258 in an inert atmosphere, 376, 629 in non-aqueous solvents, 281 aniline (and ethanolamine), D. of, 307 indicators for, 283 solvents for, 283... 

SOMe the enhancement in the meta-position is almost as large as in the para-position. The authors go on to show the applicability of op (g) values to certain solution processes, particularly those in non-aqueous solvents, but including the dissociation of thiophenols in 48% ethanol, the results of Bordwell and Andersen80 to which reference has been made earlier (Section III.A.1). A separation of field/inductive and resonance effects is also essayed for the gas-phase acidities of the phenols, and SOMe and S02Me feature in the discussion. There is reference to a oR° value of + 0.07 for SOMe as an unpublished result of Adcock, Bromilow and Taft (cf. 0.00 from Ehrenson and coworkers65 and — 0.07 from Katritzky, Topsom and colleagues128.)... 

As in electroanalysis both ionic and possible electrode aspects are of major interest, both aspects of solutes in non-aqueous solvents have to be considered this can best be done by dividing the theory of the solutions concerned into two parts, viz. (1) the exchange of ionic particles (ionotropy), which leads to acid-base systems, and (2) the exchange of electrons only, which leads to redox systems. 

To conduct meaningful mechanistic and kinetic studies in alcohol media reliable and simple measurement and control of the solution jjpH is essential. Potentiometric titration is the method of choice for obtaining acid dissociation constants or metal ion complex stability constants and in favorable cases the speciation of mixtures of metal-ion-containing complexes in solution can be proposed.20 Titrations in non-aqueous solvents are not nearly as widely reported as those in aqueous media, particularly in cases with metal ions21 and determination of pH in a non-aqueous solvent referenced to that solvent is complicated due to the lack of a way to relate the electrode EMF readings to absolute jjpH (see footnote and ref. 6) so non-aqueous solvents are generally inconvenient to use22 for detailed studies of reaction mechanisms where pH control is required. 

Copper complexes were used as efficient catalysts for selective autoxidations of flavonols (HFLA) to the corresponding o-benzoyl salicylic acid (o-BSH) and CO in non-aqueous solvents and at elevated temperatures (124-128). The oxidative cleavage of the pyrazone ring is also catalyzed by some cobalt complexes (129-131). 

Notice how we generally infer the solvated proton, H30+, each time we write a concentration as [H+], which helps explain why the concept of pH is rarely useful when considering acids dissolved in non-aqueous solvents. When comparing the battery acid with the bench acid, we say that the battery acid has a lower pH than does the bench acid, because the number of solvated protons is greater and, therefore, it is more acidic. Figure 6.1 shows the relationship between the concentration of the solvated protons and pH. We now appreciate why the pH increases as the concentration decreases. 

The ease with which the phenols are nitrated has already been discussed. The process is not satisfactory, however, even when dilute nitric acid is used, because resinous by-products are formed as a result of oxidation and condensation. Nitration with nitrogen peroxide in non-aqueous solvents such as benzene and petrol ether gives better results (Ber., 1921, 54, 1776). 

It is pertinent to observe here that the following inorganic acids almost exhibit equal strength in aqueous solutions, whereas in non-aqueous solvents, their acidity retards in the following order ... 

Abstract Titration of weak bases in non-aqueous solvents can provide valuable information about these weak bases. Some primary amines 1-aminobutane, 1-aminopropane, 2-aminoheptane, aminocyclohexane, 3-amino-l-phenylbutane were titrated with hydrochloric acid in toluene solvent. All the primary amines gave very well-shaped potentiometric titration curves. The same titrations were done with hydrochloric acid in methanol solvent to show the effect of amphiprotic solvent in the titrations with hydrochloric acid. 

Titrations in non-aqueous solvents have been traditionally an important tool for the accurate determination of various pharmaceuticals, some acids in foods, use of some acids or bases in detergents, cosmetics and textile auxiharies, in the analysis of industrial process streams, the analysis of polymers [1-7]. The determination of the pK or pK values of organic compoimds with acidity or basicity constant less than 10 can only be reahsed in non-aqueous media. Although water has excellent solvent properties, it is not suitable for such organic compotmds since the pH jump at the equivalence point in aqueous solution carmot be evalrrated with reasonable accuracy, with this resrrlt, the end point carmot be found. Moreover, most of this compotmds ate not soluble in water. For these reasons, titration in non-aqueous media has recently acqttired great importance. It is now well known that non-aqueous titrations greatly depend on the solvents used [4, 8-13]. 

Diazopyrazoles can be converted into very stable diazonium salts (chloride or bromide) upon treatment with the corresponding concentrated acid at room temperature (76JOC3781 84JHC957). Stable diazonium tetrafluoroborate [87JOC5538], platinichloride, and aurichloride [14JCS(105)435] can be isolated. Only in the case of the unstable 3-diazo-pyrazole was it necessary to operate at low temperature and in non aqueous solvents (61CB1036). 

Solubility in non-aqueous solvents.—Hydrogen chloride dissolves in many other solvents besides water—e.g. it dissolves in hydrocarbons, alcohols, aldehydes, ketones, ethers, esters, formic acid, nitrobenzene, aniline, pyridine, hydrogen... 

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. 

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