Acids in nonaqueous solutions

Reduction of carbon dioxide takes place at various metal electrodes. The main products are formic acid in aqueous solutions and oxalate, CO, and formic acid in nonaqueous solutions. An indium electrode is the most potential saving for C02 reduction. Due to the difference in optimum conditions between those for C02 reduction to formic acid and those for formic acid reduction to further reduced products, direct reduction of C02 in aqueous solutions without a catalyst to highly reduced products seems to be difficult at metal electrodes. However, catalytic effects of metal electrodes themselves have recently become more clear for example, on Cu, methane was detected, while on Ag and Au, CO was produced effectively in aqueous solutions. Furthermore, at a Mo electrode, methanol was obtained. The power efficiency is, however, still low at any electrode. 

This mechanism is entirely different from that of the homogeneous reaction catalyzed by Bronsted acid in nonaqueous solution. The latter mechanism, proposed by Bell et al. 56), is given below for comparison, with attention to the second-order dependence on the acid concentration ... 

F. Ding, J.M. Smith, and H. Wang, First-principles calculation ofpK values for organic acids in nonaqueous solution, J. Org. Chem. 74 (2009X pp-2679-2691. 

I would now like to consider the titration of acidic compounds in nonaqueous solutions. If you wish to titrate an acid in nonaqueous solution, you should choose a solvent that is not acidic and a titrant that is as strong a base as possible. The paper that really aroused people s imagination and created a lot of interest was the one published by Moss, Elliot, and Hall in 1948, in which they introduced ethylenediamine as a solvent. This compound certainly doesn t have any acidic properties and these authors showed that you can titrate phenol, which is normally too weak to titrate as an acid. In recent years, however, the trend has been away from the use of strongly basic solvents because they have a leveling effect on many bases and they are somewhat unpleasant to handle. Solvents now in use are pyridine, which is an inert solvent and a very weak base, acetonitrile, and acetone. Acetone and certain other ketones are surprisingly good. Recently we have done some work with tertiary butyl alcohol, an excellent solvent for certain cases. Sodium or potassium hydroxide can be used as tltrants, but these are not particularly... 

When chemists see a pattern in the reactions of certain substances, they formulate a definition of a class of substance that captures them all. The reactions of the substances we call acids and bases are an excellent illustration of this approach. The pattern in these reactions was first identified in aqueous solutions, and led to the Arrhenius definitions of acids and bases (Section J). However, chemists discovered that similar reactions take place in nonaqueous solutions and even in the absence of solvent. The original definitions had to be replaced by more general definitions that encompassed this new knowledge. 

Stabilization of a P-hairpin structure can be achieved in two ways, promoting a stable (or restricted) turn structure (as done with mimetics) or linking the two arms either chemically, or, more naturally, by hydrophobic interactions. In an approach to utilizing both methods, a D-Pro-Gly linkage was used to stabilize a left-handed turn (type I or II ) and various charged and hydrophobic residues were used to stabilize the molecule and enhance the interaction between arms. I252"254 Examples of these peptides studied in nonaqueous solution by IR, VCD and NMR spectroscopy exhibit characteristics of well-formed hairpins. 255 Alternatively, in aqueous solution, IR, VCD, and ECD results for related peptides agree with the NMR interpretation of conformations characterized as hairpins stabilized at the turn and frayed at the ends. 256 These latter results also have a qualitative match with theoretical simulations. Recently, examples of hydrophobically stabilized hairpins studied by NMR spectroscopy have avoided use of a nonnatural amino acid. 257,258 ... 

The oxo-TPs are weak acids (70MI3 73MI3). The pKa value of MOT is 6.4 (between carboxylic acids and phenols) isomers are less acidic. The oxo-TPs are formulated as a mesomeric anion (60JA605 74JOC3226), isolable salts with an N—metal bond (86KFZ178). MOT can be titrated in nonaqueous solution (DMF) with potassium methoxide (59JOC779). 

Semiaqueous or Nonaqueous Solutions. Although the measurement of pH in mixed solvents (e.g., water/organic solvent) is not recommended, for a solution containing more than 5% water, the classical definition of a pH measurement may still apply. In nonaqueous solution, only relative pH values can be obtained. Measurements taken in nonaqueous or partly aqueous solutions require the electrode to be frequently rehydrated (i.e soaked in water or an acidic buffer). Between measurements and after use with a nonaqueous solvent (which is immiscible with water), the electrode should first be rinsed with a solvent, which is miscible with water as well as the analyte solvent, then rinsed with water. Another potential problem with this type of medium is the risk of precipitation of the KC1 electrolyte in the junction between the reference electrode and the measuring solution. To minimize this problem, the reference electrolyte and the sample solution should be matched for mobility and solubility. For example, LiCl in ethanol or LiCl in acetic acid are often used as the reference electrode electrolyte for nonaqueous measurements. 

The advantages of the greater solubility of C02 in nonaqueous solution (e.g., acetonitrile) along with the absence of competing H2 evolution need further development. Glycolic acid is the product ofC02 reduction in acetonitrile. 

Reagents for epoxidation. Perox-yacetic acid is used in strongly acidic aqueous solutions. Alkenes are epoxidized, then opened to glycols in one step. Weakly acidic peroxyacids, such as peroxybenzoic acid, can be used in nonaqueous solutions to give good yields of epoxides. 

The electrophilic aromatic substitution reaction may be advantageously modeled by isotopic exchange reactions. Shatenshtein and his co-workers77-79 studied hydrogen exchange catalyzed by acids and bases in nonaqueous solution, and their studies throw considerable light on both electrophilic substitution and protophilic (base-induced) replacement of hydrogen in the system (see Section V, A). 

The foimation of aromatic diazonium salts from aromatic primary amines is one of the oldest synthetic procedures in organic chemistry. Methods based on nitrosation of amine with nitrous acid in aqueous solution are die best known, but diere are variants which are of particular use widi weakly basic amines and for the isolation of diazonium salts fiom nonaqueous media. General reviews include a book by Saunders and AUen and a survey of preparative methods by Schank. There ate also reviews on the diazotization of heteroaromatic primary amines and on the diazotization of weakly basic amines in strongly acidic media. The diazotization process (Scheme 11) goes by way of a primary nitrosamine. 

Aqueous methods and conversion methods In aqueous solutions, the Ce " ions would precipitate even in quite acidic solutions while the Ce " " ions would precipitate as Ce(OH)3 at much higher pH values (7-9) Ce(OH)3 precipitate could be oxidized into ceria in air. In nonaqueous solutions, ceria forms with decomposition of appropriate precursor, with or without the oxidation by air or other oxidative species. If doping is a target, the doping ions would be introduced during the synthesis, and the coprecipitation, sol-gel, combustion/spray pyrolysis, or hydrothermal techniques are usually employed for such a task. 

Conversion of mononuclear oxoanions into polyoxoanions requires the consumption of acid. Simple examples involving Bronsted acids see Acids Acidity) in aqueous solutions are given in equations (1-3), although alternative routes to polyoxoanions have been developed, for example, through the use of acidic oxides in aqueous and nonaqueous solvents, or by hydrothermal methods. 

The N -Nsc amino adds are easily synthesized and commerdally available (Hyundai Pharm./ Bucheon, Korea), crystalline, and stable compounds (Scheme 10). They are best prepared by acylation of the trimethylsilylated amino acids with 2-[4-(nitrophenyl)sulfonyl]ethyl chloroformate (Nsc-Cl)t l in nonaqueous solution (78-90% yield). 

When equations of the type (8) and (9) are applied to reactions in nonaqueous solution, qualitative rather than quantitative relationships are usually found. This can be attributed to the difficulties in unequivocally assigning catalytic constants on the basis of the experimental data. Bell has reviewed this problem and the problem of the acid-strength scale to be used (Bell, 43). In most cases the dissociation constants in water are used as the basis of acid strength, and a general discussion of this problem will follow the presentation of the other definitions of acids. 

However, if in nonaqueous solutions (discussed next) the oxidations also proceed through oxypalladation adducts, then the two mechanisms of decomposition of the oxypalladation adducts would predict diflFerent products. First, let us consider the mechanism of Jira, Sedlmeier, and Smidt (Reactions 50-53). In this case OH in II (Reaction 52) is replaced by OR. Decomposition via Reaction 55 is impossible, so II must decompose by solvolysis. This would give 1,1-disubstituted ethanes from ethylene oxidation. On the other hand, the first suggestion (Reaction 48) would probably be more consistent with formation of the vinyl compounds since hydride elimination should be completed if a rapid rearrangement of electrons to give acetaldehyde cannot occur. Evidence exists that 1,1-disubstituted ethanes are the initial products in methanol, and in acetic acid it is claimed that both vinyl acetate and 1,1-diace-toxyethane are initial products this suggests that in this solvent competition exists between palladium (II) hydride elimination and acetate attack. However, until now there have been no detailed studies of the oxidation under conditions where 1,1-disubstituted products are formed. More work is needed before the course of the reaction under these conditions is completely understood. 

---