Solvents, acidic aprotic

J. J. Lagowski, "The Chemistry of Nonaqueous Solvents," Inert Aprotic and Acidic Solvents, Vol. 3, Academic Press, Inc., New York, 1970 see also Chapt. 4. 

As would be expected, the larger titration potential ranges offer much more scope for mutually distinguishing between individual acids or bases in amphiprotic solvents, as a consequence of self-dissociation, the potential ranges are rather limited, whereas in the aprotic protophilic solvents and "aprotic inert solvents these ranges are considerably more extensive. 

If a molecule is an acid or base it will take on either a positive or negative charge upon its ionization. The type of solvent used will significantly influence the observed solubility of the charged species. If the solvent is aprotic or non-polar, the solubility... 

Similar processes also occur with 2,2 -bipyridine and 1,10-phenanthroline complexes of metals like Co, Cr, Ni and Ru. It is also known from the ESR study that, in the second step of Eq. (4.8), the electron is accepted not by the central metal ion but by the ligand, giving a radical anion of the ligand (see Section 8.2.2). The low-valency complexes are stabilized in aprotic solvents because aprotic solvents are of such weak acidity that they cannot liberate the coordinating ligand and its radical anion from the central metal ion. Aprotic solvents are suitable for studying the chemistry of low-valency metal complexes. 

For an organic compound (Q) in dipolar aprotic solvents, the half-wave potential ( 1/2) of the first reduction step tends to shift to the positive direction with an increase in solvent Lewis acidity (i.e. acceptor number). This is because, for the redox couple Q/Q, the reduced fonn (Q ) is energetically more stabilized than the oxidized fonn (Q) with increasing solvent acidity. The positive shift in E1/2 with solvent acceptor number has been observed with quinones [57 b], benzophenone [57 a, c] and anthracene [57 c], With fullerene (C60), the positive shift in E1/2 with solvent acidity parameter, ET, has been observed for the reductions of C60 to Qo, Qo to Clo, and Cf)0 to Cli, [54c], However, the positive shift in E1/2 is not apparent if the charge in Q is highly delocalized, as in the cases of perylene and fluoren-9-one [57 c]. 

As noted previously in Chapter 1, the electrophilic reactivities of acetyl salts increase dramatically as the acidity of the reaction medium increases. This was one of the observations that lead Olah and co-workers to first propose the concept of superelectrophilic activation, or protosolvation of the acetyl cation, in 1975.2 This seminal paper described the chemistry of acetyl hexafluoroantimonate (CHsCO+SbFg-) and the reaction with alkanes in various solvents. In aprotic solvents such as SO2, SO2CIF, AsF3, and CH2CI2, there was no reaction. However in HF-BF3, acetyl salts react with Ao-alkanes and efficient hydride abstraction is observed.27 This was interpreted by Olah as evidence for protonation of the acetyl... 

Since the advantage of using nonaqueous systems in electrochemistry lies in their wide electrochemical windows and low reactivity toward active electrodes, it is crucial to minimize atmospheric contaminants such as 02, H20, N2, C02, as well as possible protic contaminants such as alcoholic and acidic precursors of these solvents. In aprotic media, these contaminants may be electrochemically active on electrode surfaces, even at the ppm level. In particular, when the electrolytes comprise metallic cations (e.g., Li, Mg, Na), the reduction of all the above-mentioned atmospheric contaminants at low potentials may form surface films as the insoluble products precipitate on the electrode surfaces. In such cases, the metal-solution interface becomes much more complicated than their original design. Electron transfer, for instance, takes place through electrode-solution rate limiting interphase. Hence, the commonly distributed solvents and salts for usual R D in chemistry, even in an analytical grade, may not be sufficient for use as received in electrochemical systems. 

Figure 2.8 Specific conductivities of protic vs. aprotic ionic liquids, showing matching of concentrated lithium chloride solution conductivity by solvent-free aprotic liquids. Note that at low temperature, the conductivity of protic nitrate in excess nitric acid is higher than that of the aqueous Lid case with the same excess solvent. (From Xu and Angell [17] by permission)...

Formaldehyde is employed in Mannich reactions either as an aqueous solution ( formalin ) or in the form of paraformaldehyde or trioxanc. The amine reactant is used as a free base or hydrochloride. The most commonly adopted solvents for the reaction are alcohols (ethanol mainly, methanol, and isopropanol), water, or acetic acid. Aprotic solvents or neat conditions are also occasionally used. 

As with ketone enolate anions (see 16-34), the use of amide bases under kinetic control conditions (strong base with a weak conjugate acid, aprotic solvents, low temperatures), allows the mixed Claisen condensation to proceed. Self-condensation of the lithium enolate with the parent ester is a problem when LDA is used as a base, ° but this is minimized with LICA (lithium isopropylcyclohexyl amide).Note that solvent-free Claisen condensation reactions have been reported. ° ... 

Optimization of the experimental conditions for selective formation of 24a-d in an undivided cell was examined [96]. The product of mixed coupling, 24a, was favored (88%) by reduction in AcOH/Ac20 (1 1). Most other conditions (strongly acidic, aprotic, or basic) favored 4,4 -coupling. The dehydrated cyclic compound 24c (62%) and the hydrated form 24b (20%) were formed in strongly acidic conditions (pH 1.1). Product 24b (cisjtrans =1 1) was favored by reduction in aprotic solvents (DMF, Et4NOTs) or by basic conditions. Cyclization was best avoided by reduction at elevated temperature... 

Extensive studies have been made of solvent effects on atom transfer reactions involving ions [12]. In the case of reaction (7.3.23), the rate constant decreases from 250M s in A-methylpyrrolidinone to 3 x 10 M s in methanol. This effect can be attributed to solvation of the anionic reactant Cl and the anionic transition state [12]. Since the reactant is monoatomic, its solvation is much more important. It increases significantly with solvent acidity leading to considerable stabilization of the reactants. As a result the potential energy barrier increases and the rate decreases with increase in solvent acidity. As shown in fig. 7.7, this leads to an approximate linear relationship between the logarithm of the rate constant and the solvent s acceptor number AN, an empirical measure of solvent acidity (see section 4.9). Most of the results were obtained in aprotic solvents which have lower values of AN. The three data points at higher values of AN are for protic solvents. 

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. 

Polyimides can be prepared through several step-growth processes. One of the most common methods used for this purpose is the reaction of dianhydrides with diamines (Fig. 3.6). The first product obtained is poly(amic acid), which has the advantage of being soluble in organic solvents (normally aprotic polar solvents are used, such as dimethyl sulfoxide, DMF, dimethylacetamide, and NMP). The solution of poly(amic acid) can easily be manipulated or processed. The acid can then be cycled thermally, generating the polyamide, which is insoluble in most of the common organic solvents. 

Acid catalysis in aqueous solution is well understood, however, it has been less studied in other polar protic solvents and aprotic solvents. Although solvent effects on acidity (p a) are documented independently in the literature, the first comprehensive, overarching book in this area was published recently in 2013 by Cox. Acidities in most common solvents are discussed, however, supercritical and higher temperature fluids are not considered. Some general considerations in this area are presented briefly below, however, for more detail we direct the reader to this excellent book. 

Polyoxyalkylene-modified silane (SiHX) is combined with non-aqueous solvent and electrolyte salt to form non-aqueous solution. Preferred SiHX is 0.001 to 0.1% by volume of solution. Light metals such as Li, Na, K, Mg, Ca, and A1 are used as electrolyte salts in concentration of 0.5 to 2 M. Suitable solvents include aprotic dielectric constant compounds such as ethylene carbonate, y-butyrolactone, and aprotic low viscosity solvents such as dimethylcarbonate, sulfolane, and acetic acid esters. 

All performed in protic media, the extensive exchange experiments discussed above carry intriguing implications for aprotic solvents. Here ylid concentrations are not suppressed ndi. are their dimers cleaved as a consequence of solvent acidity, so it mould be possible to prepare a host of yet-unknown heteroatom-substituted eAylenes by the ylid route. In addition to the peraminoethylenes synthesized via ylids, in fact, several 1,2-diamino-1,2-dithioethylenes and tetrathioethylenes have already been prepared by deprotonation of thiazoUum and 1,3-dithiolium salts, respectively. 

Anticodon Sequence of three bases in a molecule of tRNA that is complementary to the codon of mRNA for a particular amino acid. Aprotic solvent A solvent that does not have easily exchangeable protons such as those bonded to oxygen of hydroxyl groups. 

Such an assumption was proposed, namely that a bridge consisting of a 0.1 mol dm tetraethylammonium picrate in acetonitrile suppresses the liquid junction potential between two different nonaqueous electrolytes [6]. The argument in favor of such a salt bridge for nonaqueous electrolytes is the similar electrical mobility of the tetraethylammonium cation and the picrate anion in acetonitrile. This assumption was later expanded to allow for other nonaqueous solvents [28]. Agreement for the electrochemical data was found if the nonaqueous solvents did not have acidic hydrogen atom(s) in the solvent molecule (aprotic solvents) [29], 0.1 mol dm solutions of either tetrabutylammonium picrate or pyridinium trifluorosulftMiate [30] were also used. 

The most commonly used protected derivatives of aldehydes and ketones are 1,3-dioxolanes and 1,3-oxathiolanes. They are obtained from the carbonyl compounds and 1,2-ethanediol or 2-mercaptoethanol, respectively, in aprotic solvents and in the presence of catalysts, e.g. BF, (L.F. Fieser, 1954 G.E. Wilson, Jr., 1968), and water scavengers, e.g. orthoesters (P. Doyle. 1965). Acid-catalyzed exchange dioxolanation with dioxolanes of low boiling ketones, e.g. acetone, which are distilled during the reaction, can also be applied (H. J. Dauben, Jr., 1954). Selective monoketalization of diketones is often used with good success (C. Mercier, 1973). Even from diketones with two keto groups of very similar reactivity monoketals may be obtained by repeated acid-catalyzed equilibration (W.S. Johnson, 1962 A.G. Hortmann, 1969). Most aldehydes are easily converted into acetals. The ketalization of ketones is more difficult for sterical reasons and often requires long reaction times at elevated temperatures. a, -Unsaturated ketones react more slowly than saturated ketones. 2-Mercaptoethanol is more reactive than 1,2-ethanediol (J. Romo, 1951 C. Djerassi, 1952 G.E. Wilson, Jr., 1968). 

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