Dimethylformamide, proton transfer

Complexes of maltose with urea, thiourea, imidazole, methanol, 2-oxazolidinone, N,N-dimethylformamide, and hexamethylphosphoric triamide have been described.131 These complexes were noncrystalline and hygroscopic. Such complex-forming reactions could be valuable in the preservation of food flavors during the dehydration process. Sugars have been shown to complex with ethylenediamine.133,133 The nature of the complex has been suggested to be that of a proton-transfer type, in which the carbohydrate moiety is the proton donor and the amine is the proton acceptor. 

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,A -Dimethylformamide [DMFH][N03] ion pairs were optimized with the 6-31G Pople basis set and the B3LYP hybrid functional [41]. Subsequently, energies with the 6-311++G basis set were obtained. The enol form of the [DMFH]+ cation was observed to form three stable conformers with the anion, while the cation of the keto form is unstable and the proton transfer occurs to form three kinds of neutral molecule pairs [41], Moreover, the neutral pairs were more stable than the ion pairs, and the ion pairs tended to tautomerize to neutral pairs without barriers, which was interpreted as decomposition of the ILs [41],... 

Scheme 2.19 depicts a typical example of the coupling of acid-base reactions, here protonations, with electron transfer. In a dry aprotic solvent [e.g., /V./V-dimethylformamide (DMF)], an aromatic hydrocarbon such as anthracene exhibits two successive reversible cyclic voltammetric waves (suspensions of neutral alumina may be used efficiently to dry the solvent... 

Organic solvents can also be classified according to their ability to accept or transfer protons (i.e., their acid-base behavior) [20,21]. Amphiprotic solvents possess donor as well as acceptor capabilities and can undergo autoprotolysis. They can be subdivided into neutral solvents that possess approximately equal donor and acceptor capabilities (water and alcohols), acidic solvents with predominantly proton donor properties (acetic acid, formic acid), and basic solvents with primarily proton acceptor characteristics (formamide, N-methylformamide, and N,N-dimethylformamide). Aprotic solvents are not capable of autoprotolysis but may be able to accept protons (ACN, DMSO, propylene carbonate). Inert solvents (hexane) neither accept nor donate protons nor are they capable of autoprotolysis. 

These chemical electron-transfer reactions are in contrast with electrode reactions. For instance, stilbene gives a mixture of 1,2-diphenylethylene with 1,2,3,4-tetraphenylbu-tane upon electrolysis (Hg) in dimethylformamide at potentials around that of the first one-electron wave. This solvent has faint proton-donor properties. Stilbene anion radical is stable under these conditions it has enough time to diffuse from the electrode into the solvent and to dimerize therein (Wawzonek et al. 1965). 

The simplest reactions involve only mass transfer of a reactant to the electrode, heterogeneous electron transfer involving nonadsorbed species, and mass transfer of the product to the bulk solution. A representative reaction of this sort is the reduction of the aromatic hydrocarbon 9,10-diphenylanthracene (DPA) to the radical anion (DPAt) in an aprotic solvent (e.g., 7V,A-dimethylformamide). More complex reaction sequences involving a series of electron transfers and protonations, branching mechanisms, parallel paths, or modifications of the electrode surface are quite common. When a steady-state current is obtained, the rates of all reaction steps in a series are the same. The magnitude of this current is often limited by the inherent sluggishness of one or more reactions called ratedetermining steps. The more facile reactions are held back from their maximum rates by... 

Acetonitrile, as noted in sect. 3.6.3, is much less basic than water, di-methylsulphoxide, or dimethylformamide. The medium effect for transfer of the proton to it from water is 4.3 1.5 (Table 3.3.3). In mixtures with methanol or ethanol, acetonitrile effectively behaves as an inert component. Anions cannot be stabilised by hydrogen-bonding them to acetonitrile and therefore undergo homo- and heteroconjuga-tion. The data in Table 3.6.2 show that homoconjugation... 

As a rule it is best to carry out electrochemical generation in media where there is a single one-electron polarographic wave this occurs under conditions where the protonation of radical anions formed in the first electron transfer is either excluded or retarded, i. e., in polar aprotic solvents (acetonitrile, dimethylformamide, tetrahydrofuran, 1,2-dimethoxyethane, and others). 

In buffered aqueous ethanolic and dimethylformamide solutions Schiff s bases (prepared from aniline and salicylaldehyde or o-tosyl-aminobenzaldehyde derivatives) with an intramolecular hydrogen bond give two one-electron waves. In aqueous ethanolic solution both waves are shifted towards negative potentials with increase in the pH value, which indicates that electron transfer is preceded by protonation. 

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