Nitromethane, dielectric constant

As noted above, there is no physical evidence for this equilibrium in pure N2O4, but the electrical conductivity is considerably enhanced when the liquid is mixed with a solvent of high dielectric constant such as nitromethane (e 37), or with donor solvents (D) such as MeC02Et, Et20, Mc2SO, or Et2NNO (diethylnitrosamdne) ... 

The effectiveness of nitromethane can be attributed to its high dielectric constant, at least in part, which tends to promote reactions which involve electron-rich intermediates. It may also result from the low solubility of the indole products in nitromethane since the indoles precipitate out of the reaction mixture in many cases. ... 

The photodecomposition and thermodecomposition of nitromethane have been extensively studied as model systems in combustion, explosion and atmosphere pollution processes[l]. On another hand, nitromethane was selected as a model solvent in experiments aimed at examining non hydrogen-bonded solvent effects in a general acid-base theory of organic molecules [2.3]. This selection is based on the electronic and structural characteristics of nitromethane that has a high dielectric constant, and at the same time cannot form hydrogen bonds with solute molecules. 

On the assumption that = 2, the theoretical values of the ion solvation energy were shown to agree well with the experimental values for univalent cations and anions in various solvents (e.g., 1,1- and 1,2-dichloroethane, tetrahydrofuran, 1,2-dimethoxyethane, ammonia, acetone, acetonitrile, nitromethane, 1-propanol, ethanol, methanol, and water). Abraham et al. [16,17] proposed an extended model in which the local solvent layer was further divided into two layers of different dielectric constants. The nonlocal electrostatic theory [9,11,12] was also presented, in which the permittivity of a medium was assumed to change continuously with the electric field around an ion. Combined with the above-mentioned Uhlig formula, it was successfully employed to elucidate the ion transfer energy at the nitrobenzene-water and 1,2-dichloroethane-water interfaces. 

It would be reasonable to assume that, in a solvent of high dielectric constant, such as nitromethane, the nitrile (84) is formed by direct attack of cyanide ion on an ion-pair (86) in which the bromide ion has the a-D orientation. Departure, assisted by metal ions, of the halide ion from 83 or 86, with possible assistance by the lone pair of the ring-oxygen atom, would lead to an oxonium ion (87) that could... 

DNs range from zero (solvents like hexane, tetrachloromethane), through modest donors (acetonitrile 14.1, acetone 17), to good donors like water (18), to superb donors like DMSO (29.8) and, best of all, HMPA (38.8) (see table 3.7). The DN enables us to rationalize why a solvent such as nitromethane, (6r= 35.8) is considered to be fairly nonpolar, although it has a higher dielectric constant than diethyl ether (Sr = 4.2) and tetrahydrofuran (Sr = 7.6) which are often thought to be more polar solvents than their dielectric constants would indicate. The DN of nitromethane is only 2.7, compared with that of 19.2 for diethyl ether and 20 for tetrahydrofuran. These ether solvents are much better electron-pair donors than nitromethane. 

D. L. Chapman, for potassium tri-iodide. 0. Gropp measured the effect of temp, on the conductivity of solid and frozen soln. of sodium iodide. For the effect of press, on the electrical properties, vide alkali chlorides. A. Reis found the free energy for the separation of the ions of K1 to be 144 lrilogrm. cals, per mol. for iN al, 158 Lil, 153 and for HI, 305. S. W. Serkofi 35 measured the conductivity of lithium iodide in methyl alcohol P. Walden, of sodium iodide in acetonitrile P. Dutoit in acetone, benzonitrite, pyridine, acetophenone. J. C. Philip and H. R. Courtman, B. B. Turner, J. Fischler, and P. Walden of potassium iodide in methyl or ethyl alcohol J. C. Philip and H. P. Courtman in nitromethane P. Dutoit in acetone. H. C. Jones, of rubidium iodide in formamide. S. von Lasczynsky and S. von Gorsky, of potassium and sodium iodides in pyridine. A. Heydweiller found the dielectric constants of powdered and compact potassium iodide to be respectively 3 00 and 5 58. 

The investigation of viscosities, electrical conductivities, refractive indexes and densities of binary liquid systems of sulphuric acid with nitromethane, nitrobenzene and 0-, m and p-nitrotoluene was made in order to obtain a clearer picture of the behaviour of these binary mixtures, regarding the stability of the addition compounds formed between the components. The application of these methods of physicochemical analysis to a number of binary systems with sulphuric acid [1, 2, 3] has enabled us to get some idea of the way in which the formation and stability of addition compounds affects the liquid phase properties of these systems. The binary systems of sulphuric acid with mononitrocompounds are particularly suitable for comparison with each other, because of the close similarity of the liquid media in these systems, due to comparable values of dielectric constants and liquid phase properties of the mononitrocompounds. The stability of the addition compounds in these systems in the crystalline phase [4] has... 

Mayer [22], the above correlations indeed work well and are quite useful for predicting values such as the free energy of salt solutions and complex formation in various solvents. Another typical example of the importance of the use of DN and AN as solvent parameters, instead of properties such as the dielectric constant, would be ion pair association constants in isodielectric solvents. For instance, as shown by Mayer [15], association constants of various perchlorates isocyanates, and halides (alkali metal, ammonium, and tetraalkyl ammonium cations) are very different in isodielectric solvents such as nitromethane (DN = 2.7), acetonitrile (DN = 14.1), and DMF (DN = 26.6), whose dielectric constant is around 26 at room temperature. 

Sn(CH3)3l dissolved in nitrobenzene as a function of concentration of various EPD solvents added (35). In noncoordinating or weakly coordinating solvents, such as hexane, earbon tetrachloride, 1,2-dichloroethane, nitrobenzene, or nitromethane, Sn(CH3)3l is present in an unionized state (tetrahedral molecules). Addition of a stronger EPD solvent to this solution provokes ionization, presumably with formation of trigonal bipju amidal cations [Sn(CH3)3 (EPD)2J. Table II reveals that the molar conductivities at a given mole ratio EPD Sn(CH3)3l are (with the exception of pyridine) in accordance with the relative solvent donicities. No relationship appears to exist between conductivities and the dipole moments or the dielectric constants of the solvents. 

Nitromethane and nitrobenzene have an even more pronounced effect on the rate, increasing it by at least two orders of magnitude. Both are very poor donors, but do have high dielectric constants. Apparently, there is a dissociative step when cyanoacrylate is polymerized by aliphatic amines in THF. 

X 10 M at 25°C for the trityl salts, and = 0.5 x 10 M at 0°C for the tropylium salts. Note that those Tables contain some misprints and the ordinal references should be consulted for the correct values. A recent study by Gogolczyk et al. is particularly interesting in this context because while it confirms the above value of for trityl salts in dichloromethane, it also reports for the first time values of the dissociation constant in nitromethane and in mixtures of the two solvents. The value in pure nitromethane is more than a hundred times higher than that in dichloromethane, as expected from the considerable increase in polarity. Obviously then, the proportion of free ions in CH3NO2 is very high and at salt concentrations below 10 M they will be the predominant species. It must be emphasised that these ionic salts are only sparingly soluble in solvents of low dielectric constant such as carbon tetrachloride and therefore polymerisations caimot be carried out in these media. 

Nitromethane is one of the few solvents useful for anodic reactions, among them anodic coupling of aromatic hydrocarbons. Its application for cathodic reactions is limited, but in some cases in which its unreactivity toward certain active halogen compounds is valuable it may be used. The liquid range of nitromethane is —29 to 101 °C its dielectric constant is 37, but the dissociation of salts is not as high as could be expected. 

Solvents which are poor donors are commonly used in glycoside synthesis, for instance dichlorometh-ane, cyclohexane or petroleum ether. These solvents favor SN2-type reactions. Solvents which are better donors, for instance ethers (diethyl ether, THF, etc.), acetonitrile, pyridine, nitromethane etc., each result in a typical change in the reaction course due to their different participation in the stabilization of the reaction intermediates. With ethers, acetonitrile and pyridine participation leads to onium-type intermediates (Scheme 5 8 and 9), which eventually provide, via fast equilibration, mainly the -anomer (8), due to their higher thermodynamic stability, based on the inverse anomeric effect .Thus a-product formation is often favored in these solvents (see Section 1.2.3.2.5). Solvents with even higher dielectric constants commonly result in lower diastereocontrol in glycoside synthesis. 

Solvent effect. Because of its high dielectric constant (39), nitromethane is recommended as solvent for Koenig-Knorr condensation of glycosyl bromides with ethanol in the presence of silver carbonate to yield jS-glycosides. In less polar solvents the o-glycoside can be the major product. 

The "modest" acceleration of the amine-epoxide reaction by nitromethane was ascribed to the influence of the high dielectric constant of the solvent. The greater influence of hydroxyl-containing compounds in accelerating this reaction has been suggested to result from the formation of a ternary intermediate complex of the reactants with hydroxyl-containing material, such as that proposed by Smith (6) or Mika and Tanaka (7) ... 

On dilution of the DMSO solvent with nitromethane, an inert solvent of similar dielectric constant, the overall rate of complex formation is increased significantly, a result expected on the above mechanism since the equilibrium will be displaced in favour of the intermediate. 

There is always a certain amount of ring-chlorinated by-product formed in the nitrations. Reactions carried out either by using an excess of aromatics as solvent (TiCU is miscible with many aromatics) or in carbon tetrachloride solution, always contain chlorinated by-products. The amount of chlorinated by-products can be decreased by using solvents with higher dielectric constants. Tetramethyiene sulfone (sulfolane) was found to be a suitable solvent for the TiCL and also for most of the other Lewis-acid-catalyzed nitrations. It has excellent solvent properties for aromatics and the catalysts as well as for nitryl halides. It is superior to other solvents that can be used, such as nitromethane. As it is completely miscible with water, the work-up of the reaction mixtures after the reactions are completed is very easy. 

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