Solvent selectivity hydrogen-bonding systems

B. Solvent Selectivity Effects—Non-Hydrogen-Bonding Systems... 

Less is known about the relative Importance of solvent-solute hydrogen bonding in affecting solvent selectivity, although several workers have postulated such effects (e.g., /. 42, 43). However, one must be cautious in accepting these various observations at face value, because solvent-solute localization effects will normally be large in systems that can ex-... 

Dielectric constants cannot explain, quantitatively, most physicochemical properties and laws of solutions, and we shall soon see that they can become unimportant. The molecules of more polar solvents, which tend to cluster around the ions and dipole ions, produce a preferential or selective solvation that is reflected in measurements of such properties as solubility, acid—base equilibria, and reaction rates. Nonelectrostatic effects, such as the basicity of some solvents, their hydrogen-bonding, and the internal cohesion and the viscosity of mixtures, probably interfere with the electrostatic effects and thus reduce their actual influence. On the other hand, mixtures of water and nonaqueous solvents are enormously complicated systems, and their effective microscopic properties may be vastly different from their macroscopic properties, varying with the solute because of selective attraction of one of the solvents for the solute. 

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. 

THF and methanol employed as organic modifiers of mobile phase provided a considerable difference in selectivity based on the polar interactions between solutes and the organic solvent molecules in the stationary phase. Acidic compounds, phenols and nitroaromatics, were preferentially retained in the THF-based mobile phase, whereas esters and ketones were preferentially retained in the methanol (a hydrogen-bond donor) containing mobile phase. The system presented here seems to be very practical because any laboratory possessing two sets of HPLC equipment and two C j g columns can attempt similar 2D HPLC by simply changing the mobile phase for the two dimensions. 

Solvent-resistant elastomers, 9 560-562 Solvent-resistant membranes, 27 656 Solvent-resistant rubber, 22 583-584 Solvent selection, in liquid-liquid extraction, 70 746-749 Solvent-solute interactions, 26 855,23 91-96 acid/base interactions in, 23 96 dispersion in, 23 92-93 electrostatic forces in, 23 91-92 hydrogen bonding in, 23 94-95 hydrophobic interactions in, 23 95 polarization in, 23 92 repulsion in, 23 93-94 Solvent strength, of pure fluids, 24 3-4 Solvent systems, for acid gas removal, 72 376-377... 

It is very instructive to compare the kinetics and plausible mechanisms of reactions catalyzed by the same or related catalyst(s) in aqueous and non-aqueous systems. A catalyst which is sufficiently soluble both in aqueous and in organic solvents (a rather rare situation) can be used in both environments without chemical modifications which could alter its catalytic properties. Even then there may be important differences in the rate and selectivity of a catalytic reaction on going from an organic to an aqueous phase. TTie most important characteristics of water in this context are the following polarity, capability of hydrogen bonding, and self-ionization (amphoteric acid-base nature). 

The selective hydrogenation of enones is also achieved in a process employing an aluminium-mckel system. This process is electrochemical in nature but does not use an external electron source. Dissolving aluminium is used as the reducing agent with a catalytic amount of nickel chloride present in the tetrahydrofuran solvent. Finely divided nickel is deposited on tlie aluminium and this sets up local corrosion cells. Aluminium dissolves and tlie released electrons are transferred to nickel where protons are reduced to hydrogen. The hydrogen-nickel system then reduces the alkene bond in the enone [153]. 

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