Solvation reactions

Four Challenges in IVIoSecula Modelling Free Energies, Solvation, Reactions and Solid-state Defects... 

Kim, H. J. and Hynes, J. T. A theoretical model for SNI ionic dissociation in solution. 2. Nonequilibriurn solvation reaction path and reaction rate, JAm.Chem.Soc., 114(1992), 10528-10537... 

A comparison is made between the gas phase and solution phase reaction pathways for a wide range of organic reactions. Examples are presented in which the gas phase and solution phase mechanisms are the same for a given set of reactants in which they differ, but attachment of the first molecule of solvent to the bare gas phase ionic reactant results in the solution phase products and in which the bare, monosolvated, and bulk-solvated reactions proceed by three different pathways for the same reactants. The various tools available to the gas phase ion chemist are discussed, and examples of their use in the probing of ionic structures and mechanisms are reported. 

Several mechanisms for the peroxide oxidation of organosilanes to alcohols are compared. Without doubt, the reaction proceeds via anionic, pentacoordinate silicate species, but a profound difference is found between in vacuo and solvated reaction profiles, as expected. In the solvents investigated (CH2CI2 and MeOH), the most favorable mechanism is addition of peroxide anion to a fluorosilane used as starting material or formed in situ, followed by a concerted migration and dissociation of hydroxide anion. In the gas phase, and possibly in very nonpolar solvents, concerted addition-migration of H2O2 to a pentacoordinate fluorosilicate is also plausible. ... 

Tetrahydrofuran (THF) is another important process solvent often used to solvate reactions involving strong bases. The workup of strong base reactions often includes an aqueous extraction, creating the problem of contaminated aqueous waste because THF is completely water-miscible. Typically, elaborate steps such as solvent exchange by distillation are taken to avoid THF contact with water. This is an energy-intensive process, and a more economical solution is desirable. Hatton s group has examined the... 

The physicochemical stage includes the chemical processes in electron excitation states, as well as the chemical transformations of the active intermediates under nonequilibrium conditions. These are the predissociation and the ion-molecular reactions that take about 1013 s the recombination of positive ions with thermalized electrons (1CT12-10 10s) and the electron-solvation reactions (10 12-10-1° s). Thus, the physicochemical stage lasts from 1CT13 to 10-I0s. 

This classification has been broadened39,40 by replacing the Brpnsted acid (proton donor) with a Lewis acid (an electron acceptor) and the Brpnsted base with a Lewis base (an electron donor). (A Brpnsted acid is a Lewis acid but not necessarily vice versa.) Solvent-proton interactions are therefore included as one subdivision of this classification, but many solvation reactions of cations with solvents also will be included as reactions of Lewis acid-base systems. This approach still does not solve the problem of fitting specific solvation interactions into the classification scheme. For example, acetonitrile behaves as a good Lewis base toward silver ion, but a poor one toward hydronium ion. The broader scheme also does not specifically take into account hydrogenbonding effects in hydroxylic and other solvents, which affect both the dielectric... 

On this conceptual premise, uptake and release of solvent can be paralleled to a solid-gas reaction, whereby the reactants are the molecules in the crystalline solid and the molecules in the gas phase and the product is the solvated crystal. Clearly, the same reasoning applies to the reverse process, i.e. generation of a new crystalline form by means of gas release. In gas-solid reactions, gases are reacted directly with crystals or amorphous phases to give complete conversion and usually quantitative yields. What would then be the difference between a solvation reaction and a reaction leading to new molecular/ionic species if not the energetic scale of the processes and the fact that in solvation processes molecules retain their chemical identity ... 

Fig. 2.30. The n dependence of differential integrated thermochemical values, AG°, AH°, and -7AS°, for three different solvation reactions. The values with n ss oo correspond to the relative thermochembal values for the bulk hydration of the ions (1 cal = 4.184 J). (Reprinted from K. Hiraoka, S. Mizuse, and S. Yamabe, J. Phys. Chem. 92 3943,1988.)...

Irradiation of aqueous solutions by ionizing radiation has been found to be a very important and selective method for the generation of ROS and RNS. Interaction of ionizing radiation with dilute aqueous solutions causes excitation and ionization of water molecules which undergo subsequent changes, mainly due to ion-molecule reactions, dissociation reactions, and solvation reactions to produce a number of radical and molecular species [Eq. (18)]. 

Many reviews of this area are available and no attempt will be made to be comprehensive here. Instead the focus will be on the relationship between photochemistry and photophysics with emphasis on the use of directly measured kinetic data (lifetimes and quantum yields) to draw mechanistic interpretations. The photochemical discussion will be limited to substitution, especially solvation, reactions. The effect of solvent motion will be explicitly treated. Reactions of a few hexacoordinated transition metal complexes will be used to illustrate the important ideas. 

If we assume that the extraction of uranium(VI) from sulphuric acid solutions with TOPO is governed by solvating reaction as in the case of the extraction from nitric and hydrochloric acid solutions ( ,3), viz. 

Warning Le Chatelier s Principle does not always predict the correct shift. Notable exceptions are solvation reactions, and pressure increase due to the addition of a nonreactive gas. The solubility of salts generally increase with increasing temperature, even when the reaction is exothermic. This is largely due to the significant entropy increase that occurs with dissolution. The entropy factor becomes more important as the temperature increases. An example of a pressure increase where... 

The equilibrium of a solvation reaction has its own equilibrium constant called the solubility product Ksp. Use Ksp the same way you would use any other equilibrium constant. Remember that solids and pure liquids have an approximate mole fraction of one and can be excluded from the equilibrium expression. Thus, solids are left out of the solubility product expression as in the example of the Ksp for barium hydroxide shown below. 

Equation (1) essentially represents a simple solvation reaction (ion-dipole interaction) the events in Eq. (2), which probably occur at the higher temperatures required for catal3dic reactions depict the labiliza-tion (polarization) of one of the hydrogens (Hi <+ ) of the solvating water molecule. The process could proceed to the extent of complete dissociation, Eq. (3), and formation of a proton, Hj.Hi could thus participate in a catalytic reaction, i.e., alcohol dehydration, and be subsequently returned to the surface hydrogen pool. It is almost certain that the populations of such entities as the right-hand members of Eqs. (2) and (3) are extremely small and that cation (M)—OH vibrations might he undetectable by infrared. 

Crerar, D.A. and Anderson, G.M., 1971. Solubility and solvation reactions of quartz in dilute hydrothermal solutions. Chem. Geol., 8 107—122. 

The unusual affinity of PPS towards a combination of ASF3 and ASF5, in view of its impenetrability by either arsenic species alone, can be explained in terms of sequential doping and solvation reactions. Without added ASF3, once the outer layers of the... 

Hydrogen can form the hydrogen ion only when its compounds are dissolved in media which solvate protons. The solvation process thus provides the energy required for bond rupture a necessary corollary of this process is that the proton, H+, never exists in condensed phases, but occurs always as solvates—H30 +, R2OH+, etc. The order of magnitude of these solvation energies can be appreciated by considering the solvation reaction in water (estimated from thermodynamic cycles) ... 

Preferential solvation in mixed solvents. In a solvent S the solvation reaction can be written as... 

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