Solvation chemistry

Solvent use in chemistry is pervasive (Nelson, 1998). Probably one of the most critical components of a reaction or chemical process is the identity of the solvent(s) used. In the emerging area of green chemistry, careful reasoning must enter into the choice of solvents. Chemists use solvents for many reasons, but primarily they are used as reaction media, in separation/purification technologies and in cleaning technologies. 

The most general effect a solvent may have on a solute dissolved in it is the solvation of the solute. Often the solute may not only be a solute foreign to the solvent, but may also be a molecule of the solvent itself. There is no limitation on the concentration of the solute, so that it may dissolve and be solvated in a solution that already contains this solute as well as other ones (Marcns, 1998b). 

The quality and suitability of reactions and chemical processes are usually dependent upon the quality of the solvent utilized. As mentioned previously, the largest volume of solvents is used in the manufacturing and cleaning industries. 

According to the end goal of green chemistry, the criteria for what determines a green solvent will vary, depending upon the use and ultimate fate of the solvent during the process. Likewise, the determination of what constitutes appropriate solvent usage in 

then constitutes an appropriate solvent for green chemistry Fundamentally, it must be a solvent which allows the chemist to accomplish his or her task, but in an environmentally conscious manner. The modem chemist is expected to consider toxicity as part of the key elements affecting choice of solvents, so that they are minimally toxic to human health and the environment, and disposed of in ways that do not contribute to pollution (DeVito, 1996). The considerations for what constitutes a safe solvent might contain some of the following considerations  

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J. Tomasi, Thirty years of continuum solvation chemistry A review, and prospects for the near future. Theor. Chem. Account 112, 184 (2004)... 

Sometimes chemical reaction or solvation chemistry is exploited by the extraction process. Often, coordination reactions render a lipophobic species lipophilic by complexation with suitable organic ligands, which cluster around the lipophobic moiety in the complex and enable it to transfer into the solvent phase. The oily organic or solvent phase in such instances usually consists of an... 

Solubility— The energy density of a semi-solid flow battery is not limited by the solvation chemistry in the electrolyte, but rather depends on the rheology of the suspension (the suspension must still be flowable). For example, 20 vol.% LiCo02 (p = 5.06 g/cm, MW = 97.9 g/mol) can be easily dispersed in the... 

J. Tomasi, Theor. Chem. Acc., 112(4), 184-203 (2004).Thirty Years of Continuum Solvation Chemistry A Review, and Prospects fot the near Future. 

Free energy perturbation (FEP) theory is now widely used as a tool in computational chemistry and biochemistry [91]. It has been applied to detennine differences in the free energies of solvation of two solutes, free energy differences in confonnational or tautomeric fonns of the same solute by mutating one molecule or fonn into the other. Figure A2.3.20 illustrates this for the mutation of CFt OFl CFt CFt [92]. 

The solute-solvent interaction in equation A2.4.19 is a measure of the solvation energy of the solute species at infinite dilution. The basic model for ionic hydration is shown in figure A2.4.3 [5] there is an iimer hydration sheath of water molecules whose orientation is essentially detemiined entirely by the field due to the central ion. The number of water molecules in this iimer sheath depends on the size and chemistry of the central ion ... 

The treatment of equilibrium solvation effects in condensed-phase kmetics on the basis of TST has a long history and the literature on this topic is extensive. As the basic ideas can be found m most physical chemistry textbooks and excellent reviews and monographs on more advanced aspects are available (see, for example, the recent review article by Tnihlar et al [6] and references therein), the following presentation will be brief and far from providing a complete picture. 

In either case, the structure of the solvation shell has to be calculated by otiier methods supplied or introduced ad hoc by some fiirther model assumptions, while charge distributions of the solute and within solvent molecules are obtained from quantum chemistry. 

Solutions of alkali metals in liquid ammonia are used in organic chemistry as reducing agents. The deep blue solutions effectively contain solvated electrons (p. 126), for example... 

Cramer C J and Truhlar D G 1995. Continuum Solvation Models Classical and Quantum Mechanical Implementations. In Lipkowitz K B and D B Boyd (Editors) Reviews in Computational Chemistry Volume 6. New York, VCH Publishers, pp. 1-72. 

Chambers C C, G D Hawkins, C J Cramer and D G Tmlilar 1996. Model for Aqueous Solvation Ba sed on Class IC Atomic Charges and First Solvation Shell Effects. Journal of Physical Chemistry 100 16385-16398. 

Klamt A 1995. Conductor-like Screening Model for Real Solvent A New Approach to the Quantitativt Calculation of Solvation Phenomena. Journal of Physical Chemistry 99 2224-2235. 

W C, A Tempcz)rrk, R C Hawley and T Hendrickson 1990. Semianalytical Treatment of Solvation for Molecular Mechanics and Dynamics. Journal of the American Chemical Society 112 6127-6129. ensson M, S Humbel, R D J Froese, T Matsubara, S Sieber and K Morokuma 1996. ONIOM A Multilayered Integrated MO + MM Method for Geometry Optimisations and Single Point Energy Predictions. A Test for Diels-Alder Reactions and Pt(P(t-Bu)3)2 + H2 Oxidative Addition. Journal of Physical Chemistry 100 19357-19363. 

Solvent effects on chemical equilibria and reactions have been an important issue in physical organic chemistry. Several empirical relationships have been proposed to characterize systematically the various types of properties in protic and aprotic solvents. One of the simplest models is the continuum reaction field characterized by the dielectric constant, e, of the solvent, which is still widely used. Taft and coworkers [30] presented more sophisticated solvent parameters that can take solute-solvent hydrogen bonding and polarity into account. Although this parameter has been successfully applied to rationalize experimentally observed solvent effects, it seems still far from satisfactory to interpret solvent effects on the basis of microscopic infomation of the solute-solvent interaction and solvation free energy. 

The central role of the concept of polarity in chemistry arises from the electrical nature of matter. In the context of solution chemistry, solvent polarity is the ability of a solvent to stabilize (by solvation) charges or dipoles. " We have already seen that the physical quantities e (dielectric constant) and p (dipole moment) are quantitative measures of properties that must be related to the qualitative concept of... 

The interaction between a solute species and solvent molecules is called solvation, or hydration in aqueous solution. This phenomenon stabilizes separated charges and makes possible heterolytic reactions in solution. Solvation is, therefore, an important subject in solution chemistry. The solvation of ions has been most thoroughly studied. 

Be sure to remind students that these frequencies are gas phase data and arc thus not the same as the more-faniiliar solution spectra (we will treat solvated systems in Chapter 9). Even so, such gas phase calculations make excellent discovery-based exercises, For example, students may be asked to explain the substituent effects observed tising basic chemistry knowledge. 

Oxygen chelates such as those of edta and polyphosphates are of importance in analytical chemistry and in removing Ca ions from hard water. There is no unique. sequence of stabilities since these depend sensitively on a variety of factors where geometrical considerations are not important the smaller ions tend to form the stronger complexes but in polydentate macrocycles steric factors can be crucial. Thus dicyclohexyl-18-crown-6 (p. 96) forms much stronger complexes with Sr and Ba than with Ca (or the alkali metals) as shown in Fig. 5.6. Structural data are also available and an example of a solvated 8-coordinate Ca complex [(benzo-l5-crown-5)-Ca(NCS)2-MeOH] is shown in Fig. 5.7. The coordination polyhedron is not regular Ca lies above the mean plane of the 5 ether oxygens... 

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