The Role of Water as a Solvent

Of the millions of chemical reactions occurring in and around you, we have examined only a tiny fraction so far, and it would be impossible to examine them all. Fortunately, it isn t necessary to catalog every reaction, because when we survey even a small percentage of reactions, a few major patterns emerge. In this chapter, we examine the underlying nature of the three most common reaction processes. Many reactions occur in aqueous solution, so first we ll highlight the importance of water. 

Concepts Skills to Review Before You Study This Chapter 

Our first step toward comprehending aqueous reactions is to understand how water acts as a solvent. The role a solvent plays in a reaction depends on its chemical nature. Some solvents play a passive role, dispersing the dissolved substances into individual molecules but doing nothing further. Water plays a much more active role, interacting strongly with the substances and, in some cases, even reacting with them. To understand this active role, we ll examine the structure of water and how it interacts with ionic and covalent solutes. 

---

The molecular mechanism of the Hoffmann elimination involving (iV-Cl)-N-methyl-ethanolamine has been theoretically characterized by using DFT at the B3LYP/ 6-31++G computing level.49 The role of water as a solvent has been analysed by using both discrete and hybrid discrete-continuum models. The rearrangement proceeds by a water-assisted asynchronous concerted mechanism. 

In summary, solvents can influence Diels-Alder reactions through a multitude of different interactions, of which the contributions to fire overall rate uniquely depend on the particular solvent-diene-dienophile combination. Scientists usually feel uncomfortable about such a situation and try to extract generalities. When limited to the most extensively studied type A Diels-Alder reactions this approach seems feasible. These Diels-Alder reactions are dominated by hydrogen bonding interactions in combination with solvophobic interactions. This observation predicts a very special role of water as a solvent for type A Diels-Alder reactions, which is described in Section 1.4. 

When exposed to a polar solvent, such as water, the ethylene oxide portion of Triton X-100 becomes lyophilic, whereas the hydrocarbon section becomes lyophobic. The roles of these two moieties will be switched in case of a nonpolar solvent. Thus, the surface active behavior for a given surfactant molecule is governed by the solvent and the eonditions of the system. This section will discuss the use of water as a solvent beeause most CMP slurries are aqueous based. 

The nse of water as a solvent in organic synthesis will play key roles in green chemistry. Despite the importance of Lewis acid-catalyzed reactions in laboratories as well as in industry, however, such reactions have not been carried ont in aqueons media, becanse Lewis acids were believed to hydrolyze rapidly in the presence of water. Contrary to this belief, we fonnd that rare earth and... 

Numerous experimental studies on micellization in various surfactant solutions indicated that the values of A 5Tmjc are usually very small and often positive [1,15]. Since a spontaneous processes is accompanied by a decrease in the system free energy, small and, moreover, positive values of A5Tmjc indicate that entropical changes play a significant role in spontaneous micellization process. Such changes are primarily related to the specific features in the structure of water as a solvent (see Chapter 11,2). 

The acid-base properties of water play an important part in biological processes because of the central role of water as a solvent. The extent of self-dissociation of water to hydrogen ion and hydroxide ion. 

Water is an excellent and green solvent for many solutes. It plays an outstanding role for many processes in life and nature. Some of the important and salient features of water as a solvent are listed here ... 

In order to improve the resolution efficiency, i. e. to increase the yield of the less-soluble three-component diastereomeric salt without any deterioration in the diastereomeric purity, the effect of water in ethanol was examined for a range of 2-75% (w/w) water contents. Table 5.10 shows that the enantiomeric excess of the amine recovered from the less-soluble diastereomeric salt increased and then decreased with decreasing water content, until finally no crystal was obtained. This result indicates that the presence of water in a solvent is essential for the formation of the less-soluble diastereomeric salt and that the three-component salt could possibly deposit in a larger quantity from a solvent less polar than ethanol. On the basis of this consideration, less polar alcohols were used as solvents in the presence of a small amount of water (Table 5.11). When 2-butanol containing two moles of water was used as a solvent, the highest resolution efficiency was achieved. The diastereomeric salt crystals, obtained in all the systems shown in Table 5.11, contained an equimolar amount of water as a component. These results obviously show that water plays a very important role in the formation of stable diastereomeric salt crystals with satisfactory diastereomeric purity. The recrystaUization of the crude salt once from aqueous 2-butanol gave the diastereomeric three-component salt with diastereomeric purity of more than 95 %. The final product (S)-3-(methylamino)-l-(2-thienyl)propan-l-ol with more than 99.5% ee was obtained upon treatment of the recrystaUized salt with aqueous sodium hydroxide, followed by extraction with 2-butanol and crystallization from toluene [21]. 

The extensive use of liquid water as a solvent and reagent in chemical reactions, the widespread occurrence of water on the planet Earth, and the unique role of water as a biological life-support system combine to make an understanding of the properties of liquid water in terms of structure a matter of central importance to chemistry, the earth sciences and biology. The focus of this chapter is to review recent research studies of the structure of water at ordinary temperature and pressure, and to present an opinion on the state of knowledge about this system considered both as a structural problem in physical chemistry and as a methodological problem in computer simulation of the liquid state. ... 

---