THE SPECIAL PROPERTIES OF WATER

Water also possesses the property of dissolving a variety of substances and causing their ionic dissociation. There are very few other compounds which compare with water in this respect, with the exception perhaps of liquid HF, HGN and HCONH2, although these compounds have been far less investigated because of the experimental difficulties encountered in using them. 

The peculiar properties of water are as important in biology as in chemistry life on this planet would be quite different were it not for these properties. For example, if ice were heavier than water, a large part of the earth would have quite a different climate. When water freezes an ice crust forms on its surface and to a large extent prevents further freezing, since the heat in the water is lost by conduction and radiation through the ice crust, a much slower process than if its surface were water. Again, the maximum in the density prevents the formation of ice. If water had a normal coefficient of expansion, cold water would sink and the warmer water rise to the surface. Actually, this process occurs only until a 

The great solvent power of water, especially for ionic compounds, is due to its dielectric constant. If this were only, say 10, instead of the actual 80, it would mean that water could dissolve only a trace of sodium chloride. This solvent action of water., naturally. plays an important role in geology. In biology, water functions as a means of conveying salts and other substances which circulate in the bodies of animals and plants. It is outside the scope of this book to discuss any further the function of water on this planet, a subject which could fill many volumes. It is important in this context that we now know water molecules to possess a dipole moment and to discover whether perhaps this fact can provide an explanation of the unique properties of water. 

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In Chapter 1 mechanistic aspects of Are Diels-Alder reaction are discussed. The literature on the effects of solvents and Lewis-acid catalysts on this reaction is surveyed. The special properties of water are reviewed and the effects of water on the Diels-Alder reaction is discussed. Finally, the effect of water on Lewis acid - Lewis base interactions is described. 

The special properties of water—the related physics and chemistry... 

We have already encountered many aspects of temperature — indeed temperature affects essentially all processes in plants. For instance, when introducing the special properties of water in Chapter 2, we noted that physiological processes generally take place within a fairly narrow temperature range, that the water vapor content of air at saturation is very temperature dependent (also see Appendix I), and that biochemical reactions... 

Chemists now use water as a solvent for synthetic reactions, which takes advantage of the selectivities that the hydrophobic effect can induce (4). Many enzyme mimics (vide infra) also use water as the medium to promote substrate binding into the catalyst. Of course, water as a solvent also has important environmental advantages over volatile organic solvents. It remains to be seen how much impact the special properties of water as a solvent wiU have within chemical synthesis and manufacturing. 

Water of course is a covalent structure H-O-H. The special properties of water is -polarity of between O-H bonds and the resultant dipole moments of the bonds and the molecule itself. 

On the other hand, reactions of high molecular mass free radicals, e.g. occurring during pyrolysis, are slowed down by a so-called cage effect caused by solvent molecules at high pressure [5]. In some cases these effects may be reinforced by the special properties of water. A similar effect may be the reason why organometaUic complexes are able to exist and even act as catalysts at rather high temperatures in supercritical water. 

It is clear that apart from the green benefits in terms of reduced hazards and toxicity, the special properties of water as a solvent can have dramatic effects on reactions. Further research in this area is very likely to produce new reactions of general applicability. 

AH biological processes are either directly or indirectly under the influence of some of the characteristic properties of water. The special properties of water are due to the fact that it retains a relatively ordered stmcture as a liquid (Zubay, 1988). 

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