Comments on some common solvent systems

1 Water. Water is infinitely the most important and, in many ways, the most difficult to understand. The special structure of liquid water which arises because of its (2 2) functionality is essentially a three-dimensional network of hydrogen bonds, each molecule generally forming four bonds directed roughly tetrahedrally (structure (1)). All molecules diffuse very rapidly and these bonds have short lifetimes. Some are strong, some are weak, some are bent and, presumably, some are broken. Much of this chapter relates to how spectroscopy has been used to shed light on this subject. 

2 Alcohols. Methanol and ethanol are important solvents, and are extensively used as mixed solvents with water. They mimic water in the way they solvate but, having a (1 2) functionality, they tend to form linear structures, with the capacity of branching. Comparisons between water and methanol are drawn extensively herein. 

3 Ethers and esters. These are relatively poor solvents, especially for electrolytes, since their functionality is (0 2). They solvate cations quite well, and polyethers are important in this respect. They are only sparingly soluble in water, so mixed solvents are not used. 

4 Amines. Although liquid ammonia is used widely for certain chemical reductions (which hinge on the solubility of alkali-metals therein), amines are not widely used. Ammonia has (3 1) functionality, ethylamine (2 1), etc. They are quite basic via the nitrogen lone-pair, but only very weakly acidic, and many N-H groups are essentially free in the pure solvents and in aqueous solutions. Thus they are good cation, but poor anion, solvators. 

Spectroscopic studies of a number of these solvents are described here. 

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