Hydrogenation, general homogenous nature

Four general methods have been used for obtaining chiral ligands resolution of a racemic mixture, use of a chiral naturally occurring product 33), and asymmetric homogeneous or heterogeneous hydrogenation. 

Hydrogenation of aromatic nitro compounds [8,18,29] and hydrogenation of benzene derivatives [2,9,21] have been generally accepted as model reactions to check the heterogeneous nature of catalyst, because homogeneous species are not believed to be active. But at least two well-studied examples show that molecular catalysts can hydrogenate benzene [36,37]. 

In addition to water, a variety of organic liquids, including amines, carboxylic acids, and hydrocarbons, have been used as solvents in the study of the homogeneous reactions of hydrogen with metal salts. In general, there is more uncertainty about the nature of the species present in such systems than in aqueous solution and, correspondingly, it is usually more difficult to elucidate the reaction mechanisms in detail. The most extensive solvent effect studies have been made on cupric, cuprous, and silver salts. A number of the more important results are considered below. 

Hydrogen ion regulation in natural waters is provided by numerous homogeneous and heterogeneous buffer systems. It is important to distinguish in these systems between intensity factors (pH) and capacity factors (e.g., the total acid- or base-neutralizing capacity). The buffer intensity is found to be an implicit function of both these factors. In this chapter, we discuss acid-base equilibria primarily from a general and didactic point of view. In Chapter 4 we address ourselves more specifically to the dissolved carbonate system. 

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