Selectivity and the concept of alternative reaction paths

One of the characteristics of many catalytic hydrogenation reactions is the ability of the catalyst to promote the formation of more than one reaction product. Thus, for example, in the hydrogenation of acetylene, ethylene may be formed as an intermediate in the production of ethane and may be the major product in the initial stages of the reaction 

Similarly, the hydrogenation of an unsaturated aldehyde, e.g. cinnamalde-hyde to 3-phenylpropanol, may proceed by two routes as shown in Fig. 2. 

Other examples of alternative reaction pathways are shown in Table 2. 

The concept of alternative reaction pathways leads to the concepts of selectivity and stereospecificity in hydrogenation catalysis. 

The general features of selectivity in catalytic systems have been considered from a kinetic standpoint by Wheeler [35] and by Waterman et al. [36,37], According to these workers, three types of selectivity can be distinguished. Type I selectivity arises when two reactants A and C are initially present 

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