Addition of H2 and X2 Reagents

In this section, we see two very different reactions of diatomic reagents, the addition of H2 and of X2. In the first reaction, the concerted addition of hydrogen to the double bond of an alkene, a catalyst is required to get things going. The second reaction, halogen addition, fits better into the pattern seen in Chapter 9, in which a two-step process is followed. 

FIGURE 10.1 Hydrogen gas and an alkene react very quickly in the presence of a metal catalyst such as palladium adsorbed on charcoal (Pd/C). The rhodium-based Wilkinson s catalyst is typical of soluble molecules that also catalyze hydrogenation. 

PROBLEM 10.1 Use bond dissociation energies (BDE) (Table 8.2, p. 337) to calculate the exothermicity or endothermicity of the hydrogenation of ethylene to ethane. 

Nonmetallic reducing agents can be used in alkene hydrogenation as well. Diimide (HN NH) is a good example. It is not stable under normal conditions and is made only as it is needed. Diimide exists in two forms, cis and trans, but only the less stable cis form is active in hydrogenation (Fig. 10.2). 

FIGURE 10.2 An effective nonmetaUic reducing agent for alkenes is diimide 

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Show the products, including regiochemistry and stereochemistry, resulting from the addition to alkynes of all of the reagents listed in this chapter. These reagents include HX, X2, Hg2+/H+/H20, disiamylborane, and H2. (Problems 11.32 and 11.33)... 

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