Summary

With the background needed to understand organic reactions now covered, this chapter has begun the systematic description of major functional groups.

Alkenes are generally prepared by an elimination reaction, such as dehydrohalogenation, the elimination of HX from an alkyl halide, or dehydration, the elimination of water from an alcohol. The converse of this elimination reaction is the addition of various substances to the alkene double bond to give saturated products.

HCl, HBr, and HI add to alkenes by a two-step electrophilic addition mechanism. Initial reaction of the nucleophilic double bond with H⁺ gives a carbocation intermediate, which then reacts with halide ion. Bromine and chlorine add to alkenes via three-membered-ring bromonium ion or chloronium ion intermediates to give addition products having anti stereochemistry. If water is present during the halogen addition reaction, a halohydrin is formed.

Hydration of an alkene—the addition of water—is carried out by either of two procedures, depending on the product desired. Oxymercuration–demercuration involves electrophilic addition of Hg²⁺ to an alkene, followed by trapping of the cation intermediate with water and subsequent treatment with NaBH₄. Hydroboration involves addition of borane (BH₃) followed by oxidation of the intermediate organoborane with alkaline H₂O₂. The two hydration methods are complementary: oxymercuration–demercuration gives the product of Markovnikov addition, whereas hydroboration–oxidation gives the product with non-Markovnikov syn stereochemistry.

Alkenes are reduced by addition of H₂ in the presence of a catalyst such as platinum or palladium to yield alkanes, a process called catalytic hydrogenation. Alkenes are also oxidized by reaction with a peroxyacid to give epoxides, which can be converted into trans-1,2-diols by acid-catalyzed hydrolysis. The corresponding cis-1,2-diols can be made directly from alkenes by hydroxylation with OsO₄. Alkenes can also be cleaved to produce carbonyl compounds by reaction with ozone, followed by reduction with zinc metal. In addition, alkenes react with divalent substances called carbenes, R₂C:, to give cyclopropanes. Nonhalogenated cyclopropanes are best prepared by treatment of the alkene with CH₂I₂ and zinc–copper, a process called the Simmons–Smith reaction.

Alkene polymers—large molecules resulting from repetitive bonding of many hundreds or thousands of small monomer units—are formed by chain-reaction polymerization of simple alkenes. Polyethylene, polypropylene, and polystyrene are examples. As a general rule, radical addition reactions are not common in the laboratory but occur frequently in biological pathways.

Many reactions give chiral products. If the reactants are optically inactive, the products are also optically inactive. If one or both of the reactants is optically active, the products can also be optically active.