Alkenes Reactions and Synthesis

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Alkcne addition reactions occur widely, both in the laboratory and in livini organisms. Although we ve studied only the addition of HX thus far, many closely related reactions also take place. In this chapter, we ll see briefly how alkenes are prepared, we ll discuss many further examples of alkene addition reactions, and we ll sec the wide v ariety of compounds that can be made from alkenes. 

Much of the background needed to understand organic reactions has now been covered, and it s time to begin a systematic description of the major fu nc-tional groups. Both in this chapter on alkenes and in future chapters on other 

Before getting to the main subject of tliis diapter—the reactions of alkenes— let s take a lirief look at liow alkenes aio prepared. The subject is a bit complex, though, so we ll return in Chapter 11 for a more detailed study. For the present, it s enough to realize that alkenes are readily available from simple precursors— Lisuallv alcohols in biological systems and either alcohols or alkyl halides in the laboratory. 

Just as the chemistry of alkenes is dominated hy addition reactions, the preparation of alkenes is dominated hy elimination reactions. Additions and eliminations are, in many respects, two sides of the same coin. That is, an addition reaction might involve the addition of HBr or H2O to an alkene to form an alkyl halide or alcohol, whereas an elimination reaction might involve the loss of HBr or H2O from an alkyl halide or alcohol to form an alkene. 

The two most common elimination reactions are dehydrohalogenation—the loss of HX from an alkyl halide— and dehydration—the loss of water from an alcohol. Dehydrohalogenation usually occurs by reaction of an alkyl halide with strong base such as potassium hydroxide. For example, bromocyclohexane yields cyclohexene when treated with KOH in ethanol solution. 

Dehydration is often carried out in the laboratory by treatment of an alcohol with a strong acid. For example, when 1-methylcyclohexanol is warmed with aqueous sulfuric acid in tetrahydrofuran (THF) solvent, loss of water occurs and 1-methylcyclohexene is formed. 

Reaction of an alkyl halide with a strong base forms an alkene, with loss of HX. 

Reaction occurs with anti stereochemistry both bromines come from opposite sides of the molecule. 

The reaction intermediate is a cyclic bromonium intermediate that is formed in a single step by interaction of an alkene with Br+. 

Alkenes add HO-X (X = Br or Cl), forming halohydrins, when they react with halogens in the presence of H20. 

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Chapter 7, Alkenes Reactions and Synthesis—Alkene epoxidation has been moved to Section 7.8, and Section 7.11 on the biological addition of radicals to alkenes has been substantially expanded. 

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