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Organic Chemistry

8.1 Preparing Alkenes: A Preview of Elimination Reactions

Organic Chemistry8.1 Preparing Alkenes: A Preview of Elimination Reactions

8.1 • Preparing Alkenes: A Preview of Elimination Reactions

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

Just as the chemistry of alkenes is dominated by addition reactions, the preparation of alkenes is dominated by 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.

In a reversible addition-elimination reaction, alkene reacts with X single bonded to Y to form a compound, in which C1 and C2 are bonded to X and Y, respectively.

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.

In a reaction, bromocyclohexane reacts with potassium hydroxide in the presence of ethanol to form cyclohexene (81 percent yield), potassium bromide, and water.

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.

In a reaction, 1-methylcyclohexanol reacts with sulfuric acid in the presence of water, tetrahydrofuran (a common solvent) at 50 degrees Celsius to form 1-methylcyclohexene (91 percent yield) and water.

In biological pathways, dehydrations rarely occur with isolated alcohols. Instead, they normally take place on substrates in which the −OH is positioned two carbons away from a C═OC═O group. In the biosynthesis of fats, for instance, β-hydroxybutyryl ACP is converted by dehydration to trans-crotonyl ACP, where ACP is an abbreviation for acyl carrier protein. We’ll see the reason for this requirement in Section 11.10.

In a reaction, beta-hydroxybutyryl A C P forms trans-crotonyl A C P and water.
Problem 8-1
One problem with elimination reactions is that mixtures of products are often formed. For example, treatment of 2-bromo-2-methylbutane with KOH in ethanol yields a mixture of two alkene products. What are their likely structures?
Problem 8-2

How many alkene products, including E,Z isomers, might be obtained by dehydration of 3-methyl-3-hexanol with aqueous sulfuric acid?

In an incomplete reaction, 3-methyl-3-hexanol reacts with sulfuric acid to form unknown product(s), depicted by a question mark.
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