9. Alkenes and Alkynes

E1 eliminations of alkyl halides are rarely useful for synthetic purposes because they give mixtures of substitution and elimination products. Explain why the sulfuric acid-catalyzed dehydration of cyclohexanol gives a good yield of cyclohexene even though the reaction goes by an E1 mechanism. (Hint: What are the nucleophiles in the reaction mixture? What products are formed if these nucleophiles attack the carbocation? What further reactions can these substitution products undergo?)

Propose mechanisms for the following reactions. In most cases, more products are formed than are shown here. You only need to explain the formation of the products shown, however.

(c)

Starting from bromobenzene and any other reagents and solvents you need, show how you would synthesize the following compounds. Any of these products may be used as starting materials in subsequent parts of this problem.

c. 1-phenylpropan-2-ol

What occurs during a dehydration synthesis reaction?

Propose mechanisms for the following reactions. Additional products may be formed, but your mechanism only needs to explain the products shown.

(a)

(Hint: Hydride shift)

What is the major product obtained when each of the following alcohols is heated in the presence of H₂SO₄?

b.

If the compound shown in the margin is heated in the presence of H₂SO₄,

a. what constitutional isomer would be formed in greatest yield?

b. what stereoisomer would be formed in greater yield?

The acid-catalyzed hydration we learned here in Chapter 8 is reversible.

(a) Propose a mechanism for the formation of an alkene from an alcohol.

Under acid catalysis, tetrahydrofurfuryl alcohol reacts to give surprisingly good yields of dihydropyran. Propose a mechanism to explain this useful synthesis.

Although 2-methyl-1,2-propanediol is an unsymmetrical vicinal diol, only one product is obtained when it is dehydrated in an acidic solution.

a. What is this product?

What alkenes would you expect to be obtained from the acid-catalyzed dehydration of 1-hexanol?

Predict the major product of the reaction

Four pairs of compounds are shown. In each pair, one of the compounds reacts more quickly, or with a more favorable equilibrium constant, than the less conjugated system. In each case, explain the enhanced reactivity.

(c)

Predict the major products of dehydration catalyzed by sulfuric acid.

(a) hexan-1-ol

(b) hexan-2-ol

(c) pentan-3-ol

Predict the product of the following pinacol rearrangements.

(b)