9. Alkenes and Alkynes

Predict the major product of the reaction

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?)

Predict the products of the following reactions. When more than one product is expected, predict which will be the major product.

Predict the major products of acid-catalyzed dehydration of the following alcohols.

(a)

(b)

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

(c)

Show the product(s) you expect from dehydration of the following alcohols when they are heated in sulfuric or phosphoric acid. In each case, use a mechanism to show how the products are formed.

(a)

Propose mechanisms for the following reactions.

(d)

Propose mechanisms for the following reactions.

(c)

Propose mechanisms for the following reactions.

(b)

Show the product(s) you expect from dehydration of the following alcohols when they are heated in sulfuric or phosphoric acid. In each case, use a mechanism to show how the products are formed.

(b)

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.

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

(d) How might you shift the equilibrium to the right?

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

(e) How might you shift the equilibrium to the left?

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?