9. Alkenes and Alkynes

Indicate which alcohol in each pair undergoes an elimination reaction more rapidly when heated with H₂SO₄.

c.

Propose mechanisms for the following reactions.

(b) 

HINT: Alcohol dehydrations usually go through E1 elimination of the protonated alcohol, with a carbocation intermediate. Rearrangements are common.

Propose mechanisms for the following reactions.

d.

HINT: Alcohol dehydrations usually go through E1 elimination of the protonated alcohol, with a carbocation intermediate. Rearrangements are common.

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

When the following substituted cycloheptanol undergoes dehydration, one of the minor products has undergone a ring contraction. Propose a mechanism to show how this ring contraction occurs.

Propose a mechanism for each reaction.

(c)

Propose a mechanism for each reaction.

(a)

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.

(b)

Write a balanced equation for each reaction, showing the major product you expect.

(a)

Write a balanced equation for each reaction, showing the major product you expect.

(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.

(c)

Write a mechanism that explains the formation of the following product. In your mechanism, explain the cause of the rearrangement, and explain the failure to form the Zaitsev product.

Predict the products of the sulfuric acid-catalyzed dehydration of the following alcohols. When more than one product is expected, label the major and minor products.

(d) 1-isopropylcyclohexanol

(e) 2-methylcyclohexanol

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

(b)

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)