9. Alkenes and Alkynes

What stereoisomers are formed in the following reactions? Which stereoisomer is the major product?

b. the acid-catalyzed dehydration of 3,4-dimethyl-3-hexanol to 3,4-dimethyl-3-hexene

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

a.

b.

Explain why (S)-2-butanol forms a racemic mixture when it is heated in sulfuric acid.

Propose a mechanism for the following reaction:

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

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

e.

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

d.

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)