9. Alkenes and Alkynes

Predict the major product of the reaction

Predict the major product of the reaction

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) <IMAGE>

Identify the alcohol(s) that would produce the following alkenes under the given conditions.

(b) <IMAGE>

Identify the alcohol(s) that would produce the following alkenes under the given conditions.

(c) <IMAGE>

(•••) When using sulfuric acid, but in the absence of other nucleophiles like water or bromide ion, less stable alkenes can be isomerized to their more stable isomer. Provide a mechanism for these acid-catalyzed isomerization reactions. [This is one illustration of the principle of microscopic reversibility.]

(a) <IMAGE>

Predict the product of the following pinacol rearrangements.

(c) <IMAGE>

(••••) Suggest a mechanism for the following reactions.

(c) Elimination :

<IMAGE>

(•••) Suggest a mechanism for the following substitution reaction. [We studied a similar reaction of radicals in Assessment 11.53.]

<IMAGE>

(••) Predict the product(s) that would result when molecules (a)–(p) are allowed to react under the following conditions: (vi) H₂SO₄. If no reaction occurs, write 'no reaction.'  

(a) <IMAGE>

(•••) THINKING AHEAD The acid-catalyzed hydration we learned here in Chapter 8 is reversible [we introduce this reaction in Chapter 13]:

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(c) Which side of the reaction would be favored by running the reaction at high temperatures?

What product is obtained when the following vicinal diol is heated in an acidic solution?

<IMAGE>

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. <IMAGE>

b. <IMAGE>

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

Propose a mechanism for the following reaction:

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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. <IMAGE>

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

d. <IMAGE>

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

c. <IMAGE>

Propose mechanisms for the following reactions.

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

(b) <IMAGE>  H2SO4, heat—> <IMAGE> + <IMAGE> + <IMAGE> 

Propose mechanisms for the following reactions.

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

d. <IMAGE>. H2SO4, heat—> <IMAGE> + <IMAGE> + <IMAGE> 

Under acid catalysis, tetrahydrofurfuryl alcohol reacts to give

surprisingly good yields of dihydropyran.

Propose a mechanism to explain this useful synthesis.

<IMAGE of reaction>

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.

<IMAGE of a reaction>

Propose a mechanism for each reaction.

(c) <IMAGE of reaction>

Propose a mechanism for each reaction.

(a) <IMAGE of reaction>

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) <IMAGE of reaction>

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

(a) CH3-CH2-CH(CH3)OH H2SO4, heat—>

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

(c) <IMAGE> H2SO4, heat—>

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) <IMAGE>

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.

<IMAGE> H2SO4, heat—> <IMAGE>

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) <IMAGE> H2SO4, heat—> <IMAGE> + <IMAGE> + <IMAGE>

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) <IMAGE>

Suggest an alkene that, in two steps, could be converted into each of the following ketones. Each sequence should involve a pinacol rearrangement.

(c) <IMAGE>

Treatment of the following alcohol was expected to give alkene A. Instead, B was produced as the major product. Suggest a mechanism by which B formed.

<IMAGE>

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

a. CH3-CH2-CH2-CH2-OH

CH2=CH-CH2CH3 (Hint: Hydride shift)

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

(c) 2-methylcyclohexanol

(d) 2,2-dimethylpropan-1-ol

A vicinal diol has OH groups on adjacent carbons. The dehydration of a vicinal diol is accompanied by a rearrangement called the pinacol rearrangement. Propose a mechanism for this reaction.

<IMAGE>

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) <IMAGE of reaction>