10. Addition Reactions

(•••) LOOKING AHEAD In Chapter 19, we discuss the reaction of enols with bromine. This reaction produces α -bromoketones in good yields. Suggest a mechanism for this reaction and justify its deviation from the dibromide product you might have expected.

<IMAGE>

Predict the product of the following addition reactions to dienes.

c. <IMAGE>

(••) Predict the product(s) that would result when the alkenes are allowed to react under the following conditions: (i) Br₂ (ii) Cl₂.

(e) <IMAGE>

Provide an arrow-pushing mechanism that rationalizes the stereospecific formation of each dihalide in Assessment 9.6.

(••) Predict the product(s) that would result when the alkenes are allowed to react under the following conditions: (i) Br₂ (ii) Cl₂

(f) <IMAGE>

Predict the product of the following haloalkane syntheses.

(a) ↖ Br₂ <IMAGE>

Predict the product(s) of each of the following reactions, making sure to indicate the relative stereochemical outcome. Indicate any racemic mixtures by drawing both enantiomers.

(••) In spite of being mechanistically similar to some of the reactions we saw in Chapter 8, rearrangement never occurred here in Chapter 9. Why doesn't rearrangement occur in the following bromination reaction despite the proximity of a more substituted carbon?

<IMAGE>

In contrast to the addition of Br₂ the addition of HBr [Section 8.3] is not stereospecific. Why?

<IMAGE>

Show how each of the following compounds can be synthesized from an alkene:

b. <IMAGE>

a. How does the first step in the reaction of propene with Br2 differ from the first step in the reaction of propene with HBr?

b. To understand why Br− adds to a carbon of the bromonium ion rather than to the positively charged ­bromine, draw the product that would be obtained if Br− did add to bromine.

What is the product of the addition of I—Cl to 1-butene? (Hint: Chlorine is more electronegative than iodine [Table 1.3]. <IMAGE>)

Using any alkene and any other reagents, how would you prepare the following compounds?

f. <IMAGE>

The reaction of 2-ethyl-1-pentene with Br2, with H2 + Pd/C, or with R2BH/THF followed by aqueous HO- + H2O2 leads to a racemic mixture. Explain why a racemic mixture is obtained in each case.

Using a sample of trans-2-pentene, how could you prove that the addition of Br2 forms a cyclic bromonium ion intermediate rather than a carbocation intermediate?

a. Draw the product or products that will be obtained from the reaction of cis-2-butene and trans-2-butene with each of the following reagents. If a

product can exist as stereoisomers, show which stereoisomers are formed.

4. Br₂ in CH₂Cl₂

What stereoisomers are obtained when (S)-3-methyl-1-pentene reacts with Cl₂?

Show how you would make the following compounds from a suitable cyclic alkene.

c. <IMAGE>

Cyclohexene is dissolved in a solution of lithium chloride in chloroform. To this solution is added one equivalent of bromine. The material isolated from this reaction contains primarily a mixture of trans-1,2-dibromocyclohexane and trans-1-bromo-2-chlorocyclohexane. Propose a mechanism to show how these compounds are formed.

We have seen many examples where halogens add to alkenes with anti stereochemistry via the halonium ion mechanism. However, when 1-phenylcyclohexene reacts with chlorine in carbon tetrachloride, a mixture of the cis and trans isomers of the product is recovered. Propose a mechanism, and explain this lack of stereospecificity.

<IMAGE>

(Once you have shown how to synthesize a compound,

you may use it as the starting material in

any later parts of this problem.)

(a) cis-cyclooctene

(b) cyclooctane

(c) trans-1,2-dibromocyclooctane

a. What is the major product obtained from the reaction of propene and Br2 plus excess Cl-?

b. Indicate the relative amounts of the stereoisomers that are obtained.