10. Addition Reactions

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

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].)

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

f.

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?

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)

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.

Using cyclooctyne as your starting material, show how you would synthesize the following compounds. (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.

When Br₂ adds to a cis alkene that has different substituents attached to each of the two sp² carbons, such as cis-2-heptene, identical amounts of the two threo enantiomers are obtained even though Br^- is more likely to add to the less sterically hindered carbon of the bromonium ion. Explain why identical amounts of the two enantiomers are obtained.

Propose a mechanism for the following reactions:

b.