10. Addition Reactions

Predict the product of the following addition reactions to dienes.

(c)

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.

(f)

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.

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

(d)

Bromination of a highly electron-rich alkene such as 2-methoxybut-2-ene has been shown to produce approximately equal mixtures of the trans- and cis-dibromide. Suggest an explanation for this observation.

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.

Give mechanisms to account for the stereochemistry of the products observed from the addition of bromine to cis- and trans-but-2-ene (Figure 8-5). Why are two products formed from the cis isomer but only one from the trans? (Making models will be ­helpful.)

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.

Predict the product of the following reactions.

(d)

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 mechanisms and predict the major products of the following reactions. Include stereochemistry where appropriate.

(c) (E)-dec-3-ene + Br₂ in CCl₄

(d) (Z)-dec-3-ene + Br₂ in CCl₄

Problem-Solving Hint: Models may be helpful whenever stereochemistry is involved. Write complete structures, including all bonds and charges, when writing mechanisms.

Predict the product of the following haloalkane syntheses.

(a)

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

In which of the two steps in the alkene halogenation mechanism does a redox reaction occur?