10. Addition Reactions

Predict the major products of the following reactions, and give the structures of any intermediates. Include stereochemistry where appropriate.

(b)

What is the major product of each of the following reactions, assuming that one equivalent of each reagent is used in each reaction?

b.

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

(d)

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

(k)

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 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.

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.

(c)

Propose a mechanism for the following reactions:

b.

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?

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

(c)

What is the major product of each of the following reactions?

h.

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?

Identify A through O:

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