16. Conjugated Systems

Which of the following compounds can be prepared in good yield by allylic halogenation of an alkene?

Show how you might synthesize the following compounds starting with bromobenzene, and alkyl or alkenyl halides of four carbon atoms or fewer.

c. dec-5-ene

Predict the products of the following reactions.

(g) 1-(bromomethyl)-2-methylcyclopentene, heated in methanol

Show how the reaction of an allylic halide with a Grignard reagent might be used to synthesize the following hydrocarbons.

a. 5-methylhex-1-ene

Show how the reaction of an allylic halide with a Grignard reagent might be used to synthesize the following hydrocarbons.

b. 2,5,5-trimethylhept-2-ene

When Br2 is added to buta-1,3-diene at –15 °C, the product mixture contains 60% of product A and 40% of product B. When the same reaction takes place at 60 °C, the product ratio is 10% A and 90% B.

a. Propose structures for products A and B. (Hint: In many cases, an allylic carbocation is more stable than a bromonium ion.)

b. Propose a mechanism to account for formation of both A and B.

c. Show why A predominates at –15 °C and B predominates at 60 °C.

A student was studying terpene synthesis, and she wanted to make the compound shown here. First she converted 3-bromo-6-methylcyclohexene to alcohol A. She heated alcohol A with sulfuric acid and purified one of the components (compound B) from the resulting mixture. Compound B has the correct molecular formula for the desired product.

a. Suggest how 3-bromo-6-methylcyclohexene might be converted to alcohol A.