16. Conjugated Systems

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.