16. Conjugated Systems

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.

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