Question

Question: When is added to buta-1,3- diene at -15⁰C, the product mixture contains 60% of product A and 40% of product B. When the same reaction takes place at 60⁰C, the product ratio is 10% A and 90% B.

a. Propose structures for products A and B (Hint: In many case, 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 -15⁰C and B predominates at 60⁰C .

d. If you had a solution of pure A, and its temperature were raised to 60⁰C , what would you expect to happen ? Propose a mechanism to support your prediction.

Short answer

Answer

(c) A is monosubstituted, whereas B is disubstituted, so it is reasonable that B product is major and thermodynamic at60⁰C .

Step-by-step solution

Kinetic and thermodynamic product

Kinetic products form faster and are irreversible, whereas thermodynamic products are stable and reversible.Kinetic products are created at lower temperatures and thermodynamic products at higher temperatures.

Mechanism for the formation of A and B

a. The resonance form has a positive charge on carbon. It is a significant resonance contributor than . Having a greater positive charge on 2⁰ carbon than on carbon, we expect an attack of bromide ion on 2⁰ carbon to have lower activation energy. Thus, A considered a kinetic product.

When the temperature is high, the last step becomes reversible, and the product stability becomes the dominator factor for determining the ratios of product formed. A is monosubstituted, whereas B is disubstituted, so it is reasonable that B product is major and thermodynamic at 60⁰C .