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

d. The mechanism when pure A compound raises its temperature to 60⁰C is given below.

Step-by-step solution

Ionization

Ionization leads to carbocation formation and is mainly expected in alkene radical cations. The nucleophile attacks that carbon which is electron deficient.

Mechanism of pure A when temperature raised to  600C

(d)The A’s ionization at temperature 60⁰C results in the same allylic carbocation as shown in (b) part. It also gives the same product ratios of A and B from butadiene.

The mechanism when pure A’s temperature raised to 60⁰C