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Chapter 15: Q8-b (page 763)

Question: When Br2 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

Expert verified

Answer

(b) The mechanism for forming products A and B are given below.

Step by step solution

01

Bromination

Bromination is the reaction involving the bromine addition and is considered an electrophilic aromatic substitution. In the case of an alkene, allylic carbocations are formed, and bromine can attack either one.

02

Mechanism of the formation of A and B

(b) When bromination of buta-1,3- diene occurs, two resonance stabilized allylic carbocations are formed. The two carbons are electron deficient, so the nucleophiles attack either one carbon.


The mechanism for the formation of A and B

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Most popular questions from this chapter

Predict the products of the following Diels-Alder reactions.

a)

b)

c)

d)

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.

Question: The central carbon atom of an allene is a member of two double bonds, and it has an interesting orbital arrangement that holds the two ends of the molecule at right angles to each other.

  1. Draw an orbital diagram of allene, showing why the two ends are perpendicular.
  2. Draw the two enantiomers of penta-2,3-diene. A model may be helpful.

Question: Draw another resonance form for each of the substituted allylic cations shown in the preceding figure, showing how the positive charge is shared by another carbon atom. In each case, state whether your second resonance form is a more important or less important resonance contributor than the first structure. (Which structure places the positive charge on the more-substituted carbon atom?)

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.
See all solutions

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