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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

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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

When methylenecyclohexane is treated with a low concentration of bromine under irradiation by a sunlamp, two substitution products are formed.

  1. Propose structures for these two products.
  2. Propose a mechanism to account for their formation.

Question: When N-bromosuccinimide is added to hex-1-ene in CCl4and a sunlamp is shone on the mixture, three products result.

  1. Give the structures of these three products.
  2. Propose a mechanism that accounts for the formation of these three products.

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.

Addition of 1-bromobut-2-ene to magnesium metal in dry ether results in formation of a Grignard reagent. Addition of water to this Grignard reagent gives a mixture of but-1-ene and but-2-ene (cis and trans). When the Grignard reagent is made using 3-bromobut-1-ene, addition of water produces exactly the same mixture of products in the same ratios. Explain this curious result.

Propose a mechanism for each reaction, showing explicitly how the observed mixtures of products are formed.

a) 3-methyl-but-2-en-1-ol+HBr→1-bromo-2-ene+3-bromo-3-methyl-1-ene

b)2-methyl-but-3-en-2a-ol+HBr→1-bromo-2-ene+3-bromo-3-methyl-1-enec)cyclopenta-1,3-diene+Br2→3,4-dibromocyclopent-1-ene+3,5-dibromocyclopent-1-ene

d)1-cholorobut-2-ene+AgNO3+H2O→but-2-en-1-ol+but-3-en-2-ol

e)3-cholorobut-1-ene+AgNO3+H2O→but-2-en-1-ol+but-3-en-2-ol
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