Mechanism of Bromine Addition to Alkenes

Only available on StudyMode
  • Download(s) : 158
  • Published : October 27, 2012
Open Document
Text Preview
Bromination of Trans-Cinnamic Acid

Christopher B. Martin CHEM 3411

1

Mechanism of Bromine Addition to Alkenes
Understanding the chemical mechanism (order of bonds broken and made as well as intermediates formed) has a great value in chemical synthesis. The chemical mechanism of a reaction will influence the rate of the reaction, the stereochemistry of the product(s), and the extent of possible undesired side reactions. Organic reaction mechanisms investigate the path towards a desired product and not just the destination of that product. In this reaction, we will examine the mechanism of the addition of bromine to an alkene, specifically trans-cinnamic acid. There are 4 possible mechanisms: Bromonium, Concerted, Carbocation, and Radical Mechanisms Christopher B. Martin CHEM 3411

2

1

1. Bromonium Intermediate
Br H Br CO2H H Br CO2H Ph Ph H Ph H Br Formation of the Bromonium Intermediate H Ph Br CO2H H Br Ph CO2H Br Ph H H CO2H Br H H Ph Br Br H erythro-dibromide (same side of Fischer Projection) Br Backside Attack and opening of the Bromonium Ion Br Trans Addition of the Bromine Atoms CO2H H Br CO2H H H Br

Christopher B. Martin CHEM 3411

3

2. Concerted Mechanism
Br H Br CO2H H Br Br CO H 2 Br Ph H Br CO2H H

Ph

H

Ph

H

Concerted addition, all bonds made and broken at the same time Br Ph H Br CO2H H

Both Bromines add to the same face CO2H Br H Ph CO2H H Br

Cis Addition of the Bromine Atoms

threo-dibromide (opposite side of Fischer Projection)

H Ph Br

H CO2H Br

H Br Ph

Br H

Christopher B. Martin CHEM 3411

4

2

3. Carbocation Intermediate
Br H Br CO2H H Ph Br CO2H H Br (top face) Br H Ph Br CO2H H Ph H

Electrophilic Addition of one Bromine

Carbocation Intermediate

Cis Addition of the Bromine Atoms Br CO2H H

H Ph

H Ph CO2H (bottom face) H Br Br Br CO2H

CO2H Br H Ph CO2H H Br Ph Br H H H H Br Br Br

Trans Addition of the Bromine Atoms

H H Ph CO2H Br Br Ph mixture of erythro and threo-dibromide isomers

Christopher B. Martin CHEM 3411

5

4. Radical Mechanism
Br H Br CO2H H Ph Br CO2H H Br (top face) Br H Ph Br CO2H H Ph H

Electrophilic Addition of one Bromine

Radical Intermediate

Cis Addition of the Bromine Atoms Br CO2H H

H Ph

H Ph CO2H (bottom face) H Br Br Br CO2H

CO2H Br H Ph CO2H H Br Ph Br H H H H Br Br Br

Trans Addition of the Bromine Atoms

H H Ph CO2H Br Br Ph mixture of erythro and threo-dibromide isomers

Christopher B. Martin CHEM 3411

6

3

Summary of Products and m.p.
Product Distribution Erythro dibomide Threo dibromide Both Erythro m.p. 204oC 95oC Mechanism Bromonium Concerted Carbocation –orRadical

???
(broad)

and Threo

You are to perform the bromination of trans-cinnamic acid and isolate the product. Obtain a melting point of the product and determine the mechanism through which the bromination reaction proceeded. Christopher B. Martin CHEM 3411

7

Procedure Notes
• You should use the bromine-acetic acid solution provided (premixed). • Be VERY CAREFUL around the bromination solution…it will cause chemical burns on your skin. • The presence of a brown color indicates excess bromine. This is desired during the reaction to ensure a complete reaction. • At the end of the reaction period, it may be necessary to add cyclohexene dropwise to quench any unreacted bromine. • Upon cooling in an ice bath, the product will crystallize. There is no need to recrystallize. • Make sure your crystals are dry by allowing them to have air passed through them on your Buchner funnel.

Christopher B. Martin CHEM 3411

8

4

tracking img