Mechanism of Bromine Addition to Alkenes

Bromination of Trans-Cinnamic Acid

Christopher B. Martin CHEM 3411

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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
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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

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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

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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