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