Bromination of trans-cinnamic acid
Alkenes are important starting materials because they undergo a variety of electrophilic addition
reactions. One such reaction is halogenation, either chlorination or bromination, which leads to a
dihalide as shown below in Figure 1. Common solvents for this reaction include dichloromethane and
Figure 1: Halogenation of an alkene
X = Br or Cl
Since the reagent, Cl2 or Br2, is symmetric, we don’t need to worry about issues of regioselectivity, i.e. a
halogen ends up on each carbon atom of the starting alkene. An interesting question, however, is the
relative orientation of the two halogens after they have added to the alkene. For instance, reaction of
cyclohexene with bromine could result in the formation of three different products (see Figure 2).
Product A is the meso stereoisomer and is called the syn addition product, i.e. the halogens both added
to the same side of the alkene. Products B and C are a pair of enantiomers that result from anti
addition, i.e. the halogens added to opposite sides of the alkene.
Figure 2: Possible products for the bromination of cyclohexene
In practice, the reaction of bromine with cyclohexene gives the enantiomeric mixture (B and C) as the
major product. Thus, this reaction goes via an anti-addition. The mechanism shown below in Figure 3
accounts for the observed stereochemical result by invoking the formation of a bromonium ion
intermediate. Subsequent reaction of bromine anion with the bromonium ion intermediate, via SN2 like
backside attack, provides the anti-addition product.
Figure 3: The mechanism for bromination of cyclohexene
Description of the Experiment:
One might wonder how general the anti-addition reaction is for bromination reactions. Cyclohexene is a
fairly simple alkene with no other functionality in the compound. In contrast, trans -cinnamic acid
(shown below) is a more complicated alkene that is found in a variety of natural sources including oil of
Bromination of trans-cinnamic acid can result in the formation of two different pairs of enantiomers (see
Figure 4). Fortunately, the pairs of enantiomers have very different melting points. Thus, we can
determine the result of this reaction by examining the melting point of the product(s) that we obtain.
Figure 4: Possible products for the bromination of trans-cinnamic acid
Pair of enantiomers
mp 93 - 95 °C
Pair of enantiomers
mp 202 - 204 °C
(2S, 3S)-dibromo3-phenylpropanoic acid
1 M bromine in dichloromethane
(2R, 3R)-dibromo3-phenylpropanoic acid
(2S, 3R)-dibromo3-phenylpropanoic acid
(2R, 3S)-dibromo3-phenylpropanoic acid
50% aqueous ethanol
- Create a table of reagents and their important physical properties in your lab notebook.
- Write out a proposed mechanism for the formation of each possible product.
- Prepare a flow chart of the steps that you will take in today’s lab.
- Which pair of enantiomers above (R,R and S,S or S,R and R,S) is the product of syn addition and
which pair is the result of anti addition?
- Dichloromethane is irritating to the skin and a potential carcinogen.
- Bromine is strongly corrosive, causes serious eye damage and is toxic to aquatic environments.
- Cyclohexene is a highly flammable liquid.
- 50% aqueous ethanol is flammable and an irritant.
- If you are exposed to any of these materials, you should wash the affected area for 10 – 15
minutes. Eye exposure should be treated by rinsing your eye(s) for at least 15 minutes. Eye
exposure to the bromine solution requires medical attention.
- The dichloromethane should be disposed of in the halogenated waste carboy.
- 50% aqueous ethanol should be disposed on in the non-halogenated waste carboy.
- Any solids should be disposed of in the solid waste drum.
Set Up the Reaction
Equip a 5-mL reaction vial with a spin vane. Add 200 mg of trans-cinnamic acid and 1.5 mL of
dichloromethane to the reaction vial and stir the solution to dissolve the solids on a stirring hot plate.
Once the solid has dissolved, raise the reaction vial, place a crystalizing dish with water on the hot plate,
lower the vial back into the water bath and add 1.5 mL of 1 M bromine in dichloromethane. Equip the
reaction vial with a water-cooled condenser and heat the water bath to about 50 °C. The reaction
should be kept at a gentle reflux for 25 - 30 minutes. As the reaction proceeds, the dark bromine color
should change to a light orange color. If the solution becomes colorless before the end of the reaction,
additional 1 M bromine in dichloromethane should be added dropwise via the top of the condenser until
the reaction stays a light orange color.
Quench any remaining bromine by the dropwise addition of cyclohexene (this should only take a few
drops of cyclohexene) until the reaction becomes colorless. Remove the reaction vial from the water
bath by first removing the condenser and then raising the vial. Turn off the heat to the water bath and
allow the system to cool to room temperature. As the reaction cools, you should note the formation of
a precipitate. After the vial has cooled to room temperature you should cool it further in an ice water
bath for 15 minutes. Isolate the solids by vacuum filtration, washing with a small quantity of ice-cold
dichloromethane. Allow the crystals to dry and remove a small quantity of the crude solid (enough for a
The product should be recrystallized from 50% aqueous ethanol, dried, and a final weight of the isolated
product should be obtained.
Obtain a melting point of the crude and purified products. An IR spectrum of the final product should
also be obtained if requested by your teaching assistant.
Lab report questions/details
- Compare the melting points from before and after recrystallization. Was there a difference, and
why might they be different?
- Report the yield and % yield of your final product. Which mixture of enantiomers did you obtain?
Write a mechanism for the formation of one of the enantiomers.
- Interpret the IR spectrum of your product.
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