Chem 241, Lab Section
Reviewed 4-12-12
EXPERIMENT 8:
STEREOCHEMISTRY OF BROMINE ADDITION TO
TRANS-CINNAMIC ACID
REACTION: Bromination of an Alkene
TECHNIQUES: Microscale Reflux, Recrystallization
In this experiment, we will learn how experimental results can be used to propose a reaction
mechanism. Specifically, we will study the mechanism of addition to an alkene using transcinnamic acid and molecular bromine.
READING ASSIGNMENTS:
Theory and Experimental Procedure (Attached to this handout also see Bruice below)
Technique 7.1: Refluxing a Reaction Mixture pp 59 - 60 in Techniques in Organic Chemistry
3rd Ed.
Bruice Sections: pp 171 – 174 and pp 236 - 237
IMPORTANT SAFETY INFORMATION
Bromine is very corrosive and causes serious burns. Its vapors are extremely toxic and
irritating to the eyes, mucous membranes, and respiratory tract.
Dichloromethane is toxic, an irritant, absorbed through the skin, and harmful if
swallowed. Wear gloves and wash your hands thoroughly after handling it. Avoid contact
with skin and eyes.
trans-Cinnamic acid is a mild irritant. Avoid skin contact and wash your hands after
handling it.
EXPERIMENTAL NOTES:
 Rewrite the microscale procedure attached to this handout in your own words!
 Once you are ready with your reaction set up ask the instructor to come over and add
bromine to your flask. You are not allowed to handle the bromine!
Your Results section should include:
 Appearance and melting point of the product.
 Yield of the product and calculation of % yield.
Chem 241, Lab Section
Reviewed 4-12-12
NOTES FOR WRITING YOUR DISCUSSION:
Your Theory subsection should include:
 An overview of the bromination reaction and 2 of 3 (you will not need to include the syn
addition mechanism but need to include anti or syn/anti) of the theoretically possible
mechanisms.
Your Analysis of Results subsection should include:
 Discuss the identity of the compound you isolated at the end.
 Discuss the % yield and purity of the product.
 Based on the identity of your product, propose which mechanism is operative in the
reaction – explain your choice.
Your Sources of Error subsection should include:
 Evaluate how reliable your results are: how certain are you of the identity and purity of
the product you isolated? Is it possible to have more than one operating mechanism?
 Propose different experiments that you could use confirm the mechanistic hypothesis you
came up with based on the results of this experiment.
Prelab Questions:
1.
This reaction you are about to perform in ran using dry dichloromethane (ie free of
water). Why must the solvent be dry in this reaction?
2. In this reaction you will use an excess of bromine. How can you easily tell if the bromine
is not in excess?
Postlab Questions:
1.
A student tried to carry out the identical reaction as you but received a weird side
product. After careful analysis the student realized that their flask had been contaminated
with potassium iodide (KI) from some careless student in a general chemistry section
offered a few hours before. Draw a structure for what the side product could be and
explain your rationale.
2. Give the product(s) for the following reaction (be sure to indicate the stereochemistry for
each product):
H
Br2
H
3.
CH2Cl2
If you ran chiral chromatography (HPLC) on the product(s) from the reaction above how
many peaks in the HPLC graph should you expect to see?
Chem 241, Lab Section
Reviewed 4-12-12
Experimental Section:
Introduction:
In this experiment, you will add bromine across the double bond of transcinnamic acid to form,
3-dibromo-3-phenylpropanoic acid. The product of this reaction contains two asymmetric
centers, therefore there are four possible stereoisomers that could possibly be formed in the
reaction.
O
Br
Br2
O
OH
OH
CH2Cl2
t rans-cinnamic acid
mp 136 oC, MW = 148.2 g/mol
Br
2,3-dibromo-3-phenylpropanoic acid
mp varies depending on stereoisomer
MW = 308.0 g/mol
The stereoisomers that are actually formed in the reaction depend on the mechanism of the
reaction. Through analysis of your product mixture, you will be able to draw conclusions about
the mechanism of bromine addition to a double bond. The four possible reaction products are
shown below and are result of syn or anti addition of Bromine to the double bond. (To review
this type of addition refer to Bruice pp 171 – 174 and pp 236 – 237).
Br
Br
O
OH
1
Br
Br
O
2
O
OH
OH
Br
Br
O
3
Br
4
Possible stereoisomers formed for Br2 additon to cinnamic acid
OH
Br
The relationship between structure 1 and structure 4 is that they are enantiomer (as both
stereocenters are inverted) and are identical in almost all their physical properties including their
melting points. Likewise, 2 and 3 are also enantiomers and also share almost identical properties
to one another as well. The relationship between 1 and 2 (or 3) is that they are diastereomers.
Since diastereomers have different physical properties, you will be able to determine which pair
of enantiomers you have formed by taking the melting point of your product.
The question you are trying answer with this experiment is whether the addition of bromine
across a double bond is a syn addition, or an anti addition. Syn addition means that the two
substituents have added to the same side of the double bond, anti addition means that the two
substituents have added to opposite sides of the double bond.
Chem 241, Lab Section
Reviewed 4-12-12
Procedure:
To a small round bottom flask (5 mL or 10 mL in size) add 100 mg of trans-cinnamic acid and
1.0 mL of dichloromethane. Place a boiling stone in the flask and cap with and air filled
condenser (no water is needed to flow in the condenser for this reaction, see Technique 7.1c in
Mohrig’s Techniques in Organic Chemistry, 3rd Ed.). At this point your instructor will add 0.35
mL of a 10% bromine in dichloromethane solution to your reaction flask. Once the bromine is
added, place the flask in a 50 oC water bath (a hot water bath can be made by placing water into a
50 mL beaker and gently heating on a hot plate). The reaction mixture will gently reflux. Once
it begins to reflux, time the reaction to go for an additional 20 minutes. If the mixture becomes
colorless (or nearly so) during this period, ask the instructor to add more of the bromine solution
dropwise until color just persists. The product should precipitate out during this period of time.
Cool the reaction mixture in an ice-water bath for 5 minutes or more, scratching the sides of the
vial to induce crystallization if necessary. Collect the product by vacuum filtration on a Hirsch
funnel and wash the solid with 3 x 0.5 mL portions of ice-cold dichloromethane. Transfer the
product to a 10 mL Erylenmer flask and add 0.5 mL of ethanol and heat the flask until the
ethanol begins to boil. If the product doesn’t completely dissolve in the hot ethanol add 0.1 mL
portions of ethanol until it does dissolve. Quickly add an equal amount of water to the ethanol,
heat until all the crystals, that may have formed from the water addition, dissolve. Cool the
crystals in an ice bath for 5 minutes then collect using a Buchner funnel. Dry the product
between two pieces of filter paper, take the weight and measure the melting point to determine
whether you have the compounds 2 and 3 (m.p. 204 oC) or compounds 1 and 4 (m.p. 95 oC).
IMPORTANT NOTE: THE Video has these values reversed.