EXPERIMENT 2
STEREOCHEMISTRY OF BROMINE ADDITION TO
TRANS-­CINNAMIC ACID
REACTION: Bromination of an Alkene
TECHNIQUES: Microscale Reflux, Recrystallization, Melting Point
The purpose of this experiment is to elucidate the operating mechanism in the addition of bromine to
an alkene. Three possible mechanisms, each with a distinct stereochemical outcome for the product,
will be evaluated. The melting range of the product obtained from the reaction will be used to
determine its stereochemistry and based on the stereochemical outcome one of the three proposed
mechanisms will be chosen as the operating one in the reaction.
READING ASSIGNMENT:
● This Handout for procedure.
● Review recrystallization technique from last week.
● Supplementary info in Janice Gorzynski Smith (2nd ed), Chapter 10, pp. 381-­384.
PRE-­LAB ASSIGNMENT:
1. Prepare the appropriate purpose, reactions, tables, etc., in your notebook
2. Re-­write the procedure in your own words in your notebook.
3. Complete your Sapling assignment.
EXPERIMENTAL NOTES:
REACTION PROCEDURE:
In a 10 mL round-­bottomed flask add 100 mg of trans-­cinnamic acid followed by 0.7 mL
dichloromethane. Add a few boiling chips to the flask and equip it with a water-­cooled condenser.
(Note: make sure the clamp is holding the neck of the flask and not the condenser!) Once you are
ready with your reaction set up ask the TA to come over and add 0.35 mL of 10% bromine in
dichloromethane solution to your flask. You are not allowed to handle the bromine! Lower
your apparatus into a warm water bath (Use a 250 mL beaker filled up with water and heat it to
40-­50 °C). Reflux the reaction for 20 min.
NOTES:
If the temperature of the bath gets too hot, add some cold water to avoid boiling off all of your
solvent. (What’s the boiling point of dichloromethane?) If you do boil off too much of your
solvent you can add additional solvent as necessary at any time. If the reaction turns from brown
to colorless during the 20 min of reflux ask your TA to add additional bromine to your reaction.
WORK-­UP:
Allow the reaction to cool to room temperature. Add cyclohexene dropwise until the color ceases
to fade further. The solution may not go completely colorless, but it should go to a faint orange.
Add only the minimum of cyclohexene you need.
Safety notice: Some students are particularly sensitive to the odor of cyclohexene and may
develop headaches or dizziness. Ensure that cyclohexene is used exclusively in the fume hood
and in the smallest necessary amounts
Cool the reaction flask in an ice bath to ensure that all of your product has crashed out of solution.
Filter the reaction mixture using a Hirsch funnel and collect the solid. Please note that Hirsch
funnel frits are NOT DISPOSABLE and should be reused. Hirsch funnel filter paper should be
placed on top of the frit prior to filtering and is disposable.
Recrystalize the solid using minimum amount of ethanol (Write a procedure for the
recrystallization in your lab notebook) and obtain a melting range for the purified product.
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. Keep bromine in the fume hood. Avoid
inhalation and contact with skin and eyes.
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.
Dichloromethane is a suspected carcinogen when inhaled in large quantities. Keep it in the fume
hood.
trans-­Cinnamic acid is a mild irritant. Avoid skin contact and wash your hands after handling it.
Cyclohexene is flammable and an irritant. Its vapors are irritating to the eyes, mucous membranes,
and respiratory tract. Keep cyclohexene in the fume hood. Avoid inhalation and contact with skin
and eyes.
LAB WRITE-­UP:
Results: Record your results in the space provided below:
● Yield of the product:
● Calculate the theoretical yield of the product and the % yield.
● Appearance of the product:
● Melting range of the product:
Mechanism:
● Propose 3 distinct possible arrow-­pushing mechanisms for the reaction of bromine with
trans-­cinnamic acid. Clearly indicate the stereochemistry of the products obtained in each
case.
● Mechanism 1:
● Mechanism 2:
● Mechanism 3:
Theory and Discussion:
● Why is it possible to use melting point to distinguish the different stereochemistry of the
products? (Hint: what is the relationship of the different stereoisomers of the product?)
● What is the melting point of the anti addition product? What is the melting point of the syn
addition product?
● Describe the melting point you expect to observe if the isolated product is the mixture of
both syn and anti addition of bromine. (There are three possible types of mixtures here,
such as syn as the major product and anti as the minor, etc. Describe the melting range for
each scenario.)
● Is the melting point you recorded consistent with one particular stereoisomer of the product
or is it a mixture of the two? If its only one stereoisomer, which one is it?
● Based on the identity of your product, propose which mechanism is operative in the
reaction.