CHEM 51LB: EXPERIMENT 3
STEREOCHEMISTRY OF BROMINE ADDITION TO ALKENES
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 or trans-stilbene with molecular bromine.
REACTION: Bromination of an Alkene
TECHNIQUES: Recrystallization, Melting Point
Take a look in any organic chemistry textbook and you will find that the accepted
mechanism for halogenation of an alkene is the anti addition mechanism. How do we know
this? In fact, how do we know any mechanism? Every mechanism you see drawn in a
textbook had to be discovered. Chemists use many different tools to find evidence for
mechanisms. In this experiment we will explore the simplest and very first step chemists
take to elucidate a mechanism. We will eliminate possible mechanisms based on the
products of a reaction.
Let’s pretend for a while that we don’t actually know the mechanism for the bromination of
an alkene. We know that one bromine atom adds to each carbon of the C-C double bond, but
we do not know the stereochemical outcome. We can imagine three possible mechanisms for
the bromination of and alkene: an anti addition mechanism, a syn addition mechanism, and a
single mechanism that can lead to a mixture of both syn and anti addition products.
(Remember, we’re pretending that we don’t know the actual mechanism!) How are the
components of the pair of anti products related to each other? How are the components of
the pair of syn products related to each other? How are the anti products related to the syn
products? You should be able to propose these three potential mechanisms.
Molecular bromine is necessary for conducting bromination of an alkene, bromine solutions
are very corrosive and dangerous to handle. Instead of adding Br2 directly, we will use
pyridinium tribromide as a source of molecular bromine. Use of pyrdinium tribromide allows
for the generation of bromine in situ by the equilibrium shown below. What does in situ
mean? If the generation of Br2 is an equilibrium process, how is enough of Br2 produced to
react with all of the alkene?
N
H
Br Br Br
N
H
+
Br Br
Br
READING ASSIGNMENT:
! Review recrystallization technique from last week.
! Supplementary information in Janice Gorzynski Smith
3rd ed or 4th ed, Chapter 10.13-10.14
PRE-LAB ASSIGNMENT:
! Follow the instructions in the “Chem 51L Notebook Guidelines” document
located on the course website to prepare your pre-lab.
! As part of your pre-lab assignment, propose and draw out three potential
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reaction mechanisms. These should be a mechanism that leads to anti
addition, a mechanism that leads to syn addition, and a mechanism that could
account for products from both syn and anti addition.
! Complete the online Sapling pre-lab questions.
IMPORTANT SAFETY INFORMATION
Pyridinium tribromide is corrosive and dangerous if absorbed through the skin. Can
cause eye damage. Avoid contact with skin and eyes.
Glacial acetic acid is corrosive and has a strong, pungent odor. Use in fume hood. Avoid
contact with skin and eyes.
trans-Cinnamic acid is a mild irritant. Avoid skin contact and wash your hands after
handling it.
Ethanol is flammable. Inhalation of vapors can be toxic. Work in the fume hood and
keep away from sparks or flames
EXPERIMENTAL NOTES:
Reaction Procedure:
To a 25 mL round-bottomed flask containing a magnetic stir bar, add 250 mg of
trans-cinnamic acid followed by 2.5 mL glacial acetic acid. Make a water bath using a 250
mL beaker and DI water. Heat the water bath to ~40-50 °C. Lower your flask into the water
bath. Heat and stir until the solution is homogenous. What does homogeneous mean? Raise
the flask out of the water bath and add 432 mg pyridinium tribromide to the flask. Rinse
solid from sides of flask into the reaction mixture with an additional 2.5 mL glacial acetic
acid. Lower the reaction flask into the water bath and increase the heating until the water is
boiling. Continue heating the reaction mixture in the water bath for 15 minutes.
Work-up:
Remove the reaction flask from the water bath and allow it to cool to room temperature. Add
5 mL DI water to reaction flask. Transfer the reaction flask to an ice bath and cool for 15
minutes to ensure full precipitation of product. Collect the solid product by vacuum filtration
using a Hirsch funnel. What do you think is in the filtrate? (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.) Rinse the solid with small portions of icecold water until most (if not all) or the orange color is gone. If a significant amount of solid
begins to form in the filtrate, you can isolate this second crop of crystals by conducting a
second, separate filtration. Do not combine your crops of crystals until you are certain that
they are the same compound! How can you tell if both crops of crystals are your product?
Why would additional solid precipitate in your filtrate?
Purification:
Recrystallize the solid by dissolving it in a minimum amount of hot (nearly boiling) ethanol
Add hot water slowly until a slight cloudiness persists after stirring. Add a few drops of hot
ethanol just until all cloudiness disappears. Proceed with the recrystallization as you
normally would. After filtering and drying the solid, obtain a melting range for the purified
product.
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Data Analysis:
Use the data you collect to identify your product.
LAB WRITE-UP:
Complete the Experiment 3 Report Scaffold worksheet provided on the class website and
turn in to your TA at the specified time. Be sure to read and follow the instructions provided
on the worksheet!
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