Experiment 8:
BROMINATION OF STILBENE:
A GREEN SYNTHESIS
Objectives

To perform a green chemistry bromination of
trans-stilbene with HBr and H2O2 to form
dibromostilbene.

To purify the product by recrystallization.

To analyze the purity of the product using TLC
analysis.
Before coming to lab…

Review the techniques of:

TLC

Suction filtration
Introduction to Green Chemistry

Green Chemistry: the design of chemical products
and processes that reduce or eliminate the use and
generation of hazardous substances.

A “PERFECT” chemical reaction would be one that
is:



Selective: forming the desired product ONLY.
Efficient: converting all atoms of starting materials and
reagents to product.
Non-hazardous: is safer to the chemist and the
environment.
Typical Bromination Reaction

Elemental bromine (Br2) and chlorinated solvents
are used!

Both are extremely hazardous!
Br
H
H
C
H
C
H
Br2
C
C
CH2Cl2
Br
“Greener”
Bromination Reaction

We use ethanol, a much safer solvent.

The Br2 will be generated “in situ”, through the
oxidation of HBr with H2O2.
H2O2 + HBr
Br
H
H
C
H
C
H
Br2
CH3CH2OH
C
C
Br
CHEMICAL EQUATION
H
+ 2 HBr
H
+
Br
H2O2
CH3CH2OH
H
+ 2 H2O
Br
H
Bromination Mechanism
2 H-Br
Ph
H
Br
C C
H
+ H2O2
Ph
p electrons
form bond to
d+ bromine
atom…
Br
Br-Br
Br
+
Ph
C
H
… to form
bromonium ion
intermediate.
H
C
+
2 H2O
Br
+ Br
-
Ph
… d- bromine atom
forms bond to
carbon on opposite
side of bromonium
ion…
Ph
H
C
H
C
Ph
Br
… to result in
trans addition
product.
Bromination Mechanism

The following link will allow you to view an
animation of this mechanism:
http://www.youtube.com/watch?v=Ni8Ufb3HlL0
OVERVIEW

Heat alkene, acid, and peroxide under reflux to
synthesize products.

Neutralize acid, then suction filter to isolate solid
product from unreacted starting materials.

Analyze product to determine identity and purity
using TLC analysis.
Experimental Procedure
(Synthesis)
water out
• Add Stilbene and ethanol to the flask.
• Begin water flow and heating with VR
@ 20. Heat solution until it refluxes.
• Add 48% HBr during the reflux,
followed by 30% H2O2. This should
result in the appearance of a yellow
color. LOWER SASH TO HOOD.
water in
heating mantle
to voltage regulator
iron ring
• As the reflux proceeds, the solution
will return to colorless.
• Neutralize excess HBr using NaOH.
Experimental Procedure
(Product Isolation)
• Set up a suction filtration
apparatus to isolate the solid.
• Remember to clamp the filter
flask to the ring stand and connect
the red hose to the vacuum line!
• Once isolated, determine the
purity of the solid using TLC
Analysis.
• Prior to calculating percent yield,
the solid must then be dried in a
warm oven for several minutes.
Experimental Procedure
(TLC Analysis)

Prepare TLC plate and TLC chamber.

Apply standard solutions and sample solution to plate.

Develop TLC plate in chamber.

Visualize plate using UV lamp!

Circle spots.

Calculate TLC Rf value for each spot.

If all of the stilbene has reacted, there will be no evidence
of it in the product lane. This indicates a successful
reaction!
stilbene
standard
your sample
dibromostilbene
standard
Theoretical Yield
• In order to determine the theoretical yield, one must
first determine which reactant is the limiting reagent
by converting the amount of each reactant used to
moles of product that can be produced.
• A simple analogy is the synthesis of a cheese
sandwich.
•You have six slices of bread and four slices of cheese.
The recipe for a cheese sandwich is:
bread
2
+ 1 cheese
1
sandwich
Theoretical Yield
o Question: How many sandwiches can you make, and
which ingredient runs out first?
o
Answer: You can make three sandwiches and the bread runs
out first:

6 slices of bread *
1 sandwich
= 3 sandwiches
2 slices of bread

4 slices of cheese *

The bread is the limiting reagent. The number of
slices of bread or cheese is analogous to the number
of moles of each of the reactants in a chemical
equation.
1 sandwich
1 slice of cheese
= 4 sandwiches
Limiting Reagent

Stilbene = 0.50g x 1 mol stilbene x 1 mol dibromostilbene
180 g
1 mol stilbene
= 2.78 x 10-3 mol
dibromostilbene
(based on stilbene used)

If you used 0.8 mL of H2O2 and 1.2 mL of HBr:


H2O2:
(0.8 mL) x 1.11 g x 0.30 g x 1 mol x 1 mol dibromostilbene = 7.8 x 10-3 mol
mL
1.0 g 34 g 1 mol H2O2
dibromostilbene
(based on H2O2 used)


HBr:
(1.2 mL) x 1.49 g x 0.48 g x 1 mol x 1 mol dibromostilbene
mL
1.0 g 81 g 2 mol HBr
= 5.3 x 10-3 mol
dibromostilbene
(based on HBr used)
• The reactant which produces the least amount of the product is the LIMITING
REAGENT! We now calculate the theoretical yield based on it!
Theoretical Yield

The last step is to convert the number of moles of
product based on the limiting reagent to a mass in
grams of product. This is your THEORETICAL YIELD!

2.78 x 10-3 mol X 340 g
mol
= ? g product
Molecular weight
of product!!!
Table 8.1:
Experimental Results
Final Mass of product (g)
Obtained at end of this lab!
Theoretical Yield (g)
Calculated
% Yield
Actual mass/theoretical yield
Product Appearance
Physical state & color
Table 8.2: TLC Results
Compound
TLC Rf values
Standard
Stilbene
Dibromostilbene
• Rf values are UNITLESS!
• 2 decimal places ONLY!
Sample
SAFETY CONCERNS

Goggles are required at all times during the lab!

30 % Hydrogen peroxide is a very strong oxidizer
and will burn your skin if you get any on yourself.
One person per group should handle the H2O2 using
gloves
WASTE MANAGEMENT

Place all liquid waste from recrystallization and TLC
analysis in container labeled “LIQUID WASTE”.

Place all solid waste in container labeled “SOLID
WASTE”.

Place used TLC capillaries in broken glass
container.

Place TLC plates in yellow trashcan under supply
hood.
IN LAB QUESTION
(The following question should be answered in laboratory notebook.)

Draw the complete mechanism for the synthesis of
the product formed from the addition of Br2 to 1,2dimethylcyclohexene using curved arrows to indicate
the movement of electrons.
CH3
CH3
+
MW: 110.2 g/mol
Br
Br
?
IN LAB QUESTION
(The following question should be answered in laboratory notebook.)

Based on the previous question, calculate the theoretical
yield based on 1.0 g of the 1,2-dimethylcyclohexene. The
molecular weight of the starting alkene is given, but the
molecular weight of the product must be determined
using the product structure. Be sure to include units.