Experiment 8:
BROMINATION OF STILBENE:
A GREEN SYNTHESIS
Objectives



To perform a Green Chemistry synthesis
and determine chemical yield.
To purify and isolate the product by
recrystallization and vacuum filtration.
To determine the degree of product purity
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
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
Experimental Procedure
(Synthesis)
• Add Stilbene and ethanol to the flask.
water out
• 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
• As the reflux proceeds, the solution will
return to colorless.
heating mantle
to voltage regulator
iron ring
• 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.
stilbene
standard
your sample
If all of the stilbene has reacted, there will
be no evidence of it in the product lane. This
indicates a successful reaction!
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
cheese
2
+ 1
1
sandwich
Theoretical Yield
o Question: How many sandwiches can you make,
and which ingredient runs out first?

Answer: You can make three sandwiches and
the bread runs out first:

6 slices of bread *

4 slices of cheese *

1 sandwich
2 slices of bread
1 sandwich
1 slice of cheese
= 3 sandwiches
= 4 sandwiches
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.
Limiting Reagent






Stilbene = 0.50g x 1 mol stilbene x 1 mol dibromostilbene = 2.78 x 10-3 mol
180 g
1 mol stilbene
dibromostilbene
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
HBr:
(1.2 mL) x 1.49 g x 0.48 g x 1 mol x 1 mol dibromostilbene = 5.3 x 10-3 mol
mL
1.0 g
81 g
2 mol HBr
dibromostilbene
• 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 = ? g product
mol
Table 8.1
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 Rf values
Compound
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 “ORGANIC WASTE”.
Place all solid waste in container labeled
“SOLID ORGANIC WASTE”.
Place used TLC capillaries in broken glass
container.
Place TLC plates in yellow trashcan under
supply hood.