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!

C CH

H

H

H

Br2

CH2Cl2C C

Br

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.

C CH

H

H

H

Br2

CH3CH2OHC C

Br

Br

H2O2 + HBr

CHEMICAL EQUATION

H

H

H

Br

Br

HCH3CH2OH

+ 2 HBr + H2O2+ 2 H2O

Bromination Mechanism

C C

HPh

PhH

Br Br CH

CPh

Ph

Br+

+ Br-

CH

CPh

Ph

Br

BrH H

2 H-Br + H2O2 Br-Br + 2 H2O

electrons form bond to +

bromine atom…

… to form bromonium ion intermediate.

… - bromine atom forms bond to carbon on opposite side of bromonium ion…

… 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

water in

heating mantle

iron ring

to voltage regulator

• 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.

• 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: 2 + 1 1

bread cheese

sandwich

Theoretical Yieldo 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 * 1 sandwich = 4 sandwiches 1 slice 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.

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

(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 = 5.3 x

10-3 mol mL 1.0 g 81 g 2 mol HBr 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 = ? g product mol

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

CompoundTLC Rf values

Standard Sample

Stilbene

Dibromostilbene

• Rf values are UNITLESS!

• 2 decimal places ONLY!

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 1,2-dimethylcyclohexene using curved arrows to indicate the movement of electrons. CH3

CH3

+ Br

MW: 110.2 g/mol

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.