Experiment 8
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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.

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Experiment 8 l.jpg

Experiment 8:



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  • 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.

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Before coming to lab…

  • Review the techniques of:

    • TLC

    • Suction filtration

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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.

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Typical Bromination Reaction

  • Elemental bromine (Br2) and chlorinated solvents are used!

  • Both are extremely hazardous!

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“Greener” Bromination Reaction

  • We use ethanol, a much safer solvent.

  • The Br2 will be generated “in situ”, through the oxidation of HBr with H2O2.

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Bromination Mechanism

p electrons form bond to d+ bromine atom…

… to form bromonium ion intermediate.

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

… to result in trans addition product.

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Bromination Mechanism

  • The following link will allow you to view an animation of this mechanism:


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Experimental Procedure(Synthesis)

  • 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.

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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.

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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!

Experimental Procedure(TLC Analysis)

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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




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Theoretical Yield

  • 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 * 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.

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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!

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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


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Table 8.2

  • Rf values are UNITLESS!

  • 2 decimal places ONLY!

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  • Gogglesare requiredat all timesduring 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

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  • 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.