Experiment A2b: The Bromination of (E)-Stilbene

Experiment A2 b: The Bromination of (E)-Stilbene: meso-Stilbene Dibromide Objective The purpose of this experiment is to synthesize the second intermediate in the b series of Sequential Reactions. The product of this experiment, meso-stilbene dibromide, is created through the bromination of (E)-Stilbene and isolated through vacuum filtration. Experimental Procedure The experiment A2 b was performed according to the procedure in Mayo, pages 454-455, with the modifications listed in the Canvas document. Modifications include: ● Run the reaction on a ⅙ scale, using ⅙ the amount of initial reactants and solvents ● The reaction will take place in a 10 mL round bottom flask ● Use a water-jacketed reflux condenser instead of an air condenser ● Wash with cold methanol Reaction Scheme Calculations Theoretical Yield Limiting Reactant ---- (E)-Stilbene, 110 mg 110 mg× 1𝑔 1000 𝑚𝑔 −4 6. 10 × 10 × 1 𝑚𝑜𝑙 180.25 𝑔 𝑚𝑜𝑙𝑠 × = 6.10 x 10-4 moles 1 𝑚𝑜𝑙 𝑀𝑒𝑠𝑜. 1 𝑚𝑜𝑙 (𝐸)−𝑆 × 340.06 𝑔 1 𝑚𝑜𝑙 𝑀𝑒𝑠𝑜. = 0.207 grams of meso-Stilbene dibromide Actual Yield Final Mass of meso-Stilbene dibromide product 0.126 grams % Yield (Experimental Yield) / (Theoretical Yield) X 100 = 0.126 𝑔 0.207 𝑔 × 100 = 60.87% Yield Data and Results Compound MW Amount 1)TY 2)EY Mmol 1)TY 2)EY mp (℃) 1) Known 2) Exp. (E)-Stilbene 180.25 110 mg 0.610 122-124 Acetic Acid P. B. Perbromide Meso-stilbene 2 mL 319.83 209 mg 340.06 1) 207 mg 2) 126 mg 0.653 205 1) 0.610 2) 0.370 1) 241.0 2) 241.5 bp (℃) d 118 1.049 Observations ● After adding initial reactants, the solution became an orange/yellow color ● Upon heating, the condenser and flask turned a darker red/orange color ● The mixture began to boil around 117 °C ● Areas in the flask exposed to the liquid and left to dry turned white ● Upon isolation, the meso product was a crystalline white powder Questions 1) Mechanism for the bromination reaction of (E)-stilbene to form meso-stilbene dibromide 2) Water Jacketed reflux condenser a) 116-118 °C b) The jacket was needed for this experiment so that the vapors released from heating the solution can be cooled and condensed back down into a liquid before it escapes the apparatus and diffuses into the air. This is necessary for two reasons. One, the vapors released are volatile and toxic and therefore needs to be contained. Cooling the gas before it can escape ensures that it won't combust or be accidentally inhaled in large quantities. Additionally, cooling the vapors escaping from the boiling solution helps to maintain a constant internal temperature within the reaction. c) The condenser provides a way to adequately cool hot vapors that rise from applying heat. The cold water rushes into the condenser and chills the glass tube the vapors are exposed to. This, in turn, absorbs energy from the vapor, condensing it back into a liquid that will run down the side of the tube back into the flask. This reflux cycle of heating and cooling allows for the reaction to proceed at a constant temperature. 3) Stereochemistry a) Stilbene dibromide has three possible isomers. b) In this experiment, the meso isomer was formed. There are two other possible isomers shown below. c) The two other isomers of stilbene dibromide are nonsuperimposable mirror images of each other; a set of enantiomers. d) These isomers are diastereomers of the synthesized product. 4) Experimental Data a) For the product synthesized in the experiment, the mass of the meso-stilbene and a melting point were obtained. b) Mass of Product - 126 mg Melting Point - 241.5 °C The measured melting point of the product would suggest that the product synthesized was indeed meso-stilbene. The accepted melting point of this molecule is 241 °C; the experimental melting point was 241.5 °C ,only 0.5 °C off. The small difference in temperatures would verify the isolated product was meso-stilbene.