Bromination

Green Bromination of Stilbene

Introduction:
The experiment performed is halogenation of a π bond. This is done by a procedure called refluxing. Refluxing is the process of heating a product to the boiling point and recondensing the vapor continuously. The procedure halogenation is the addition of a halogen to a π bond forming a haloalkane. In this synthetic reaction bromine was used in the process called bromination. The bromine is acting first like an electrophile, and then after bromine has broaken the π bond, a carbocation has formed, and a bromide ion has been created, the bromide ion then acts as the nucleophile and forms a bond with the carbocation. cyclic intermediate: remember? There is a cation but it's the bromonium ion. The first step in this reaction is the generation of bromine. Elemental bromine is very reactive and toxic so the bromine for this experiment is generated in situ by oxidizing hydrobromic acid with hydrogen peroxide. Ethanol is used for the solvent. The reagent is hydrogen peroxide (%30) and hydrobromic acid. The product of bromination of (E)-stilbene is the formation of 1,2-dibromo-1,2-diphenylethane (dibromostilbene). Did you think about the stereochemistry of this reaction at all? You generate chiral centers in your product.

reflux apparatus
 * Procedure: **

nice photo. a picture is worth a thousand words!
 * 1) A stirrer/hot plate is placed on top of a jack stand to allow the separation of the reaction vessel from the heating apparatus.
 * 2) The hot water bath is prepared using a shallow water dish placed on top of the stirrer/hot plate.
 * 3) Magnetic stir bar is placed in a 100ml round bottom flask.
 * 4) A clamp is attached to the flask and it is then lowered into the water bath so that the magnet is low enough to the stirrer/hot plate to be activated.
 * 5) (E)-stilbene (0.5 g) is weighed and emptied into the round bottom flask. Then 10ml of ethanol is added.
 * 6) Place the condensing column into the flask mouth and clamp in place. Attach the cold water supply to the lower water jacket port and then attach a drain tube to the upper water jacket port.
 * 7) Begin heating the water bath setting hot stirrer/hot plate temp to 285°. Start stirring the mixture. Turn cold water supply on to the condensing column and adjust flow to a minimal rate.
 * 8) Continue heating and stirring the mixture until most of the solid (E)-stilbene has dissolved.
 * 9) Very cautiously and slowly add 1.2 mL of concentrated aqueous hydrobromic acid to the flask using a long pipette and placing it down the condensation apparatus so that the aqueous HBr is released directly into the reaction flask.
 * 10) In a clean pipette draw up 0.8 mL of 30% hydrogen peroxide and add it directly to the reaction flask by separating the round bottom flask from the condensation tube if the pipette is not long enough to reach the mixture down though the condensation apparatus. The color will change to a dark golden-yellow.
 * 11) Continue stirring and heating the mixture at reflux for approximately 20 minutes or until the color fades to a milky white.
 * 12) Turn off the heat and remove from hot water bath. Begin cooling the mixture to room temperature. Check the pH of the solution and adjust to a pH of 5-7 by adding concentrated aqueous NaHCO3.
 * 13) Place the round bottom flask in an ice bath to precipitate more product out of the solution.
 * 14) Assemble the vacuum filtration apparatus and empty mixture into the funnel. Rinse reaction flask with a small amount of de-ionized water and add to the filtration funnel. Continue to apply vacuum until the solid is dry. You may need to press between two pieces of filter paper to draw off the last bit of moisture from you sample.
 * 15) Weigh the final product and determine your MP. After satisfactory measurements have been recorded, discard your solid dibromostilbene in the appropriate dish in the vent hood for safe disposal.

//Procedure adapted from Green Organic Chemistry – Strategies, Tools, and Laboratory Experiments by K.M. Doxsee and J.E. Hutchison, Thompson Brooks/Cole, 2004; pg 125-128.//


 * Data: **

The Stilbene was a white powder textured substance that was emptied into the reaction flask. When the clear ethanol was added the stilbene did not initially dissolve. After a short period of heating and stirring constantly the white powder dissolved in the ethanol however not completely. The mixture was a clear slightly cloudy aqueous solution. After the addition of hydrobromic acid the solution became slightly orange at first and slowly faded opaque. After adding the hydrogen peroxide the mixture turned bright yellow/orange and then slowly became more orange and dark orange before fading to and opaque milky white color. The mixture separated into two layers with the precipitate on the bottom. The precipitate was cloudy white and the liquid layer on top of the precipitate was clear. After vacuum filtration and pressing between filter paper the solid product was a paste consistency and milky white in color.

1.2 ml HBr (aq), 0.8 ml H2O2 (30%) || NaHCO3 7-1/2 ml added to produce pH of 6.5-7 ||
 * Solvent || Reactants || Neutralizing agent ||
 * 10.0 ml ethanol || 0.500 g (E)-stilbene,


 * Stilbene/ Product || Weight ||
 * stilbene initial weight || 0.500 g ||
 * Product recovered weight || 1.199 g ||
 * Final pure product recovered weight || 0.665 g ||

always always always report melting point RANGES. =**Analysis:**= Percent Yield (E)-Stilbene molecular weight=185.25 g/M (180.25, is what you have below.)
 * Melting Point Data || Temp. ||
 * Literature Value || 241°C ||
 * Recovery product || 225°C ||

Calculations: 0.500 g of (E)-stlibene x (1 mol (E)-stilbene/180.25g (E)-stilbene)=0.00287 mol (E)-stilbene 0.00287 mol x (340.05g Bromostilbene/1 mol Bromostilbene)=0.943g Bromostilbene

Recovered yield/ Theoretical yield x 100= Percent Yield (0.665 g stilbene/0.943 g Bromostilbene)=0.705 x 100=**70.5% yield** of 1,2-dibromo-1,2-diphenylethane good job here on sig figs! calculation is good.

=Conclusion/Discussion:= The reason for this laboratory experiment was to offer a greener and safer approach to the bromination of an alkene (E)-Stilbene to form 1,2-dibromo-1,2-diphenylethane. The previous method appeared to be more riskier and dangerous, this method reduces hazards compared with traditional bromination that use of corrosive liquid bromine and hazardous chlorinated solvents (Hutchison).

refer to figure below reaction of bromination of stilbene to 1,2-dibromo-1,2-diphenylethane.



Once the experiment was conducted the final data was 0.665 grams of the ending product which yielded 70.5% of expected mass. This means the reason for having a much smaller yield is due to the fact that when the reaction was occurring, durring melting some of the mixture was not dissolved and the adding of hydrobromic acid could have precipitated some of the product. Also while transferring the product from the flask to vacuum filtration funnel some of the product could have been lost through the sides of the filter paper in the vacuum. Overall there were many errors when trying to collect the finished product through transfer, filtration, heating, not cooling long enough in ice bath, not letting the recovered product dry. According to the procedure, the literature melting point for dibromostilbene is 241 degrees Celsius. The ending melting point of the recovered product was 225 degrees Celsius. Speculations point to the reason for the recovered products lower melting point, is the fact there is a presence of impurities in the recovered product. There could also have been other products present that melted at a much lower melting point causing the product to not be as pure. Overall this experiment gave students a greener approach at producing a pure product of bromination of (E)-stilbene, according to other students finding they recovered relatively lower yields than expected, experiment showed it is possible to collect a higher yield, of course with less errors.This method is safer, better for the environment and results show possible higher yields recovered of pure products. nice conclusion. =References:=

K.M. Doxsee and J.E. Hutchison, Experiment 3: A Greener Bromination of Stillbene, Organic Chemistry- strategies, Tools, and Laboratory Experiments, Print 2004; pp 125-128.