Bromination+of+Stilbene

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**Objective:** To complete a addition synthesis reaction of Bromine using a greener method that eliminates the need for using elemental bromine in the lab because of the safety concerns it presents. ======

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**Introduction:** One of the very first electrophilic addition reactions learned was bromination of an alkene using elemental bromine (Br2). It turns out that bromine is very dangerous to work with in the laboratory since it is volatile and highly corrosive, causing severe burns upon contact with skin. In this experiment (Br2) is produced in the reaction mixture (it is prepared in situ), thus avoiding its hazards. The process is to brominate stilbene by using Br2 generated by oxidizing hydrobromic acid with hydrogen peroxide. The mechanism is shown below. ======

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Safety Precautions: Hydrobromic acid is corrosive. Avoid contact and inhalation of vapors. Clean up any spills with a paper towel with a little sodium bicarbonate solution. Hydrogen peroxide (30%) is a strong oxidizer and will damage your skin and clothing (the hydrogen peroxide found at the local supermarkets is only 3% concentrated) ======

**Procedure:**

 * 1) ======Prepare a hot water bath using a medium sized crystallization dish on a stirrer/hot plate. Make sure to have the hot plate sitting on a lab jack to allow for the removal of heat immediately from the reaction flask to prevent overheating. ======
 * 2) ======Place a magnetic stir bar, 0.5g of (E)-stilbene, and 10 ml of ethanol in a 100 ml round-bottom flask. Fit the flask with a water-cooled reflux condenser. ======
 * 3) ======Clamp the flask to a ring stand so that is may be heated and stirred in the hot water bath. Also, clamp the condenser to the ring stand to keep the glassware secure. Stir while heating the mixture to reflux, and continue heating and stirring until the majority of the solid has dissolved (approximately 10 minutes). ======
 * 4) ======After most of the solid has dissolved; slowly add 1.2 ml of concentrated aqueous hydrobromic acid. This will probably cause some of the stilbene to precipitate, but continued heating and stirring should cause the majority of the solid to redissolve. (continue to the next step even if some remains undissolved) ======
 * 5) ======Measure out 0.8 ml of 30% hydrogen peroxide and add it dropwise to the reaction mixture. The initially colorless mixture will change in color to a dark golden-yellow. ======
 * 6) ======Continue to stir and heat the reaction mixture at reflux until the yellow color fades and the mixture becomes a cloudy white. This typically takes roughly 20 minutes at reflux. ======
 * 7) ======Remove the flask from the hot water bath and allow it to cool to room temperature. Checking with pH paper, carefully adjust the pH of the solution to a pH 5 to 7 through the addition of concentrated aqueous NaHCO3. In some cases very little NaHCO3 is required. ======
 * 8) ======Cool the reaction mixture in an ice bath to bring more product out of the solution. Collect the solid that forms by vacuum filtration, rinsing with cold water. A wash with very cold ethanol can help to remove traces of impurities, but care must be used to avoid dissolving inordinate amounts of the product. Continue to draw air through the product until dry. ======

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**Observations/Data:** After heating and stirring the 0.5g. (I need more digits on measurements of this type) of (E)-stilbene and 10ml. of ethanol, most of the solid had dissolved. Then 1.2ml. of hydrobromic acid was added to the reaction mixture and little was observed to change, except a small portion of stilbene had precipitated, but redissolved quickly back into solution. While the 0.8ml. of hydrogen peroxide was added, the mixture immediately changed color from white to a yellow-orange and then to a creamy white after about 20 minutes of continued heating and stirring. After the solution became a creamy white color, the mixture was placed in an ice bath, then was removed from impurities with vaccum filtration. The product had still appeared to be damp after filtration, so it was set on a watch glass to dry, then placed between two paper towels and filter paper to dry completely. As shown below, the melting point of trial one and two were determined. The melting point for trial one was 215.2 C-228.1 C and the melting point for trial two was 217.7 C-231.2 C. The melting point of trial one was prepared with product that had not been dried completely with the paper towels and the melting point for trial two was prepared with product that had been dried with the paper towels and filter paper. The melting point had increased as the product had dried more, but the range of the melting point had widened. The literature value for the product is 241 C.The percent yield for this experiment was 86% and after adding 4.1ml. of NaHCO3, the pH was determined to be 7. ======

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**Step 1:** Calculate the molar masses for (E)-stilbene (C14H12) and 1,2-dibromo-1,2-diphenylethane (C14H12Br2). The bromine has no effect on the answer because bromine is in excess. ======

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<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;"> Percent yield = 100[actual yield/ predicted yield] = 100[.846 grams C14H12Br2 (product recovered from experiment) / .989 g C14H12Br2 (calculate yield of product] = **86%** <span style="color: #800000; font-family: Arial,Helvetica,sans-serif; font-size: 90%;">sig figs error. calc otherwise okay. <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;"> ======


 * < ======<span style="font-family: Arial,Helvetica,sans-serif;">**Before Reaction** ====== ||< ======<span style="font-family: Arial,Helvetica,sans-serif;">**After Reaction** ====== ||
 * ======<span style="font-family: Arial,Helvetica,sans-serif;">Amount of (E)-stilbene: 0.525g. ====== || ======<span style="font-family: Arial,Helvetica,sans-serif;">Amount of product recovered: 0.846g. ====== ||
 * || ======<span style="font-family: Arial,Helvetica,sans-serif;">**Melting Point:** __Trial 1:__ 215.2 C- 228.1 C __Trial 2:__ 217.7 C- 231.2 C ======

<span style="font-family: Arial,Helvetica,sans-serif;">(after spreading product out and allowing to dry further, the melting point increased)
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 * || ======<span style="font-family: Arial,Helvetica,sans-serif;">**% yield:** 86% ====== ||
 * || ======<span style="font-family: Arial,Helvetica,sans-serif;">pH was decreased to 7 after 4.1ml. of NaHCO3 was added to the solution. ====== ||

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<span style="font-family: Arial,Helvetica,sans-serif;">**Source of Error:** After adding the hydrogen peroxide, the solution was heated and stirred for over 20 minutes, but still appeared to have a yellowish-creamy color, instead of milky white, which could suggest that the Br2 had not reacted completely and resulted in a lower percent yield. <span style="color: #800000; font-family: Arial,Helvetica,sans-serif;">good analytical thinking here. <span style="font-family: Arial,Helvetica,sans-serif;">After using vaccum filtration on the solution, the melting point was observed and recorded, although the product was still damp. So the melting point of the product was observed and recorded once more after drying the product by placing it between filter paper and two paper towels and gently pressing them together. The melting point had increased after drying further, but the product still appeared to not have dried all the way. In addition, the final product was weighed to determine the percent yield before it had been dried further with paper towels, which could have increased the percent yield as well. ======

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<span style="font-family: Arial,Helvetica,sans-serif;">**Conclusion:** This experiment offers a greener approach to the bromination of an alkene (stilbene). Although bromination of alkenes is an important reaction in organic chemistry, the standard procedures present significant hazards in the laboratory and to the environment. In this experiment, stilbene is brominated in ethanol using a brominating agent prepared in situ by mixing hydrobromic acid and hydrogen peroxide. The solvent and reagents provide significant hazard reductions compared with previous approaches. ======

<span style="font-family: Arial,Helvetica,sans-serif;"> Nice job. Time to learn sig figs, before I steal more points from you!