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Experiment 22: Addition and Elimination Reactions: Synthesis of Diphenylacetylene from trans-Stilbene Certain acetylenes are synthesized readily by an addition-elimination sequence starting with the corresponding alkene. You will employ this method to prepare diphenylacetylene from trans-stilbene. On mechanistic grounds, which stereoisomeric dibromide do you expect to obtain from the addition reaction, the meso isomer or the racemic mixture of enantiomeric R, R and S, S isomers? (The experiment will provide the opportunity to test whether your expectation is correct, since the meso dibromide melts at 238°C while the racemic mixture of dibromides melts at 114°C. ) You will use pyridinium bromide perbromide (also known as pyridinium tribromide – structure above first arrow in scheme above and below, right) as the brominating agent; it serves as an in situ source of bromine through the following equilibrium: Pyridinium tribromide is a stable, crystalline salt is more convenient and safer to handle than bromine (safety: can be fatal if inhaled, or absorbed through skin contact, lachrymator, causes extreme burns), and has been found to possess far greater stereo-selectivity than bromine itself in the bromination of alkenes such as cis- and trans-stilbenes. Note that each mole of the reagent (MW 320) releases one mole of Br2. Though it is a solid and much less hazardous, you must still handle it with extreme care. Wear gloves, goggles, use a spatula and alert your instructor if you have any spills. Do not try to sample the smell of the compound. Treat it like it is bromine because it easily generates bromine. The second step requires a very high temperature in order to complete the reaction in a reasonable time. This is accomplished by carrying out this step in a solvent of very high boiling point and heating in an oil bath at 170°C Caution is necessary when working around such a hot bath. Procedure 1. Place 0.0055 mole of trans-stilbene in 20 mL of acetic acid in a 50 mL Erlenmeyer flask. Heat the mixture on a steam bath until all of the stilbene dissolves. Then add 0.0063 mole of pyridinium bromide perbromide (Caution: corrosive; wear gloves) and mix the reactants by swirling, If necessary, use an additional small amount of acetic acid to rinse into solution any crystals of the reagent that have adhered to the walls of the flask. Continue heating on the steam bath for approximately ten minutes. The solution does not normally change much over the course of the reaction. It normally is orange throughout the reaction though in principle the orange color should diminish as the bromine is consumed. This procedure can be scaled down according to the amount of material you actually have. When scaling to half scale, for example, cut all the quantities in half and use a smaller flask (why?). When scaling down though, you do not cut the reaction times down (why?). 2. Cool the reaction mixture in cold water (do not use an ice bath or the acetic acid will freeze). Collect the product by vacuum filtration and wash it with a small amount of cold methanol to remove all or most of the yellowish color. Save a small amount of this stilbene dibromide to take a melting point after it is dry. Weigh the remainder of your product now and continue with the next step. If you have less than 0.5 g, consult your TA or instructor. 3. Please note that the remaining procedure is already scaled. The following quantities take into account the reductio n of material from the bromination step. Do not scale down these quantities if you have 0.5 grams or more of material. If you have less than 0.5 grams consult your TA or instructor about scaling the procedure. In a large test tube which you obtain from your TA place the remainder of your stilbene dibromide (at least 0.5 gram) along with 3 pellets of potassium hydroxide. (These consist of about 85% KOH and 15% water. Three pellets weigh about 0.5 g; it is not necessary to weigh them since you are using an excess.) If you have more than 1.5 g stilbene dibromide, use 4 KOH pellets. Caution: Potassium hydroxide is very hygroscopic and very caustic; measure it out quickly and recap the bottle immediately. Clean up any spills with a damp paper towel. Do not pick up the pellets with your hands (gloved or otherwise). Use a spatula. 4. Add 4 mL of triethylene glycol. 5. As a heat source for the dehydrobromination reaction, use a sand bath that has been regulated to 170-180°C. To prepare your sand bath, fill your heating mantle with sand. Set the step control at 60 volts early in the lab and work on getting it regulated. It is helpful to stir the bath with a glass rod (not the thermometer). Do not over- heat it or leave it unattended, If the sand gets too hot, the thermometer can explode. Thermometers break very easily if used as a stir rod. 6. When the temperature is regulated, use a clamp to mount the test tube containing your reaction mixture in the sand bath, occasionally mixing the contents of the test tube with a glass stirring rod while maintaining the sand bath at 170-180°C. Caution: Hot sand can cause painful burns!!! Heat the reaction for about twenty minutes. It should turn a fairly translucent reddish- brown. Remove the test tube from the sand bath (Reminder: the test tube is hot!) and cool it to room temperature in room temperature water. Then, place the test tube in an ice bath. 7. Add 20 mL of cold water to the cooled reaction mixture, allow the reaction to cool for ten or fifteen minutes and collect the resulting nearly colorless diphenylacetylene by vacuum filtration. Do not rush this step. Diphenyl acetylene is a low melting solid and if contaminated, can separate as an oil (melting point depression). Give it time to crystallize. Isolate the crude produce using an appropriately small vacuum filtration apparatus. Allow the crude product to dry and measure its melting point. If the melting point is within five degrees of the literature value (see below) , do not recrystallize the material. 8. If the melting point is not close to the literature value, it can be recrystallized directly from 95% ethanol in the hood. Review the procedure for recrystallization. Essentially, you dissolve your product in the minimum necessary volume of hot ethanol, carry out a hot gravity filtration, and let the hot ethanol solution stand undisturbed while it cools slowly; this should lead to the formation of large colorless crystals. After the first crop of crystals has been collected by vacuum filtration, concentrate the mother liquor by boiling it to obtain a second crop of pure material, which is also collected by vacuum filtration. Combine the two crops of crystals, and after allowing the product to dry determine the yield and melting point, and submit the sample to your TA. The reported melting point of diphenylacetylene is 61°C. Your Lab Report should include answers to the following questions: 1. What results would have been obtained in this experiment if cis- stilbene had been used instead of trans-stilbene as the starting material? That is, would the intermediate or final products have been different, and if so, how? 2. How could you determine whether the elimination step occurs by the E1, or the E2 mechanism? 3. In this synthesis why do we first add two bromine atoms to trans- stilbene if we are just going to take them off again? 4. Why does the double elimination step require such a high temperature to proceed? To figure this out, work carefully through the anti-stereochemistry of the successive bimolecular eliminations.
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