Grignard - Chemistry 251 7th and 8th Lab Period Synthesis...

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Unformatted text preview: Chemistry 251 7th and 8th Lab Period Synthesis of Triphenylmethanol Reading: F&F, pp. 156-160 Please note that there is NO prelab for this experiment. You were also NOT provided with a sample flowchart for the experiment in class. Since this is the first time you are "on your own ” with the flowchart, your notebook score for this experiment will count twice in the calculation of your final grade. During the next two laboratory periods you will carry out the synthesis of triphenyl- methanol through the use of a Grignard reaction. ether / A Grignard Reagent OH O O triphenylmethanol The Grignard reaction is a very useful and versatile reaction. It is one of a small class of organic reactions which result in the formation of a new carbon—carbon bond with high yield. The major problem with this reaction is that it is extremely sensitive to the presence of water. Even trace amounts of water can be enough to either stop the reaction from occurring or markedly reduce the yield of expected product. In this experiment it is essential that your entire apparatus be clean and dry. DO NOT wash your glassware on the day of the experiment. Glassware should be washed the week before the experiment, rinsed with a small portion of acetone, and left in your drawer to dry until the following week. ‘ All of the reagents used in this experiment must be anhydrous. This is especially true of the ether (solvent) and the methyl benzoate (reactant). DO NOT use the stock (reagent grade) materials found in the lab. The bottles for these materials must be labeled ANHYDROUS or DRY. Failure to pay attention to this detail will, at best, cause your reaction to fail to start, or, at worst, give little or no expected product after two weeks of work. Be sure to replace the caps on the bottles immediately after use, or the solvents will no longer be anhydrous! CAUTION: Ether is extremely volatile and highly flammable. Care must be taken to avoid flames or electrical sparks. Be sure to “vent” your separatory funnel often during any extraction involving ether. Preparation of Phenylmagnesium Bromide (Grignard reagent) Procedure: Set up the Grignard "Glassware Tree" as shown below. Use only your previously cleaned and dried glassware for this purpose. The "Grignard Glassware Tree" Add 0.96 g of magnesium turnings and a small crystal of iodine to a dry 100,mL reaction flask. Making sure the stopcock on the sepratory (addition) funnel is closed, pour the entire sample of bromobenzene and 25 mL of DRY ether into the funnel. Be sure these two liquids have mixed thoroughly before proceeding. When you are ready to begin the - reaction, add 2-3 mL (just enough liquid to completely cover the metal turnings) of the bromobenzene/ether mixture from the addition funnel to the reaction flask. Swirl the solution until the iodine dissolves and watch for the reaction to begin, as evidenced by the disappearance of the brown iodine color and spontaneous boiling of the reaction mixture. Note that it may take several minutes for the reaction to start. (If, after a suitable period of time, your reaction mixture does not begin to boil - seek help from your TA). When boiling has been achieved, allow the solution in the addition funnel to flow slowly, drop by drop, into the reaction flask at such a rate that the ether continues to boil without external heating. After all of the bromobenzene/ether mixture has been added to the reaction flask, you should close the stopcock on the addition funnel and apply gentle heat to the reaction flask to maintain a reflux (boiling) of the reaction for 30 minutes. You may use the thermowell for this purpose but BE SURE to use a minimum setting on the controller. (Set the voltage to just past the point where the light on the controller comes on. It does not take much heat to boil this ether solution). Preparation of Triphenylmethanol After the reaction mixture has refluxed for 30 minutes, remove the heat and cool the flask down using an ice/water bath in a beaker. Place into the separatory funnel 2.6 mL of pure methyl benzoate and 10 mL of DRY anhydrous ether. Allow this mixture to SLOWLY drip into the reaction flask containing the grignard reagent with continuous agitation. This reaction is very exothermic and if boiling becomes excessive, slow down the addition or cool the flask with ice before continuing. After all of the methyl benzoate has been added, close the addition funnel stopcock and allow the reaction mixture to sit at room temperature for at least 30 minutes. During this time, a solid should form and settle down to the bottom of the flask. This solid is the magnesium bromide salt of your final product which is not very soluble in either water (too many carbons) or ether (too polar). - + O MgBr All that is left now is to replace the MgBr+ with a proton and the final alcohol product will be formed. To accomplish this 0 we will react the salt with an aqueous solution of diluted sulfuric acid. 0 Pour the contents of the flask into an erlynmeyer flask containing a mixture of about 20 grams of crushed ice, 40 mL of water and 2-3 mL of concentrated sulfuric acid (caution — corrosive!) To this mixture, add 1 mL of sodium bisulfite solution which will remove any residual iodine from the solution. There may be some gas bubbles produced by the reaction of left over magnesium metal with the sulfuric acid. After the gas formation has ceased, you may stopper the flask with a cork, wrap the flask neck and cork with parafilm and stop here for the week. Should time permit, you might also attempt to complete the extractions (your choice). Once the bubbbling has ceased and all of the solid material in the flask has been dissolved, transfer the resulting solution to a separatory tunnel for the preliminary purification of the triphenylmethanol. DO NOT add the solution to the separatory funnel if it contains ANY undissolved solids. If there is still some solid material in your flask, try adding another 5-10 ml of ether and see if it dissolves. If not - ask your TA for assistance. Perform the following extractions on your reaction mixture: 1) Add about 20 mL of standard grade ether (from this point on it is not necessary to use the more expensive anhydrous ether) to the separatory funnel, shake, and drain the aqueous layer from the funnel into a waste beaker. 2) Extract the ether/organic layer with a 10 mL portion of dilute (1 M) sulfuric acid. DO NOT use concentrated acid for this extraction. 3) Repeat the extraction with another 10 mL portion of dilute sulfuric acid. 4) Wash (extract) the organic layer with ~10mL of water 5) Wash the organic layer with sufficient aqueous sodium bicarbonate (~ 10 mL at a time) until any remaining acid present is neutralized. 6) Finally, wash the organic layer with about 10mL of saturated sodium chloride solution. This final wash is usually done with plain water, but because of the properties of ether as an extraction solvent - it is preferable to use sodium chloride solution. This will be discussed in lecture. Dry the ether layer over anhydrous magnesium sulfate. This is another potential stopping point for the first week. After a sufficient drying time, use a filter tube to remove the magnesium sulfate and transfer the ether solution of your product into a 100 mL round bottom flask. Add a couple of boiling chips and, using a steam bath, distill off the ether into an ice-cooled collection flask. Distill off as much ether as possible, leaving behind a "dry" yellow solid in the flask. This solid contains your product, triphenylmethanol, along with any impurities formed during the reaction (biphenyl) and also small amounts (hopefully) of unreacted starting materials. The impurities along with the biphenyl side product can be removed by washing the solid thoroughly with a non-polar solvent like hexane. (SAVE some of the hexane filtrate for TLC.) Stir the cooled solid residue with ~10 mL of hexane and isolate the remaining solid by vacuum filtration. (SAVE a small amount of this solid for TLC.) Complete the purification of the triphenylmethanol by recrystallizing it from 2-propanol. Vacuum filter the crystals and store in your drawer to complete the drying process. You will take a weight and melting point of the crystals next week. Chemistry 251 Name TA, Day, Time, Report on Triphenylmethanol (Triphenylcarbinol) Weight of Magnesium Moles of magnesium Weight of recrystallized triphenylmethanol Theoretical yield of triphenylmethanol (show work below) 9 % Yield of triphenylmethanol (show work) % (Calculate the theoretical yield and your percent yield assuming your sample of bromobenzene contained exactly 4.4 mL of pure material.) Comment on any unusual observations. (Did your Grignard start without much difficulty, etc.) Question: Two of the side products that could be produced in this reaction are benzene (CsHe) and biphenyl (CeHs-CGH5). Explain briefly how each of these would have formed (mechanism) and suggest experimental conditions that would minimize their formation. ...
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