Macroscale Grignard Synthesis of Triphenylmethanol. Reactions of Triphenylmethanol. IMPORTANT: Everyone will do Chapter 38, Exp’s 4, 5, and 7 and Chapter 33, Exp’s 1, 2, and 8. New techniques will be used in this experiment. These include use of ground glass equipment, a water-cooled condenser, and a separatory funnel. Read about the use of a separatory (sep) funnel in Chapt 7. When you have a moment in the lab period before the sep funnel is needed, to become familiar with its operation, do a trial run with the sep funnel using only water. The new techniques will be demonstrated. Treat this experiment as one three-week, 30 point experiment. Note that these handouts are not stand-alone procedures. These are simply suggestions that should be incorporated into your prelab outline, which is based mainly on the lab text. This is a multistep reaction. Great care must be used throughout to ensure obtaining a sufficient quantity of material to proceed to subsequent steps. A four step reaction wherein each step gives a 70% yield will result in a 24% overall yield (.7 x .7 x .7 x .7). Imagine a 15 step synthesis, which is common in many syntheses of complex products. Assuming a 70% yield at each step, one would obtain a 0.5 % yield of final product. Translated into grams, assuming that the MW of the starting material is 1/2 that of final product, to obtain 1 g of final product, one would have to start with 16,384 g of starting material! Percent yields are indeed important. Summary: you will add bromobenzene to a flask containing anyhydrous ether and magnesium, initiate the reaction by scratching the surface of the magnesium, and once the reaction begins, dilute the solution with more ether. The exothermic reaction will cause the ether to boil and the water-cooled condenser will condense the ether vapor and return the liquid ether to the flask, thereby preventing the ether from evaporating. The process of returning condensed vapor to a flask as in this case is called refluxing. A drying tube is used to protect the contents of the flask from water vapor. If the reaction gets out of control, it can be moderated by cooling in an ice-water bath. CAUTION: diethyl ether is extremely flammable. Warning. Water is very detrimental to this experiment until you purposely add it later in the procedure. Even though HOH may not be visible on glassware, enough may be adsorbed onto the surface to prevent the reaction from working. Therefore, any glassware to be used in the formation and reaction of the Grignard reagent cannot be washed with water on the same day that the experiment is run. In all operations be very careful to NOT allow water to contact the equipment or reagents or solvent and be especially careful to not allow the reagent and solvent containers to become contaminated with water. Even your fingers are moist enough to contaminate the glassware. Do not handle anything that will come into contact with the reaction mixture (such as the stirring end of the stirring rod) with your bare fingers. Exp 4. Grignard Preparation Get together the following equipment, which has been oven-dried: CAUTION: it will be very HOT. A 10 mL and 25 OR 50 mL graduated cylinder, a 100 mL 3-necked round-bottomed (RB) flask, 2 glass stoppers, a drying tube, a condenser, 2 pieces of condenser tubing (caution- these may have water inside and if you are not careful you may get the other equipment wet), a glass stirring rod, and a corked 50 mL Erlenmeyer flask.Apparatus: The apparatus will be set up as in Fig. 38.8 but with the following modifications: a three-necked flask will be used, with two of the necks stoppered with ground glass stoppers. The heating device will not be present but the flask should be clamped above the benchtop in case the ice bath is needed. The other two openings in the flask will be stoppered with ground-glass stoppers. Condenser Prep: Just so you know about how much to turn on the water valve when the time comes, turn the valve slightly, just until a very slow steady flow begins. Note that the amount to turn the valve is very slight. Remember this for later. Make sure the pieces of tubing will be long enough to reach the water source and trough. Slip the rubber tubing onto the condenser (slightly wetting the end of the tubing allows it to be pushed onto the condenser more easily, but don’t slop the water around). Connect the tubing from the lower condenser connection to the water source. Run the upper tubing directly into the cup sink in the fume hood, being sure that it is pushed far enough into the sink to prevent its coming out when the water is turned on. Do not turn on the water yet (moisture may condense on the inside of the condenser). Drying Tube Prep: Place a small loose plug of cotton into the drying tube to block the inner opening, and using the powder funnel and spoon while working right over the large plastic container (to catch stray calcium chloride pellets), slowly (so as not to clog the funnel) add enough anhydrous calcium chloride pellets to half-fill the tube, add another piece of loosely-packed cotton, then finally the one-holed stopper. Measure Reagents and Solvents: Measure out the magnesium (Mg°) onto weighing paper and add it to the RB flask and using a small 3-pronged clamp, clamp the flask securely to a ring stand. Stopper the two outer openings, place the condenser into the center opening (do NOT use grease on the ground glass connections), and place the drying tube on the top of the condenser. Do not yet turn on the water. Pour directly (no pipets) into the larger graduated cyclinder 25 mL of absolute ether, add it to the 50 mL Erlenmeyer flask and cork it. Pour directly (no pipets) into the larger graduated cylinder 15 mL of anhydrous ether and add it to the Mg° in the RB flask. Using the attached pipet, measure out 9 mL of bromobenzene into the small grad cylinder. Prepare an ice-water bath and have it handy in case the reaction becomes overzealous. Note that a mixture of ice and water is much more effective at cooling a flask than ice alone. If the reaction needs moderation, just cool the bottom of the RB flask slightly. Do this only as a last resort and only for a very short period. Otherwise the reaction may stop. In all cases keep the ice-water bath away from the reaction set-up and be very careful to not spill water near the reaction set-up or equipment being used. Keep your hands very dry. - Note