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Please make sure that you read the chapter about how to set up glassware for reaction under inert gas (page 87-95 in reader, Winter 2008) with emphasis on the basic reaction setup and the use of the Schlenk line. Those are important topics that should be familiar to you after Thursday's introduction.
Step 1: Grignard formation
The setup has to be flame dried and refilled with nitrogen gas. The Mg-turnings should be crushed before being placed in the (250 mL) three-necked flask.
Dry diethyl ether (from solvent still) is required to get a good yield of the Grignard. The experimenter should make sure to take care about this while preparing the equipment.
Like in Chem 30 CL, the bromobenzene is dissolved in diethyl ether. The solution is then poured into the addition funnel. If the bromobenzene is added first and the ether afterwards, a layering of the two liquids is observed!
The reaction is started by adding a small quantity (5-8 mL) of the ethereal PhBr solution. The reaction mixture turns milky if the reaction initiated. If this does not occur after 10 min, the mixture is heated gently with a heat gun. A clear liquid will drip back from the reflux condenser then. After the heat source is removed, the reaction should maintain a gentle boil on its own. If this does not occur, the heating has to be repeated. In case this does not lead to success, a crystal of iodine is added and the mixture heated again. At this point, the reaction should initiate. If not, the mixture should be discarded.
After the reaction initiated, the remaining PhBr solution is added at a rate that the reaction maintains a gentle boil (it will turn brown at some point). After completion of the addition, the mixture is refluxed for an additional ~15 minutes to complete the reaction.
Step 2: Assay
In order to determine the assay, a small portion of the Grignard reagent formed is hydrolyzed. A 2 mL aliquot is placed into an Erlenmeyer flask with 50 mL of water. The mixture is stirred for 10 minutes. Then three drops of a phenolphthalein solution are added (which color will it have?). As titre, a diluted solution of HCl is used (~0.1 M). The number of moles of Grignard in the prepared solution.
Step 3: Formation of PPh4Br
In this experiment (like in the SnPh4) it is important to have the correct ratio of RMgBr to PCl3. At least four equivalents of Grignard reagent are required for the formation of the one PPh4-cation. In order to be on the safe side, a ratio of 1:4.5 equivalents will be assumed. It is always easier to quench the excess Grignard reagent, than separating complicated mixtures of compounds.
A glass tube with a wide opening (~ 1-1.5 cm in diameter) will be used in the oxidation step to avoid clogging. The stream of oxygen should not be too high because it carries away the solvent (diethyl ether) as well.
The addition of ice and the conc. hydrobromic acid should be done slowly since the quenching of the excess Grignard is exothermic. The entire setup from the formation of the Grignard reagent should remain assembled until this step is completed. The two layers are then separated: the aqueous layer plus the oil which is the product and the ether layer, which can be placed aside. The oil and the aqueous layer are transferred into a bigger flask and additional water is added. Upon heating most (or all) of the oil dissolves. The solution is decanted into a clean container, and ~90 g of solid sodium bromide (NaBr) are added. The crude product is allowed to crystallize, filtered, washed with ice-cold ethanol and ether, and then dried in vacuum for 15-30 minutes.
It is then re-dissolved in a minimum amount of hot 95% ethanol. Upon cooling to ~30 oC, about 50% of the volume of ether is added. The product is allowed to precipitate, filtered and dried.
Step 4: Characterization
IR spectrum (KBr pellet), m.p., 1H, 13C and 31P-NMR, analysis of MS spectrum of PPh3 (in the reader)
Step 5: Problems to be considered
1. How can you prepare such a solution fast from conc. HCl? What molar concentration does conc. HCl have?
2. Estimate of the concentration of the Grignard solution assuming that the Mg metal reacted completely and that there are ~120 mL of solution after the reaction. Does your experimentally determined value from above make sense?
3. Why is the formation of Ph(3-x)PClx (x=1,2) a problem in the reaction? How can these species be detected in the mixture?
4. Why is the not necessary to rinse the addition funnel with toluene in this experiment as compared to the SnPh4 experiment?
5. Which solvent should be used for the characterization using NMR spectroscopy?