Preparation and Reaction of Phenylmagnesium Bromide

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CHEM 3720L Preparation and Reaction of Phenylmagnesium Bromide O Br MgBr + Mg H , H2O C C ether O MgBr OH Separatory Funnel 1. Assemble the apparatus shown at left. Dry with a Calcium Chloride heat gun (CAUTION:HOT, see lab instructor for Drying Tube instructions on proper use and safety). 2. Place 0.5 g (20.6 mmol) Mg metal in the 100 mL round bottom flask. Thermometer Adapter 3. Add one or two Iodine crystals with the Mg. 4. Allow the apparatus to cool to room temperature. 5. Prepare an ice/water bath in an appropriately Claisen Adapter sized beaker (large enough for the reaction flask to fit into). 6. Add a solution of bromobenzene (2.1 mL, 19.9 mmol) in dry ether (10 mL) through the separatory Reflux Condenser funnel. The reaction may need to be cooled or warmed to maintain a gentle reflux. You may warm the reaction mixture by using a hot water bath (start with a beaker of hot tap water, which may be further warmed with one of the hot water baths in the lab). DO NOT USE A HOT PLATE!!!! FIRE HAZARD. 100 mL Round Bottom Flask 7. Once the reaction has started, and there is a gentle reflux, add 7 mL ether through the separatory funnel. 8. Once the Grignard Reagent has completely formed (this may take up to one hour), it must be used immediately. Add a solution of 2.4 g (13.2 mmol) of benzophenone in 9 mL dry ether through the separatory funnel SLOWLY! The reaction may need to be cooled or warmed to maintain a gentle reflux. 9. After the spontaneous reflux has subsided, heat the reaction mixture with a hot water bath for 15 min. This is a good stopping point. You may disassemble the apparatus, and stopper the flask containing the reaction mixture. 10. (Week 2) Follow the procedure for the HYDROLYSIS, EXTRACTION and CRYSTALLIZATION found in Experiment 33A (Triphenylmethanol) of the laboratory text. .

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Addition of a Grignard Reagent to an Ester: Formation of a Tertiary Alcohol
Addition of a Grignard Reagent to an Ester: Formation of a Tertiary Alcohol Introduction: Grignard reagents are important and versatile reagents in organic chemistry. In this experiment, you will prepare a Grignard reagent and react it with an ester to prepare a tertiary alcohol. Specifically, in this reaction you will prepare phenyl magnesium bromide from bromobenzene and magnesium metal and then react it with methyl benzoate to prepare triphenylmethanol (Figure 1). In this reaction, two equivalents of phenylmagnesium bromide add to methyl benzoate. Note: Grignard reagents are sensitive to water. ALL glassware must be completely clean and dry. Reagents have been stored over molecular sieves to keep them dry in storage, but you should also keep containers closed as much as possible. Finally, water in the air can also react with the Grignard reagents over time. To successfully prepare triphenylmethanol in this experiment, you will need to be well organized and not take too much time between steps. Figure 1. O 1) Me O OH Br Mg MgBr THF + 2) H3O triphenylmethanol Mechanism: The formation of Grignard reagents from aryl halides is a complex mechanism, which takes place on the surface of the magnesium metal. You do not need to concern yourself with the details of this step of the mechanism. Figure 2. MgBr O Bu MgBr Bu MgBr O + OH O Bu MgBr O H3O Me OEt Bu Me OEt Me Bu Me Bu Me –EtOMgBr Bu Bu The mechanism of the reaction of a Grignard reagent with an ester is shown in Figure 2 (using methyl propionate and butylmagnesium bromide).
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