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Winthrop CHEM 304 - The Grignard Reaction

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Winthrop University Organic Chemistry Lab IIDepartment of Chemistry CHEM 304The Grignard ReactionThe Grignard reaction is a simple acyl addition reaction between a nucleophilicorganomagnesium reagent (a Grignard reagent) and an electrophilic carbonyl. It issynthetically a very useful reaction, as it is one of few that form a carbon-carbon bond.Grignard reagents are prepared by reacting alkyl halides with magnesium metal. Likemany organometallic compounds, Grignard reagents are very water sensitive, and specialprecautions must be taken to minimize the amount of moisture they are exposed to. Thehumidity of the air can be sufficient to ruin a Grignard reagent fairly quickly. Typically,carefully dried reagents and solvents are used in preparing and using Grignard reagents,and the glassware used is flame or oven dried. In this experiment, you will be preparingphenylmagnesium bromide from bromobenzene, and reacting this Grignard reagent withbenzophenone to synthesize triphenylmethanol.Br MgBrMg, etherMgBr1) etherO+2) HCl, H2OOHSafety:- Grignard reagents are powerful bases and can cause serious burns.- Hydrochloric acid is a strong acid and can cause serious burns.- Ether is extremely flammable.Procedure:A. Flame dry all glassware before proceeding. Obtain 20 mL of diethyl ether in a 50mL Erlenmeyer flask to use for the experiment. Assemble a Claisen tube, rubber septum,drying tube, 25 mL round bottom flask and spin vane as shown below. Place 150 mg ofmagnesium turnings and a spin vane into the round bottom flask. Add a small crystal ofiodine and attach reattach the flask to the assembly. In a clean, DRY 5 mL conical vial,dissolve 1.04 g of bromobenzene in 4 mL of diethyl ether. Add 1 mL of this solution tothe flask of magnesium using a syringe and warm the flask over a hotplate while stirring.Once the reaction begins to boil, add the remaining bromobenzene solution drop-wiseRevised 6-08Winthrop University Organic Chemistry Lab IIDepartment of Chemistry CHEM 304over the span of 10 minutes. Continue to warm the reaction for an additional 10 minutesafter all of the bromobenzene has been added, then cool to room temperature.B. In a 5 mL reaction vial, dissolve 1.09 g of benzophenone in 2 mL of diethyl ether.Using a syringe, add this benzophenone solution dropwise to the stirred Grignard reagentprepared in Part A. Stir the reaction for an additional 10 minutes after all of thebenzophenone has been added. If the reaction stops stirring, break up the solids with aspatula. Quench the reaction by adding 6 mL of 6M HCl, dropwise at first. C. Pour the contents of the beaker into a separatory funnel, rinsing the reaction flaskout with another 2 mL of diethyl ether. Drain off the lower aqueous layer and then washthe remaining organic layer with 20 mL of saturated NaHCO3. Drain the lower aqueouslayer and pour the organic layer into a 25 mL Erlenmeyer flask. Dry the solution by overanhydrous Na2SO4. Decant the dried solution into a small beaker and evaporate thediethyl ether by warming gently on a hotplate under a hood. D. Add 3 mL of petroleum ether to the solid and warm the mixture gently todissolve any biphenyl byproduct. Allow the mixture to cool and collect the solid viaRevised 6-08Winthrop University Organic Chemistry Lab IIDepartment of Chemistry CHEM 304vacuum filtration. Determine the weight of your product, and obtain a melting point andIR spectrum.Write-Up:Calculate the percent yield (show the calculation) and elaborate on the accuracy andsignificance of the melting point of your product. Interpret your IR spectrum.Questions:1. Write a balanced equation for the reaction of phenylmagnesium bromide withwater.2. How much water (in grams) would it take to completely destroy 5 mL of a 1Mphenylmagnesium bromide solution?3. Provide a synthesis of triphenylmethanol that uses a Grignard reaction and doesnot start with benzophenone.Revised


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