Notes Week 9

Chemistry 254 Lab Week 9 - Reminders ?? Monday- March 1 ? Lecture ? Grignard Reactions ? Experimental Design and Retrosynthesis and Mechanisms ?? Tuesday? March 2? Synthesis of Triphenylmethanol (CH 18) ?? ONLINE PRELAB DUE - Synthesis of Triphenylmethanol ?? Thursday? March 4 ? Synthesis of Triphenylmethanol (CH 18) - IN LAB Report Write - Up -Bromination of Aromatics Report DUE ? Individual and Group -Spectroscopy Unknown DUE - + Extra Credit. NEXT WEEK IN LECTURE ? MIDTERM REVIEW SESSION CH 254 ? MIDTERM ? Tuesday, March 9 in LAB (120 min.) Chemistry 254 Synthesis of Triphenylmethanol (CH 32 )O MgBr bromenz Br Mg (0 MgBr phenyl magesiu brod O + benzophe O HCl, 2 O OH triphenylmtaol Chemistry 254 Formation of a Grignard Reagent ?? Grignard reagents are stabilized by the solvent. It is important to use ethers as the solvent. Diethyl ether or tetrahydrofuran (THF) are the most commonly used.! (CH 2 )O bromenz Br M (0 MBr phenyl magesiu ro MgBr O C 23CH32 CH 23CH32 O CH 23CH32 Tetrahydofuran (F) Diethyl tr Chemistry 254 Formation of a Grignard Reagent Reactivity with water.! Grignard reagents are strong bases and readily react with water in an acid-base reaction.! ?? The glassware that you use must be ?dry? and water must be excluded from the reaction. All ?dry? glassware has a layer of water on the surface. This water can be removed by either by heating or by wiping the internal surface thoroughly with a Kimwipe. You will dry your glassware by the wiping with a Kimwipe. Your TA will show you how to do this. ! MgBr H O H MgBrO + + Recal thMgBrRMgBrR ! "-= stogbse Chemistry 254 Formation of a Grignard Reagent Make certain that there is no acetone present on your glassware before you begin the reaction. 1. The acetone used in the lab contains a lot of water which will destroy your Grignard reagent (as discussed above). 2. Acetone, like benzophenone, is a ketone, which can react with your Grignard reagent. MgBr + O O Aceton MgBr Chemistry 254 Formation of a Grignard Reagent The formation of phenyl magnesium bromide involves a heterogeneous reaction between the magnesium metal and the bromobenzene. The surface of the metal must be clean and free from oxides (MgO or Mg(OH) 2 ). ?Cleaning? the surface can be achieved by vigorous stirring of the reaction mixture or by reaction with some reagents (I 2 or BrCH 2 CH 2 Br). It is best to use magnesium that is shiny, indicating that it is relatively free of oxides. An excess of magnesium is used. The Grignard reagent is ether-soluble, but there will be some insoluble magnesium metal left. A by-product of this reaction is biphenyl. This can be easily removed at the end of the reaction sequence. (CH 2 )O bromenz Br M (0 MBr phenyl magesiu brod Chemistry 254 Reaction Apparatus Chemistry 254 Procedure 1.? Place a spin vane and 0.05 g of magnesium turnings in a 5 mL conical vial. 2.? Attach the Claisen adapter, the drying tube with calcium chloride and a teflon spetum and screwcap. 3.? In a second 5 mL conical vial add 4 mL of anhydrous diethyl ether. 4.? To a 3 mL conical vial dispense 0.35 mL of bromobenzene and 1.0 mL of diethyl ether from the stock solution vial. 5.? Draw the solution into your syringe and add 0.35 mL of the solution to the reaction vial. 6.? The reaction should become cloudy or pale yellow/brown.! Chemistry 254 Procedure 7.? If the reaction does not change color insert your microspatula and crush the magnesium against the glass surface. 8.? Once you observe a color change slowly add the resulting solution of bromobenze via syringe over a period of 5 minutes. 9.? Rinse the vial containing the bromobenzene with 0.5 mL of diethyl ether. 10.? Let the reaction mixture stir for 20-25 minutes.! Chemistry 254 Reaction of the Grignard with Benzophenone MgBr MgBr phenyl magesiu brod O + benzophe O exothrmic An excess of the Grignard reagent is used. Why?! Chemistry 254 Procedure 1.? Prepare a solution of 0.273 g of benzophenone in 0.50 mL of diethyl ether in your remaining 3 mL conical vial. 2.? Draw the benzophenone solution into your syringe and add it to the reaction mixture as rapidly as possible. 3.? The reaction mixture should turn pinkisk-red and gradually solidify. 4.? Rinse the vial that contained the benzophenone solution with 0.2 mL of diethyl ether and add it to the reaction mixture. 5.? Let the reaction stir for another 15 minutes.! Chemistry 254 Procedure - Workup ?? At the end of the reaction, the following will be in the reaction mixture:! MgBr MgBr O Mg(0)Diethyl tr Chemistry 254 The reaction mixture is first treated with aqueous acid. This will:! 1. !Destroy the excess Grignard reagent, giving benzene. ! MgBr H HCl 2 O MgBrCl 2. !Protonate the alkoxide, giving MgBrCl and triphenylmethanol. ! O OH MgBr HCl 2 O MgBrCl Chemistry 254 3. Destroy the excess Mg(0), giving MgCl 2 and H 2 .! Mg(0) MgCl 2 +H (g) HCl 2 O Chemistry 254 Workup ?? The reaction mixture will then have the following:! MgCl 2 gBrl HO Diethyl tr Organic solube, Ntvolati Wte raicsle, Vlatie (cn bremovd byeaportin) Chemistry 254 Workup - Extraction ?? After addition of the 6M HCl, two layers should be present in the vial. The following steps are carried out: 1. Remove the organic layer. 2. Extract the water layer with diethyl ether to ?pull? any triphenylmethanol in the water layer into the organic layer. 3. Combine organic layers, dry with Na 2 SO 4 , and then evaporate. Do this in an Erlenmeyer flask. ?? At this point, the solid that you have will be a mixture of triphenylmethanol and biphenyl. Save some of this solid for TLC analysis.! Chemistry 254 Workup - Trituration The biphenyl is removed by trituration, a type of solid-liquid extraction. Process: 1. Add petroleum ether (different than diethyl ether!) to the solid in the Erlenmeyer flask and heatwhile stirring the solid. This will remove the biphenyl from the solid by extraction. 2. Isolate the solid by vacuum filtration using a Hirsch Funnel. Save some of this solid for TLC analysis. 3. The liquid you recover is called the triturant. Save this for TLC analysis.! HO triphenyltaol (olursue) bipheyl (trlum tr subl) Chemistry 254 Purification and Characterization Using semi-microscale, recrystallize the product from isopropyl alcohol. Measure the melting point and check purity by TLC. On one TLC plate you will need to compare the following. Make sure you save the appropriate materials as you carry out the trituration and recrystallization. 1. Crude triphenylmethanol (after evaporation of the organic layer). 2. Petroleum ether triturant. 3. Solid obtained after trituration. 4. Mother liquor from recrystallization. 5. Recrystallized triphenylmethanol. Chemistry 254 IN LAB REPORT WRITE-UP - Reaction Equation - Procedure (In the style of a formal lab report) - Answer Questions on Purity Analysis by TLC Chemistry 254 Grignard Reactions O H MgBr 1. 2. H3O + O 2. H3O + MgBr 1. OH HO O 2. H3O + MgBr 1. OH Christopher Callam 254-Week9TR.ppt