Grignard Synthesis of Triphenylmethanol
Objective: The purpose of this experiment is to synthesize triphenylmethanol from benzophenone via Grignard
reaction. The product will be isolated through extractions and purified by recrystallization. Reaction efficiency will be evaluated through percent yield, percent recovery, and the purity of the final product will be determined by IR, TLC, and mp determination.
Chemicals: bromobenzene, magnesium turnings, diethyl ether, benzophenone, biphenyl, triphenylmethanol, iodine,
6 M HCl, brine, anhydrous MgSO4 or Na2SO4, 10:90 EtOAc/hexanes.
Glassware and equipment: 100 mL RBF, air condenser, Claisen adaptor, 60 and 125 mL addition ...view middle of the document...
This product is formed as a result of quick addition of bromobenzene to magnesium turnings.
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R-MgX MgBr + Biphenyl, major byproduct + R'-X Br R-R' + MgX2 (Wurtz coupling)
2. Benzene (Ph-H), resulting from protonation of the carbanion. The carbanion is very basic, so if there is any water in the solvent or in the glassware, or if moist air is allowed to enter the reaction mixture, some of the carbanion will be protonated. Formation of this product results from presence of any moisture in the reaction mixture: so, please, do not sneeze, cough, or spit on your reaction!!!
R-MgX R-MgX + + H2O R'-OH RH R'H + + MgXOH MgXOH
3. Activation of magnesium: Pure magnesium is an active metal, so active that any magnesium that has been exposed to air is inevitably coated with a film of magnesium oxide on its surface. This oxide film blocks the bromobenzene from actually contacting active magnesium, and thus prevents the requisite electron transfer. One way to expose active surface is to simply break several turnings before placing them into reaction flask. The other way is to break the turning inside the flask using your glass stirring rod: this procedure is not the safest as it often results in breaking the fragile glassware. The third way is addition of an activator such as a small iodine crystal. The iodine serves two functions. a. The first is as an indicator. The color will disappear when the magnesium is activated and is able to do redox chemistry with bromobenzene. b. The second is as an activator. Iodine is sometimes able to chemically “clean” the surface of the magnesium so that fresh, active magnesium is exposed so that it can do redox chemistry with bromobenzene. However, it doesn’t often work! 4. Unreacted starting material (Could be the Ph-Br, the Mg, and/or the ester).
First class period, Step 1: Preparation of the Grignard Reagent
Glassware needed for the first reaction setup: please wash and dry during the previous class period and store in your drawer. a. 100-mL RBF b. “Claisen” adapter c. Air condenser d. 125mL separatory funnel with stopper e. Two 50mL Erlenmeyer flasks f. 10 mL graduated cylinder The reaction must be done in the hood!!! 1. Clamp the 100-mL round-bottomed flask equipped with a stir bar to a vertical rod using a two-finger clamp. 2. Weigh out about 0.5g of magnesium metal. (Record weight) 3. Break a couple of magnesium turnings, pour them into RBF, add 10 mL of diethyl ether and place Claisen head into the joint. 4. Add 125 mL sep funnel on one side and the air condenser to the other joint of the Claisen head. Clamp the glassware
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with keck clamps. Temporary stopper both outlets. 5. Pour 20 mL of ether into the separatory funnel and put stopper back on. 6. Measure out 2.1 mL of bromobenzene in a graduated cylinder. Record the volume as...