Transcript Slide 1
Experiment 16 Preparation of Diphenylmethanol by Reduction of Benzophenone Objectives Gain experience in preparative reduction of an organic compound To use TLC to assess the purity of your synthesized product To gain experience in using mole quantities in calculating % yield Reactions of Alcohols OH O O ox 1o alcohols R H red ox R OH H O 2o alcohols ox R1 R red R OH 3o alcohols cannot be oxidised R 2 R R1 R1 red R OH Oxidation of 1° Alcohols One of the most valuable reactions Can be accomplished by a large number of oxidizing agents Certain reagents oxidize too rapidly for the aldehyde to be isolated R O O Na2CrO7 OH R H R OH over-oxidation O PCC R oxidation as far as aldehyde only OH R H Reduction to Alcohols Aldehydes, ketones and carboxylic acids can all be reduced to give 1° and 2° alcohols Carboxylic acids can be reduced using BH3/THF or LiAlH3 Aldehydes and ketones by NaBH4 or LiAlH3 O BH3/THF R R OH R OH OH O R 1 R R1 = H or R NaBH4 or LiAlH3 Hydride Reagents Act as a source of nucleophilic hydrogen Carbonyl carbons are electrophilic Readily attacked by hydride reagents O O OH H Ph Ph benzophenone H3O+ Ph H Ph Ph Ph diphenylmethanol Safety Notes Sodium Borohydride – irrirtant, corrosive, toxic and highly reactive with water – gives off highly flammable gas Dichloromethane is toxic Benzophenone irritating to the eyes and respiratory system Experimental Dissolve 0.4 g of benzophenone in 7ml of ethanol Dissolve 0.16 g of NaBH4 in 2ml of cold water in a separate flask Add the hydride solution dropwise to the benzophenone solution Allow to react at room temperature for 30 mins – shaking occasionally Experimental Pour the reaction mixture on to 15ml of ice water and 1ml conc HCl – gas is liberated Allow to react for a few minutes then isolate the white precipitate on a Hirsch funnel – wash with cold water Dry on the funnel for 15 mins Leave to dry in your locker Experimental Check the purity of your product by TLC Dissolve 10 mg of the crude product in a few drops of dicloromethane Spot this solution on to a TLC plate along with a reference solution of benzophenone Develop the plate in 10% ethyl acetate/hexane Visualise the plate by staining with iodine TLC plate Ideally, the spot from your crude crystals should not contain any benzophenone The reference spot of starting material allows for comparison Based on the TLC make a conclusion about the purity of your product Report Calculate the % yield of alcohol product Use numbers of moles of starting material and product in calculations Melting point of your product Calculate Rf values of starting matereial and product Intro, discussion & question in manual