Preparation of Benzopinacol via Photoreduction of Benzophenone ...

Experiment 2, Version 1.0nature with their reactivity modeled by alkoxy radicals; hence, their tendency toparticipate in hydrogen abstraction reactions. 2In the preparation of benzopinacol, the relevant reactions which occur afterphotoexcitation are: 3,41. hydrogen abstraction from isopropanol by the n→π* triplet state ofbenzophenone to yield the diphenyl ketyl and dimethyl ketyl radicals3O*+OHH 3 C CHH 3OH+OHH 3 C CH 32. radical transfer from the dimethyl ketyl radical to benzophenone to yieldacetone and another diphenyl ketyl radicalO+OHH 3 C CH 3OH+H 3 COCH 33. dimerization of diphenyl ketyl radicals to yield benzopinacolOHOH+ HO OHEquipment and Chemicalstest tubes, cork stoppers, parafilm, heavy-walled suction filtering flask, buchnerfunnel, filter paper, melting point apparatus, IR spectrometer (optional), 1 H NMRinstrument (optional), benzophenone, benzopinacol, 2-propanol, methylenechloride, petroleum etherProcedure1. Prepare a solution of benzophenone (0.8 g, 4.36 mmol), 2-propanol (8 mL,104 mmol) and glacial acetic acid (1 drop) in a test tube. In order to achievecomplete dissolution, gentle heating in a water bath is required.2. Tightly stopper the test tube using either a cork which is then wrapped inparafilm, or a rubber stopper which has been wrapped in aluminum foil. (It isimportant that the solution not come into direct contact with the rubber as itmay leach out some compounds which can quench benzophenone excitedstates.)Copyright 2002 Luzchem Research Inc Page 2 of 3