SYNTHESIS OF BENZOPINACOL

Method A: Irradiation with sunlight 

• Dissolve 10 g (0.055 mol) of benzophe­none in 50 ml of propan-2-ol in a 100-ml round-bottomed Pyrex flask by slight warming, and add one drop of glacial acetic acid. 

• Add further quanti­ties of propan-2-ol, cooling to room temperature, until the solution is about 6 mm below the bottom of the flask joint. 

• Stopper the flask, taking care that none of the solution contaminates the joint, cover the stopper and joint with aluminium foil and place the flask in direct sunlight. 

• Colourless crystals begin to separate within 24 hours. 

• Allow the flask to remain in the sunlight until no further solid appears to separate (about 8 days). 

• Cool the solution in ice-water and collect the product by suction filtration, wash it with about 10 ml of ice-cold propan-2-ol and dry. 

• About 9.28 g (92%) of almost pure benzo­pinacol, m.p. 180-182 °C, is obtained. 

• It may be recrystallised from glacial acetic acid (about 80 ml) from which it separates as colourless needles, m.p. 185-186 °C; the yield is 8.1 g (81%).

Method B: Irradiation with a mercury arc lamp 

• Use the 100 watt medium-pressure mercury arc lamp with the Pyrex outer and inner jackets and a reaction vessel of approximately 110 ml capacity. 

• Make up a solution of 10g (0.055 mol) of benzophenone in about 110 ml of propan-2-ol containing one drop of glacial acetic acid as described above. 

• Place this solution in the reactor vessel, together with a magnetic fol­lower bar, and irradiate the vigorously stirred solution under an atmosphere of nitrogen. 

• Benzopinacol begins to separate within half an hour. 

• As the quantity of product increases it gradually collects on the surface of the Pyrex jacket which thus restricts the amount of light reaching the solution. 

• There­fore, after about 2-3 hours, switch off the lamp, raise the lamp insert from the reaction mixture and carefully scrape off the solid into the reactor vessel. 

• Collect the product (about 4 g) by vacuum filtration, wash it with a few ml of cold propan-2-ol and dry. 

• Return the filtrate to the reaction vessel. 

• Clean the lamp insert with paper tissue moistened with ethanol and dry, and replace it in the reaction vessel. 

• Continue the irradiation as above until a further appre­ciable quantity of product has separated (about 2 hours) and collect as before.Repeat this procedure until no further solid separates from the reac­tion mixture. 

• About four crops of material will be obtained during a total irradiation period of 8 hours. 

• The yield of benzopinacol, m.p. 178-182 °C, is about 8.65 g (75%). It may be purified by recrystallisation from glacial acetic acid as described above.