SYNTHESIS OF DIPHENYLMETHANE

CAUTION: This preparation should be conducted in an efficient fume cupboard.

• Fit a 500-ml three-necked round-bottomed flask with a sealed mechanical stirrer, attach a gas absorption device to one of the side-necks and stopper the third neck. 

• Place 38g (35 ml, 0.3 mol) of redistilled benzyl chloride and 150 ml of dry benzene (CAUTION) in the flask. 

• Weigh out 2g (0.015 mol) of anhydrous aluminium chloride into a dry capped specimen tube with the minimum exposure to the atmosphere. 

• Cool the flask in a bath of crushed ice and add about one-fifth of the aluminium chloride. 

• Stir the mixture; a vigorous reaction will set in within a few minutes and hydrogen chloride is evolved. 

• When the reaction has subsided, add a further portion of the aluminium chloride and repeat the process until all has been introduced. 

• The mixture should be kept well shaken and immersed in the ice bath during the addition. 

• After 15 minutes cautiously add lOOg of crushed ice, followed by 100 ml of water in order to decompose the aluminium complex. 

• Shake the mixture well, transfer to a separatory funnel and run off the lower aqueous layer. 

• Wash the upper layer successively with dilute hydrochloric acid and water and dry it with anhydrous calcium chloride. 

• Remove the benzene with the aid of the apparatus. 

• Distil the remaining liquid through an air condenser either with a free flame or from an air bath. 

• Collect the diphenylmethane at 250-275 °C (the pure substance boils at 262 °C) (1). 

• The distillate should solidify on cooling in ice and scratching with a glass rod, or by seeding with a crystal of the pure material. 

• If it does not crystallise, redistil from a small flask and collect the fraction, b.p. 255-267°C; this generally crystallises on cooling and has m.p. 24-25°C. The yield is 25g (50%).

Notes to Keep in Mind:

1. Alternatively the distillation may be conducted under diminished pressure; the fraction, b.p. 125-1 30 °C/10mmHg, is collected.

Cognate preparation: Triphenylmethane

• The apparatus is similar to that described above, but incorporating a reflux condenser to the outlet of which is fitted the gas absorption device. 

• Place a mixture of 200g (230 ml, 2.57 mol) of dry benzene (CAUTION) and 40g (26 ml, 0.33 mol) of dry chloroform in the flask, and add 35g (0.26 mol) of anhydrous aluminium chloride in portions of about 6g and at intervals of 5 minutes with constant stirring. 

• The reaction sets in upon the addition of the aluminium chloride and the liquid boils with the evolution of hydrogen chloride. Complete the reaction by refluxing for 30 minutes on a water bath. 

• When cold, pour the contents of the flask very cautiously on to 250g of crushed ice and 10 ml of concentrated hydrochloric acid. 

• Separate the upper benzene layer, dry it with anhydrous calcium chloride or with magnesium sulphate and remove the benzene by flash distillation. 

• Attach a Claisen still-head connected to a short air condenser and distil the remaining oil under reduced pressure; collect the fraction, b.p. 190-215 °C/10mmHg. 

• This is crude triphenylmethane which solidifies on cooling. 

• Recrystallise it from about four times its weight of ethanol; triphenylmethane separates in needles and melts at 92 °C. The yield is 30g (37%).