SYNTHESIS OF BIPHENYL-2,2'-DICARBOXYLIC ACID (Diphenic acid)

• The reducing agent (a solution containing cupro-ammonia ions) is first prepared. 

• Dissolve 63g (0.25 mol) of crystallised copper(n) sulphate in 250 ml of water in a 1 -litre beaker, add 100 ml of concentrated ammonium hydroxide solution (d 0.88) and cool the solution to 10 °C. 

• Dissolve 17.8g (0.256 mol) of hydroxylammonium chloride or 21g (0.256 mol) of hydroxylammonium sulphate in 60 ml of water, cool to 10 °C and add 42.5 ml of 6 m sodium hydroxide solution; if the resulting solution of hydroxylamine is not clear, filter it at the pump. 

• Without delay add the hydroxylamine solution, with stirring, to the ammoniacal copper(n) sulphate solution. 

• Reduction occurs at once, a gas is evolved and the solution assumes a pale blue colour. 

• Protect the reducing agent from the air if it is not used immediately. 

• Grind 25g (0.1 8 mol) of anthranilic acid with 46 ml of concentrated hydrochloric acid and 75 ml of water in a glass mortar, and transfer the suspension to a 500-ml round-bottomed flask which is provided with a mechanical stirrer. 

• Cool the contents of the flask in an ice bath to 0-5°C, and add a solution of 13.0g (0.19 mol) of sodium nitrite in 175 ml of water from a dropping funnel during about 20 minutes. 

• Keep the diazonium solution below 5 °C and, if it is not clear, filter it by suction through a chilled Buchner funnel immediately before use. 

• Surround the reducing solution in the 1-litre beaker (which is equipped with a mechanical stirrer) with a bath of crushed ice so that the temperature of the solution is about 10 °C. 

• Attach, by means of a short length of rubber tubing, to the stem of a dropping funnel a glass tube which dips well below the surface of the solution and is bent upwards at the end and constricted so that the opening is about 2 mm (this arrangement ensures that the diazonium solution reacts with the ammoniacal solution in the beaker and prevents the latter rising in the stem of the funnel). 

• Place about 45 ml of the cold diazonium solution in the funnel and add it at the rate of about 10 ml per minute while the mixture is stirred. 

• Add the remainder of the diazonium solution at the same rate; continue the stirring for 5 minutes after the addition is complete. 

• Heat the solution rapidly to boiling and carefully acidify with 125 ml of concentrated hydrochloric acid; the diphenic acid precipitates as pale brown crystals. 

• Allow to stand overnight and filter with suction; wash the crude diphenic acid with about 25 ml of cold water. 

• Suspend the crude acid in 100 ml of water and add 20g of solid sodium hydrogen carbonate. 

• Filter the resulting solution by gravity, and then boil with about 0.5g of decolourising carbon; filter and acidify the filtrate while still hot with excess of dilute hydrochloric acid (1:1). 

• Collect the precipitated diphenic acid on a Buchner funnel, wash it with 20 ml of cold water and dry at 100 °C. The yield of diphenic acid is 18g (82%); it melts at 227-228 °C and usually possesses a light cream colour.