Reaction:

Phenol.

Salicylic acid.

At the present day salicylic acid is made exclusively by the Kolbe-Schmitt method, which consists in treating sodium phenate with dry carbonic acid at first at the ordinary temperature, and then at 125o under 4-7 atms. pressure. The preparation is practically quantitative if the salt is absolutely dry and very finely divided, which may be attained by drying and grinding the substance in a vacuum.

93 Gms. of pure phenol are placed in an autoclave provided with a valve for the introduction of carbonic acid (Plates I. and XIII.), together with 1 gm.-molecule(40.1 gms. 100%) caustic soda,free from carbonate, dissolved in 100 gms. water.1 The solution is evaporated at 100° with continuous stirring under reduced pressure, until no more water comes off. The dried phenate is then removed from the autoclave and powdered as rapidly as possible in a previously heated porcelain basin. In order to protect it from moisture it is at once reintroduced into the autoclave together with 5-10 balls of about 14 mms. diameter made of iron or stone, which serve to pulverize it further during the stirring; the mass is again heated to 165o in vacuo until absolutely dry, which requires 5-6 hours, after which it is cooled down to 30o, and then carbon dioxide is led into the apparatus from a cylinder, with continuous stirring. . By means of the reducing valve on the cylinder, the pressure is regulated so that it does not rise higher than 1 atm. After 2 hours the pressure is slowly increased to 5 atms., and the temperature to 125o; after a further hour the tube is disconnected and the pressure let off. When the product has cooled the powdery yellowish salicylate is dissolved in 400 c.cs. water and precipitated with 125 gms. hydrochloric acid (3° %). The salicylic acid, which comes out in a practically pure form, is filtered at 300, and the traces of phenol washed away with a little water. For the further purification it may either be distilled with super-heated steam at 140o, or be recrystallized from hot water, after precipitating the impurities by means of 50 % of its weight of stannous chloride.1

93 gms. Phenol.

40.1 gms. 100 % NaOH.

400 c.cs. H2O. 125 gms. Hc1.

1 The caustic soda is completely freed from carbonate by dissolving in its own weight of water and allowing to stand for a day at 500. The solution, after filtering through asbestos, is titrated, using phenolphthalein as indicator.

The yield of pure distilled salicylic acid is 125 gms. from 93 gms. phenol.

Notes on Works Technique and Practice. - The equipment used in the works is modelled on the laboratory apparatus, but very powerful stirring-gear and grinding balk are used from the start, so as to make it unnecessary to remove the salt from the autoclave for powdering. Special stirring-gears are also made for this purpose with interlacing arms, which render the balls superfluous. To purify the salicylic acid it may be sublimed in a current of hot air, a beautiful product being obtained in this manner, which is, however, not quite pure. The yield of colours obtained from the distilled acid is always better. The process is almost quantitative, up to 137 kilos salicylic acid being obtained from 93 kilos phenol.

In a similar manner, ortho-cresotinic acid is obtained from ortho-cresol. In this case, however, the operation must be carried right through from start to finish without interruption as the sodium salt of ortho-cresol is spontaneously inflammable. About 20 % of the ortho-cresol is recovered unchanged, and the cresotinic acid must be reprecipitated from water.2 In spite of this, however, it is no more expensive than salicylic acid, as the poor yields are made up for by the cheapness of the cresol.

5% SnCl2.