Patent GB-273043-A (GB)
Title: Electrolytic desulphonation of anthraquinone sulphonic acids
IPC: C07C309/38
Filed: 1926-04-15
Abstract: 273,043. British Dyestuffs Corporation, Ltd., and Hailwood, A. J. April 15,1926. Non-metallic substances, obtaining.-Sodium or potassium salts of anthraquinone sulphonic acids in alkaline solution are subjected to electrolytic reduction in a divided cell, whereby. those acid groups occupying a-positions are in great part removed and replaced by hydrogen while those in #-positions are scarcely affected. The carbonyl groups of the anthraquinone residue are reduced in the cathode chamber, leaving the product from which the alpha-sulphonic acid groups have been removed as an oxanthrol. A current of air oxidizes this to anthraquinone. Mercury, amalgamated copper, or other metals or alloys may be used as the cathode, an advantageous current density being from 2 to 3 amperes per square decimeter. The temperature is preferably higher than atmospheric. The separation of the cathode and anode solutions may be achieved either by a porous division of the pot type, or by an impermeable division which dips into a mercury trap connecting the two solutions and which serves as the cathode. In an example, 25 gr. of disodium anthraquinone-1 5-disulphonate are dissolved in 1 litre of hot water containing 14 gr. of sodium hydrate in solution. The solution so obtained is introduced into the cathode chamber, which consists of a porous pot on the bottom of which is a layer of mercury serving as the cathode. The porous pot is fitted with an airtight lid through which passes the. stirrer, cathode lead, and a tube to carry off excess hydrogen. The cathode chamber is surrounded by a 25 per cent aqueous solution of sodium hydrate in which is a carbon or iron plate or other anode. Current at a density of from 2 to 3 amperes per square decimeter is passed until about 13 to 17 ampere-hours have passed through the cell. The catholyte is removed and a current of air blown through it until the red colour due to oxanthrol is discharged. The anthraquinone is filtered off, washed, and dried, and may be purified by sublimation or recrystallization from toluene. In another example, 11 gr. of the dipotassium salt of anthraquinone- 1 : 8-disulphonic acid, 6 gr. of potassium chloride and 800 gr. of water forms the catholyte and a 10 per cent aqueous solution of potassium carbonate forms the anolyte. Six ampere-hours of electricity at a current density of 2 amperes per square decimeter are passed, and the product worked up as in the previous example. In a third example, 300 c.c. of a 2.2 per cent solution of the disodium salts of 1 : 6 and 1 : 7 disulphonic acids of anthraquinone, containing small quantities of the sodium salts of the 1 : 5 and 1 : 8 disulphonic acids obtained as a waste product, are treated with 4.2 gr. of sodium hydrate and subjected to electrolytic reduction in the cathode chamber of the cell described above, 3.5 amperehours of electricity being used. The brownish-red solution is then oxidized with a current of air and filtered from a small quantity of anthraquinone. The filfrate containing #-mono sulphonic acid is acidified with hydrochloric acid and the sulphur dioxide boiled off, and to the hot solution an aqueous solution of #-naphthylamine hydrochloride in excess added. The #-naphthylamineanthraquinone-#-sulphonate that crystallizes out is filtered off and decomposed with sodium hydrate solution to give #-naphthylamine and sodium anthraquinone-#-sulphonate, which are separated by their different solubilities in water. In another example, 5 gr. of the sodium salt of anthraquinone-a-sulphonic acids are dissolved in 300 c.c. of water containing 4.2 gr. of sodium hydrate in solution and the product is subjected to electrolysis, 2<1>/3ampere-hours of electricity being used. The product is oxidized by air and the. anthraquinone filtered off, washed, and dried. The process is applicable to anthraquinonesulphonic acids containing halogen, amino, or other substituent groups. For example, from 1- chloranthraquinone-5-sulphonic acid, a-chloranthraquinone is obtained by electrolytic reduction followed by oxidation.