GB627700A - Improvements in processes for the preparation of laevo-ascorbic acid and related compounds - Google Patents

Abstract

l-Ascorbic acid or its analogues is prepared by re-arranging 2-keto-l-gulonic acid or an analogue or derivative thereof by heating with concentrated aqueous hydrohalide acid and an inert organic solvent immiscible with the concentrated aqueous acid. By " concentrated " is meant 20 per cent concentration or more. Preferably the mixture is distilled azeotropically under atmospheric or reduced pressure to remove a part of the organic solvent, which is added continuously or periodically to maintain a constant volume, and to remove substantially all other volatile components. The re-arranged product is recovered from the resulting crystalline suspension or slurry by filtration. The re-arrangement may take place concurrently with distillation in a continuous process. Non-polar solvents such as benzene, toluene, xylene, " Solvesso " (Registered Trade Mark), chlorbenzene and tetrachlorethane are used although polar solvents such as butyl or amyl alcohol may be admixed to dissolve tarry substances and to ensure that the solvent layer floats on the acid solution to prevent foaming and air oxidation. The polar solvent may be admixed before or during the re-arrangement or distillation or used to wash the crystalline product. In an example diacetone 2-keto-l-gulonic acid dissolved in an equal weight of 38 per cent hydrochloric acid is fed continuously to a distillation apparatus containing boiling toluene at atmospheric pressure. Acetone is liberated and re-arrangement is completed in the 2 to 3 minute interval required for azeotropic removal of hydrochloric acid. The hydrochloric acid solution is added at approximately one-fifth of the distillation rate of the azeotropic mixture and the volume of distilland maintained constant by continuous addition of toluene, preferably hot or in the vapour state and the continuous removal of crystalline slurry. The crude l-ascorbic acid obtained by filtration is washed with n-butyl alcohol to remove coloured impurities. In other examples, the initial materials are 2-keto-l-gulonic acid, methyl 2-keto-l-gulonic acid, 2-keto-l-galactonic acid and diacetone-2-keto-d-gluconic acid-the lastnamed yielding d-arascorbic acid-and the organic solvents are xylene, benzene, chlorbenzene and a mixture of tetrachlorethane and isobutyl alcohol.ALSO:l-Ascorbic acid or its analogues is prepared by re-arranging 2-keto-l-gulonic acid or an analogue or derivative thereof by heating with concentrated aqueous hydrohalide acid and an inert organic solvent immiscible with the conconcentrated aqueous acid. By "concentrated" is meant 20 per cent concentration or more. Preferably the mixture is distilled azeotropically under atmospheric or reduced pressure to remove a part of the organic solvent, which is added continuously or periodically to maintain a constant volume, and to remove substantially all other volatile components. The re-arranged product is recovered from the resulting crystalline suspension or slurry by filtration. The re-arrangement may take place concurrently with distillation in a continuous process. Non-polar solvents such as benzene, toluene, xylene, "Solvesso" (Registered Trade Mark), chlorbenzene and tetrachlorethane are used although polar solvents such as butyl or amyl alcohol may be admixed to dissolve tarry substances and to ensure that the solvent layer floats on the acid solution to prevent foaming and air oxidation. The polar solvent may be admixed before or during the re-arrangement or distillation or used to wash the crystalline product. In an example, diacetone 2-keto-l-gulonic acid dissolved in an equal weight of 38 per cent hydrochloric acid is fed continuously to a distillation apparatus containing boiling toluene at atmospheric pressure. Acetone is liberated and re-arrangement is completed in the 2 to 3 minute interval required for azeotropic removal of hydrochloric acid. The hydrochloric acid solution is added at approximately one-fifth of the distillation rate of the azeotropic mixture and the volume of distilland maintained constant by continuous addition of toluene, preferably hot or in the vapour state and the continuous removal of crystalline slurry. The crude l-ascorbic acid obtained by filtration is washed with n-butyl alcohol to remove coloured impurities. In other examples, the initial materials are 2-keto-l-gulonic acid, methyl 2-keto-l-gulonic acid, 2-keto-l-galactonic acid and diacetone-2-keto-d-gluconic acid, the last-named yielding d-arascorbic acid, and the organic solvents are xylene, benzene, chlorbenzene and a mixture of tetrachlorethane and isobutyl alcohol.