FIELD OF INVENTION
The present invention relates to improved process of preparing ascorbic acid from 2-keto- L-gulonic acid.
BACKGROUND OF INVENTION
Ascorbic acid or vitamin C is an essential nutrient which is required in human diet. It is available in tablet form and as an additive of food stuffs. It has an anti-oxidant property and act as a co-factor in many enzymatic reactions.
Ascorbic acid has been industrially produced by many processes. Generally, 2-keto-L-gulonic acid is reacted with an aliphatic ketone and an acid chloride in an acidic environment. Aliphatic ketone is an essential feature of this process so as to maintain a high concentration of hydrochloric acid in reaction system. The major drawback of this process is the formation of undesirable side products which form due to undesirable reactions with aliphatic ketone. These undesirable side products produce foul smell and coloration in reaction system. Also, the recovery ratio of the aliphatic ketone employed for the synthesis is also not necessarily high. For such processes, a purification mechanism is required to produce pure ascorbic acid. Hence, the presence of undesirable side products and additional costs due to purification poses serious hurdles for an industrial scale manufacturing.
Ascorbic acid is also produced by borohydride reduction of 2, 5-diketogluconic acid results in presence of borate impurities in the reaction product containing the 2-keto acids. The presence of such borate impurities in the 2-keto acid containing reaction product results in lower yields of ascorbic acid and erythorbic acid when the corresponding alkyl esters are heated with base to obtain desired final products.
In another method, Ascorbic acid is produced by reacting 2-keto-L-gulonic acid with surfactant and further treated with acidic catalyst to produce final product but drawback of this process is free water which is generated in reaction processing condition. Increase the amount of free water increases reaction time or decreases yield and this undesirable effect is mainly due to the decomposition of ascorbic acid which take place in the presence of an aqueous acid.
Consequently, there was thus an imperative requirement to develop an alternative industrially viable synthesis of ascorbic acid. Hence, an improved process has been developed which overcomes the problems of the prior art processes. Furthermore it is economical in operation and can be scaled up to an industrial level production.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an improved process for preparation of ascorbic acid of formula (I).

Formula (I)
It is still another object of the invention to provide a process for the preparation of ascorbic acid which is industrially feasible.
DETAILED DESCRIPTION OF THE INVENTION
The various aspects of the present invention leading to an improved process for the preparation of ascorbic acid is detailed below
In embodiment of the invention, 2-keto-L-gulonic acid is reacted with a mineral acid, an aldehyde and an acid chloride in presence of inert organic solvent and an acid preferably but not limited to hydrochloric acid.
In another embodiment of the invention, an aldehyde is dissolved in an inert solvent. The inert organic solvent herein refers to a solvent which does not react with the aldehyde, 2-keto-L-gulonic acid or L-ascorbic acid. Furthermore, an aldehyde is added to the reaction mixture. This solvent mixture is then added to a solution of 2-keto-L-gulonic acid followed by addition of an acid chloride which results in formation of ascorbic acid.
In another embodiment of the present invention inert organic solvent usable for the present invention comprises from the group but not limited to aromatic hydrocarbons which may be substituted with halogen or alkyl, such as benzene, toluene, xylene and chlorobenzene, halogenated aliphatic hydrocarbons such as chloroform and ethylene chloride, aliphatic hydrocarbons such as hexane, heptane and octane, and ethers such as tetrahydrofuran, dioxane and isopropyl ether, or a mixture of them.
In another embodiment of the present invention, an aldehyde used in the preparation of ascorbic acid comprises from the group but not limited to saturated aliphatic aldehydes, unsaturated aliphatic aldehydes, linear aliphatic aldehydes, branched aliphatic aldehydes and alicyclic aliphatic aldehydes, aromatic aldehydes.
In another embodiment of the present invention, acid chloride used in the preparation of ascorbic acid comprises from the group but not limited to an aliphatic carboxylic acid chloride.
In a yet another embodiment of the present invention, aldehyde, 2-keto-L-gulonic acid and acid chloride are added to the reaction mixture in calculated quantities.
For separating ascorbic acid from the reaction mixture, per se conventional separation and crystallization procedures can be employed. Further, if essential, advanced purification techniques can also be utilized to obtain high purity ascorbic acid.
The process of the present invention produces ascorbic acid in better yield in comparison to the prior art processes. In addition, ascorbic acid so obtained from the process as described by the present invention is accompanied by fewer impurities owing to absence of undesirable side products. Also, solvents such as but not limited to water, organic solvents such alcohols or combinations thereof required for dissolution of crude compound, can be easily distilled out. Hence, such a process also entails a high recovery ratio of solvents. Furthermore, the crude compound so obtained is directly dissolved in methanol water mixture, thus saving on the cost required for the isolation and purification of the crude compound.
Therefore the process of ascorbic acid preparation as embodied herein is particularly efficacious and proves to generate significant feasibility for an industrial scale production of ascorbic acid.
While the present invention has been described in terms of their specific embodiments, certain modifications in the process and various crystalline forms of the subject compound including specially designed equipments used are intended to be included within the scope of the present invention.
The following Examples are given for the purpose of illustration only, and are not intended to limit the scope of the invention.
EXAMPLES:
Example 1
2-keto-L-gulonic acid (100 g) was dissolved in ethylene dichloride (205 mL) and conc. hydrochloric acid (18.3 mL). This mixture was constantly stirred while adding orthochlorobenzaldehyde (30 mL) and acetyl chloride (10.3 g). 30% of Ethylene Chloride is recovered and fresh Ethylene chloride is added. The reaction was allowed to complete and crude compound was obtained which was dissolved without isolating further in any suitable solvent preferably methanol, water or methanol water mixture. The final product is recovered through recovery procedures and washed with methanol (250 mL), EDTA (0.5 g) and demineralised water to give pure L-ascorbic acid. Crystallization is carried out at 0-5 ºC followed by drying at 50-60 ºC to produce crystalline L-ascorbic acid.