PROCESS FOR THE PREPARATION OF L-ASCORBIC ACID

PROCEDE DE PREPARATION D'ACIDE L-ASCORBIQUE

English Abstract

A process is disclosed for preparing L-ascorbic acid by reaction of 2-keto-L-
gulonic acid with concentrated mineral acid at temperatures of between 40 and
80 ~C.

French Abstract

L'invention concerne un procédé de préparation d'acide L-ascorbique par réaction d'acide 2-cétone-L-gulonique avec de l'acide minéral concentré à des températures comprises entre 40 et 80 ·C.

Claims

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Claims
1. Process for the preparation of L-ascorbic acid,
characterized in that 2-keto-L-gulonic acid is reacted
with concentrated mineral acid at temperatures between
40 and 80°C.
2. Process according to Claim 1, characterized in
that the reaction is carried out at temperatures
between 50 and 70°C.
3. Process according to Claim 1 or 2,
characterized in that the 2-keto-L-gulonic acid and the
mineral acid are employed in the ratio 1:3.
4. Process according to one of Claims 1-3,
characterized in that the mineral acid used is
concentrated hydrochloric acid.
5. Process according to one of Claims 1-4,
characterized in that a mixture consisting of 2-keto-L-gulonic
acid and concentrated mineral acid is brought
to the desired reaction temperature.

Description

- CA 02299364 2000-02-04
Process for the preparation of L-ascorbic acid
The invention relates to a process for the
preparation of L-ascorbic acid by reaction of 2-keto-L-
gulonic acid with concentrated mineral acid at
temperatures between 40 and 80°C.
The invention relates in particular to a
process for the preparation of L-ascorbic acid,
characterized in that the reaction is carried out at
temperatures between 50 and 70°C.
Processes for the preparation of ascorbic acid
in one step by reaction of 2-keto-L-gulonic acid with
an acid are known. US 2,185,383 describes the reaction
of 2-keto-L-gulonic acid with concentrated hydrochloric
acid and acetic acid as a solvent . Japanese Unexamined
Application 58-177986 describes a process for the
preparation of L-ascorbic acid in which a solution of
sodium 2-keto-L-gulonate in ethanol and acetone is
first neutralized with hydrochloric acid. Japanese
Examined Application 48-15931 describes the reaction of
2-keto-L-gulonic acid with a mineral acid in an inert
solvent and in the presence of a surface-active
substance. Likewise, the preparation of L-ascorbic acid
starting from 2-keto-L-gulonic anhydride in the
presence of a surface-active substance is described in
WO 87/00839. EP 0 324 261 and GB 2,205,567 describe the
reaction of 2-keto-L-gulonic acid with an acid in a
mixture of inert solvents in the presence of a surface-
active substance.
The reaction of 2-keto-L-gulonic acid with 36%
hydrochloric acid is described in DE 2939052. Here, the
L-ascorbic acid is obtained in a yield of 87o after a
reaction at 100°C and after distilling off the
hydrochloric acid.
Surprisingly, investigations in the context of
L-ascorbic acid synthesis showed that high yields are
obtained if the reaction is carried out at temperatures
between 40 and 80°, in particular between 50 and 70°C.

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A temperature range from 57 to 63°C has proven
particularly preferable for carrying out the reaction.
The novel process according to the invention
enables L-ascorbic acid to be obtained in high yields
by lactonization of 2-keto-L-gulonic acid with
concentrated mineral acid.
It has proven particularly advantageous that at the
lower temperatures the decomposition of the ascorbic
acid obtained takes place significantly more slowly
than in the processes known from the prior art (e.g. DE
2939052; process at 100°C).
A further advantage proves to be the lower energy
consumption associated therewith and thus lower
environmental pollution.
Moreover, it proves advantageous in comparison
with the prior art that the reaction can be carried out
without organic solvents.
The process according to the invention proves
to be advantageous if the 2-keto-L-gulonic acid and the
hydrochloric acid are employed in a quantitative ratio
of 1:3.
The invention thus also relates to a process in which
the 2-keto-L-gulonic acid and the mineral acid are
employed in the ratio 1:3.
However, the concentration can be varied within
wide ranges. A more concentrated and also a more dilute
procedure can be carried out.
The weight ratio 2-keto-L-gulonic acid/concentrated
mineral acid can be, for example, between 10:1 and
1:10. The range is particularly preferably between 1:2
and 1:4, and a ratio of 1:3 is very particularly
advantageous.
Mineral acid means, for example, hydrochloric
acid, phosphoric acid or sulfuric acid. The
concentrated mineral acids are preferably employed.
The invention also relates to a process, as
described, characterized in that the mineral acid used
is concentrated hydrochloric acid.

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The invention also relates to a process, as
described, characterized in that a mixture consisting
of 2-keto-L-gulonic acid and concentrated mineral acid
is heated to the desired reaction temperature.
The reaction time is customarily between 1 and
5 hours, in particular between 2 and 4 hours, and a
reaction time of approximately 3 hours is particularly
preferred.
Working up is carried out by methods known per
se, e.g. by active carbon treatment, filtration,
removal of the solvent and crystallization. In
accordance with the process according to the invention,
the yields of L-ascorbic acid are, as a rule, over 90%,
such that complicated purification steps can be
dispensed with.
Above and below, all temperatures are indicated in °C.

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Example 1
100 g of 2-keto-L-gulonic acid are introduced
into 300 g of 37% hydrochloric acid. The batch is then
heated to 58° and kept at this temperature for 3 hours.
After removal of the hydrochloric acid and work-up, a
yield of L-ascorbic acid of 91.4% of theory is
obtained.
Example 2
100 g of 2-keto-L-gulonic acid are introduced
into 300 g of 37 o hydrochloric acid. The batch is then
heated to 59-60° and kept at this temperature for 3
hours.
After removal of the hydrochloric acid and work-up, a
yield of L-ascorbic acid of 90.2% of theory is
obtained.
Example 3
100 g of 2-keto-L-gulonic acid are introduced
into 300 g of 37% hydrochloric acid. The batch is then
heated to 59-60° and kept at this temperature for about
1.5 hours.
After removal of the hydrochloric acid and work-up, a
yield of L-ascorbic acid of 88.4% of theory is
obtained.