Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
2002-10-03
2004-03-30
Keys, Rosalynd (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
Reexamination Certificate
active
06713656
ABSTRACT:
The present invention relates to a process for preparing diacetals of glyoxal by reacting from 40 to 75% by weight aqueous glyoxal with monohydric alcohols in the presence of an acid catalyst.
Diacetals of glyoxal, which are sometimes also referred to as tetraacetals, are interesting precursors for organic synthesis. The most important preparative method for diacetals of glyoxal is the acid-catalyzed acetalization of glyoxal using monohydric alcohols R—OH according to the following scheme:
The acid-catalyzed acetalization of glyoxal using monohydric alcohols is a complex reaction which, in addition to the monoacetal and diacetal, may also form a multitude of oligomers and/or cyclic by-products (see, for example, J. M. Kliegmann et al. in J. Org. Chem. Vol. 38 (1973) p. 556; J. Org. Chem. Vol. 37 (1972) p. 1276ff; A. Stambouli et al., Bull. Soc. Chimique France (1983) II p. 33-40.
U.S. Pat. No. 2,360,959 describes the preparation of diacetals of glyoxal from alcohols which are water-immiscible. To this end, aqueous glyoxal and a small quantity of acid catalyst are heated with at least 4 mol of the water-immiscible monohydric alcohol, an azeotropic mixture of water and alcohol is distilled off and the alcohol, after removal of the water, is recycled to the reaction. The reaction of glyoxal with methanol likewise described there is carried out in a similar way, although to separate water from alcohol, an additional, complicated fractional distillation has to be carried out. In the case of water-miscible alkanols such as methanol or ethanol, however, this process only delivers the corresponding 1,1,2,2-tetraalkoxyethane in low yields (38% in the case tetramethoxyethane).
GB 559,362 recommended the use of a water-immiscible, liquid, inert solvent such as benzene, toluene, xylene, hexane, dichloroethane or isopropyl ether in the acid catalyzed acetalization of glyoxal with monohydric alcohols. However, the use of azeotroping agents leads to additional process costs. Also, at least the halogenated azeotroping agents are toxicologically or ecologically unsafe.
F. H. Sangsari et al., Synth. Commun. 18(12) (1988) p. 1343-1348 recommended acetalization in the presence of an azeotroping agent such as chloroform using a Soxhlett apparatus filled with a drying agent to remove the water of reaction formed in preparing 1,1,2,2-tetramethoxyethane and 1,1,2,2-tetraethoxyethane.
The prior art processes for preparing diacetals of glyoxal have the disadvantage that when water-miscible alcohols such as methanol or ethanol are used, the yield of diacetal, i.e. of 1,1,2,2-tetraalkoxyethane, is low. Also, all processes require distillative measures during the conversion to remove the water of reaction which leads to increased energy and equipment costs and complicates the continuous operation of the reaction. The use of an azeotroping agent to remove the water of reaction in turn has the disadvantage that the concomitantly distilled alcohol and the azeotroping agent have to be separated.
It is an object of the present invention to provide a process for preparing diacetals of glyoxal which, even when water-miscible alcohols are used, delivers the appropriate diacetal in good yields and may be made continuous in a simple manner.
We have found that, surprisingly, this object is achieved by a process which comprises contacting a liquid mixture which, at the beginning of the reaction, comprises alcohol and glyoxal in a molar ratio of at least 15:1 and also water in a concentration of not more than 8% by weight with an acid catalyst until an approximate reaction equilibrium has been attained. According to the invention, it is believed that at least approximate reaction equilibrium has been attained when the concentration in the reaction mixture of the diacetal formed is at least 70% of the equilibrium concentration of diacetal applying to the particular composition.
Accordingly, the present invention relates to a process for preparing diacetals of glyoxal by reacting from 40 to 75% by weight aqueous glyoxal with monohydric alcohols in the presence of an acid catalyst, which comprises contacting a liquid mixture which, at the beginning of the reaction, comprises alcohol and glyoxal in a molar ratio of at least 15:1 and also water in a concentration of not more than 8% by weight with the acid catalyst until concentration in the reaction mixture of the diacetal formed reaches at least 70%, preferably at least 80%, in particular at least 90% and more preferably at least 95%, of the equilibrium concentration without more than 5% by weight of the alcohol used having already been distilled off.
The water concentration in the liquid mixture at the beginning of the reaction is preferably in the range from 2 to 8% by weight and in particular is not more than 7% by weight, for example from 3 to 7% by weight, and more preferably not more than 6% by weight, for example from 3 to 6% by weight, based in each case on the total weight of the liquid mixture. The total weight of the liquid mixture is calculated from the sum of all liquid components contained in the mixture and components dissolved therein. It does not include components which are not dissolved in the mixture such as heterogeneous catalysts. In principle, the water concentration may be adjusted in different ways:
a) for example, glyoxal can be used which has a content of at least 50% by weight and preferably at least 60% by weight. The glyoxal content of the aqueous glyoxal solution will preferably not exceed a value of 75% by weight.
b) a second possibility is to use a large excess of alcohol, for example, more than 30 mol of alcohol per mole of glyoxal, for example, from 30 to 50 mol of alcohol/mol of glyoxal in the liquid mixture.
c) a third possibility for adjusting the water content at the beginning of the reaction is adding an inert substance to the liquid mixture which is completely soluble in and/or completely miscible with the liquid mixture. When an inert substance is used, it is generally used in a quantity of at least 1% by weight, for example from 1 to 25% by weight, preferably from 2 to 20% by weight.
Preference is given to the measures a) and c), in particular a). It will be appreciated that the abovementioned measures may also be combined with each other, preferably measure a) with measure b) or measure a) with measure c).
In principle, inert substances include aprotic organic solvents and also neutral or weakly acidic salts which do not catalyze the acetalization. Preference is given to neutral or slightly acidic salts. Examples of useful salts include the halides, sulfates, nitrates, monoalkylsulfates, arylsulfonates and alkylsulfonates of metals of the first and second main group, for example, of Na, K, Li or Mg, of quaternary ammonium cations and also of iron(II) and iron(III) ions. Preferred salts include the sulfates, monoalkylsulfates, arylsulfonates and alkylsulfonates of the metals mentioned or of quaternary ammonium cations. A particular example of an alkylsulfate is methylsulfate. Particular examples of arylsulfonates include the phenylsulfonates and the tolylsulfonates. An example of an alkylsulfonate is methylsulfonate.
Examples of quaternary ammonium cations include the tetrakis-C
1
-C
10
-alkylammonium cations such as methyltriethylammonium and methyltributylammonium and also the phenyl- and benzyl-tris-C
1
-C
4
-alkylammonium cations such as benzyltrimethylammonium or benzyltriethylammonium.
Examples of useful salts which are sufficiently soluble in the liquid mixture and are inert under the reaction conditions, i.e. do not catalyze the acetalization, include in particular the tetrakis-C
1
-C
10
-alkylammonium alkylsulfates such as methyltriethylammonium methylsulfate and methyltributylammonium methylsulfate.
When a heterogeneous acetalization catalyst is used, in particular a strongly acidic ion exchange resin, the reaction mixture customarily contains no inert salt.
When aqueous glyoxal having a glyoxal content of at least 50% by weight and preferably at least 60 to 75% by weight is used in the process according to the inven
Aust Nicola Christiane
Botzem Jörg
Fischer Andreas
Heydrich Gunnar
Pütter Hermann
BASF - Aktiengesellschaft
Keil & Weinkauf
Keys Rosalynd
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