DE2337158C2 - Process for the preparation of alkanecarboxylic acids - Google Patents

Description

The invention relates to a process for the preparation of alkanecarboxylic acids having 4 to 24 C atoms by reacting olefins with a carbonyl compound which has at least one hydrogen atom on the α-carbon atom in the presence of a trivalent manganese compound.

An alkanecarboxylic acid is an aliphatic carboxylic acid with 4-24 C atoms and an alkene is an aliphatic olefin.

Such a process is described in Belgian Patent Specification No. 7 34 184. Although a good yield of alkanoic or aralkanoic acids can be achieved by this known process, further analysis shows that the acid consists to a large extent of a reaction product in which two carbonyl compounds have been bonded to each ethylene group of the alkene or aralkene; one of these compounds can be removed by hydrolysis. If the carbonyl compound is a combination of acetic acid and acetic anhydride, the reaction of an α-olefin will mainly form γ-acetoxyalkanoic acid instead of the economically much more important alkanoic acid without an acetoxy group in the γ-position. In Example 1 of the above-mentioned patent, in which 1-octene and a mixture of 500 ml of acetic acid and 100 ml of acetic anhydride with manganese dioxide as coupling reagent are used as starting materials, a yield of 75% of n-decanoic acid is mentioned. However, this yield value was only arrived at by misinterpretation of the analysis results, as can easily be seen. The same applies to Example 9 of the above-mentioned patent.

In order to increase the uneconomical yield of alkanecarboxylic acids resulting from the procedure of BE-PS 7 34 184, it was necessary to clarify the reaction mechanism, which is described in more detail below. It follows from the radical mechanism that the above-mentioned acids can only be produced in an economical yield if the following combination is met: a) the above-mentioned carbonyl compound must consist of at least 60% by weight, preferably at least 95% by weight, acetic anhydride, b) the concentration of trivalent manganese must be kept exceptionally low and c) the olefin concentration must also be low.

In the following, concentration, expressed in moles per litre (mol/l), is understood to mean the number of gram molecules per litre.

The invention consists in using a carbonyl compound which consists of at least 60% by weight of acetic anhydride in the process of the type mentioned above and known per se, and in maintaining the concentration of the trivalent manganese compound at 10 ^-3 to 10 ^-10 moles per liter and the concentration of the olefin at less than 0.1 mole per liter during the reaction. In this way it is possible to produce an alkanoic acid which has only one carboxyl group per ethylene group with a yield of more than 70%. Another important advantage of the process according to the invention is that more than 95% of the divalent manganese compound formed during the reaction can be easily removed by simple filtration.

The invention is based on the finding that the reaction proceeds according to a radical mechanism, which can be described as follows.

HOAc and Ac ₂ O mean acetic acid and acetic anhydride respectively. The initial reaction proceeds as follows: &udf53;np100&udf54;&udf53;vu10&udf54;&udf53;vz9&udf54;&udf53;vu10&udf54;

It was found that the radical &udf53;np100&udf54;&udf53;vu10&udf54;&udf53;vz9&udf54;&udf53;vu10&udf54;

can be easily added to an ethylene group according to the following equation: &udf53;np100&udf54;&udf53;vu10&udf54;&udf53;vz9&udf54;&udf53;vu10&udf54;

The addition product is again a radical, so that it also shows the tendency to react quickly with another molecule. Since the reaction mixture is formed from different components, different reactions are also possible. It is understood that the larger the proportion of molecules of a certain compound present, the greater the probability that the radical in question will react with it. In the presence of a large amount of acetic anhydride, the mixed anhydride of the desired alkanecarboxylic acid will form, immediately forming a new radical of the formula &udf53;np100&udf54;&udf53;vu10&udf54;&udf53;vz9&udf54;&udf53;vu10&udf54;

is created: &udf53;np100&udf54;&udf53;vu10&udf54;&udf53;vz9&udf54;&udf53;vu10&udf54;

This shows the great importance of a high carboxylic anhydride concentration.

With trivalent manganese the following reaction will occur: &udf53;np100&udf54;&udf53;vu10&udf54;&udf53;vz9&udf54;&udf53;vu10&udf54;&udf53;np100&udf54;&udf53;vu10&udf54;&udf53;vz9&udf54;&udf53;vu10&udf54;&udf53;np100&udf54;&udf53;vu10&udf54;&udf53;vz9&udf54;&udf53;vu10&udf54;

In order to counteract this undesirable side reaction as much as possible, it is desirable to keep the concentration of trivalent manganese low.

Another important side reaction that can occur assuming a reaction mechanism according to the invention is telomerization, whereby the radical &udf53;np100&udf54;&udf53;vu10&udf54;&udf53;vz9&udf54;&udf53;vu10&udf54;

reacts with an alkene (e.g. an α-olefin) to form a dimer: &udf53;np100&udf54;&udf53;vu10&udf54;&udf53;vz9&udf54;&udf53;vu10&udf54;

This dimer radical can then react again with acetic anhydride to form a dimeric branched acid; it can also react with trivalent manganese. This shows the great importance of not having too high an alkene concentration.

If the carbonyl compound in the form of acetic anhydride is present in amounts of less than 60 wt.%, relatively large amounts of by-products are formed by dimerization or lactone formation, so that the proposed process becomes economically less attractive.

The same applies to a concentration of trivalent manganese compound which is greater than 10 ^-3 mol/l, whereas at a concentration which is less than 10 ^-10 mol/l the reaction rate decreases so much that the process is no longer interesting from an economic point of view.

The alkene concentration should also not be higher than 0.1 mol/litre, as this would result in too many by-products being formed, which in this case consist largely of dimers and other telomeres.

It has been found that the most favourable results are obtained when the process according to the invention is carried out in such a way that the concentration of the trivalent manganese compound is between 10 ^-5 and 10 ^-7 mol/l. The concentration is preferably approximately 10 ^-6 mol/l.

Although the alkene concentration is somewhat less critical than the concentration of the trivalent manganese compound, it was found that optimal results are obtained in a concentration range of 10 ^-4 and 10 ^-5 mol/l.

The alkenes suitable for carrying out the process according to the invention include both cyclic and acyclic olefins. They can be substituted or unsubstituted and contain several unsaturated ethylene groups. It is important that they are used within the temperature range in which the reaction proceeds at an acceptable rate, namely between about 50 and 250°C. Acetic anhydride is soluble. Examples include all α-olefins from C₂ to C₂₂, 1,3-butadiene, 1,5-hexadiene, allylbenzene, cyclooctene and cyclohexene.

In general, a product with as homogeneous a composition as possible is desired. Therefore, the trivalent manganese compounds are preferably used in the form of a salt of the carboxylic acid from which the carboxylic anhydride used is derived. In this way, the formation of multiple acid anhydrides is counteracted as far as possible.

The process according to the invention can be carried out in various ways. In order to prevent the subsequent work-up of the reaction mixture from becoming too expensive due to the large amounts of solvent as a result of the highly diluted concentration of the components participating in the reaction, the trivalent manganese compound is preferably added gradually. During the addition, the carboxylic acid formed separates out in the form of an anhydride without large amounts of undesirable by-products being formed as a result of side reactions.

By stirring the reaction mixture well, a high degree of conversion can be obtained per unit volume of the reactor despite the low concentrations of one of the reaction components, given the high speed at which the reaction takes place. Good mixing is also important to avoid a sudden local increase in concentration that could lead to the side reactions mentioned above.

Although, as already stated above, the concentration of alkene is less critical than the concentration of trivalent manganese, again the best results are obtained when the alkene is added gradually to the reaction mixture.

The temperature at which the process according to the invention can be carried out successfully is about 50 to 250°C and depends on the reaction components used. Temperatures between 70 and 200°C are preferred.

When the acetic anhydride contains relatively large amounts of acetic acid and water and/or when Mn(OAc)₃·2H₂O is used instead of anhydrous manganese(III) acetate, the reaction temperature should preferably be above the boiling point of water and above the boiling point of the carboxylic acid derived from the anhydride used, but below the boiling point of the carboxylic anhydride used.

The reaction can of course also be carried out in an autoclave under increased pressure. In connection with the boiling points of the reaction components, it may also be desirable in certain cases to carry out the reaction under reduced pressure.

The reaction can be carried out either discontinuously or continuously. In a continuous mode, it can be advantageous to slowly add the olefin, the trivalent manganese compound and the acetic anhydride to the reaction mixture, which mainly contains acetic anhydride, in a reactor with good stirring, to remove any water and carboxylic acid formed via the gas phase at the top of the reactor and to remove, cool, filter and isolate the carboxylic anhydride, which contains the reaction product, at the bottom of the reactor. The acetic anhydride released during this separation can be fed back into the reaction mixture.

Example I (Comparative example according to Belgian patent specification 7 34 184)

According to Example 9 of Belgian Patent Specification 7 34 184, 0.318 mol of n-octene (50 ml) and 0.1 mol (23.2 g) of Mn(OAc)₃ are mixed with 600 ml of acetic acid and 100 ml of acetic anhydride in a nitrogen atmosphere. The mixture is heated to 110°C until the colour of the trivalent manganese has disappeared. When the mixture is cooled, no divalent manganese compound separates out. After evaporating the acetic anhydride mixture, water is added to the residue and the mixture is then kept for 48 hours. After acidification with sulphuric acid, it is extracted with ether. The ether extracts are separated off and evaporated. A residue of 10.65 g is obtained which contains 5.5% by weight of decanoic acid. This means that the yield of decanoic acid is only 1.1% of the theoretical. Th. (based on the n-octene used). Gas chromatographic analyses show the presence of a large number of by-products.

Example 2

A slurry of 0.025 moles of Mn(OAc)₃ in 200 ml of acetic anhydride is slowly added with good stirring under a nitrogen atmosphere to 300 ml of acetic anhydride, the temperature of which is kept constant at 122°C. At the same time, 0.2 moles (22.4 g) of n-octene is added to the acetic anhydride in small amounts. The rate at which the manganese acetate is added to the reaction mixture is such that the mixture remains pale pink. The reaction mixture is then cooled to room temperature, whereupon the Mn(OAc)₂ precipitates almost quantitatively (97.5 wt.%), and it is then filtered off. The acetic anhydride is removed from the filtrate by distillation. An infrared spectrum of the residue shows a strong absorption of anhydride groups. The residue is heated to reflux with acetic acid, adding a drop of a mineral acid to the mixture.

After 2 ½ hours, the mixture of acetic acid and acetic anhydride is removed by distillation. The weight of the residue is 28.2 g. An infrared spectrum shows strong absorption bands of a carboxyl group.

A gas chromatographic analysis shows that the residue contains decanoic acid in an amount of 58.7 % This corresponds to a yield of 48.1% of theory (based on the n-octene used).

Gel permeation chromatography analyses indicated the presence of telomeres, which were identified by mass spectroscopy, nuclear magnetic resonance measurements and infrared spectroscopy. The dimer was a gamma-branched C₁₈ acid.

Example 3

The experiment described in Example 2 is repeated, but n-dodecene is used instead of n-octene. After the reaction has finished, acetic anhydride is distilled off and the residue is heated under reflux with acetic acid.

After evaporation of the acetic anhydride mixture, 36.1 g of a solid white product are obtained which contains 72% by weight of myristic acid. This corresponds to a yield of 57% of theory (based on the n-dodecene used).

Example 4

This example shows that the production of decanoic acid according to Example 2 also produces good results on an industrial scale.

50 kg of acetic anhydride are placed in a reaction vessel equipped with a stirrer with a power of 1 kW/m³. 1120 g of 1-octene are placed in one dosing vessel and 10 kg of acetic acid in which 232 g of manganese(III) acetate have been dissolved are placed in a second dosing vessel. The reaction vessel and the manganese acetate dosing vessel are thoroughly flushed with nitrogen to keep the oxygen concentration below 50 ppm. The reaction vessel is then heated to 120°C and the olefin dosing begins, which is spread over 3 hours and 45 minutes. The manganese acetate dosing takes place at the same time, but over a period of 4 ½ hours. After the reaction has ended, the reaction mixture is filtered, leaving 170 g of manganese acetate on the filter. After acidification with mineral acid, the reaction mixture is freed from the volatile components by distillation. The reaction product (1500 g) consists of 77% by weight of decanoic acid. This is 67% of theory (based on the l-octene used).

Example 5

This example shows the conversion of n-hexane at a temperature of 110°C.

A solution of 0.02 mol of Mn(III) acetate in 99% aqueous acetic acid is added to a reaction vessel containing 1 l of acetic anhydride over a period of 3 hours and 35 minutes with thorough stirring and under a nitrogen atmosphere.

At the same time, but over a period of 3 hours and 20 minutes, a solution of 0.2 mol of hexene in 213 ml of acetic anhydride is added. After the reaction has finished, the reaction mixture is purified of manganese acetate by filtration and the anhydride mixture is hydrolyzed under the influence of a little mineral acid. The reaction mixture is freed of volatile components by distillation. A residue remains which consists of 81% by weight of octanoic acid.

Example 6

50 kg of acetic anhydride are placed in the same reaction vessel as in Example 5. A dosing vessel contains 350 g of manganese(III) acetate in 6 kg of acetic anhydride. Both vessels are thoroughly flushed with nitrogen to obtain an oxygen concentration of less than 50 ppm. The nitrogen is then expelled using gaseous ethylene. The temperature in the reaction vessel is simultaneously raised to 110°C. After the manganese acetate has been added, the mixture is added for 4 hours. The course of the reaction can be monitored by monitoring the ethylene gas uptake. After the addition of the mixture, the reaction mixture is left to stand for a few minutes until all of the manganese acetate has reacted. The reaction mixture is filtered, separating 280 g of manganese acetate. 440 g of ethylene gas were absorbed. After hydrolysis with acetic acid and water, the volatile components are separated off. The residue contains 83% by weight of butyric acid. The yield was 90.8% of theory (based on the ethylene used).

Example 7 (comparative example)

This example is carried out in a manner analogous to that given for example 2. However, the manganese acetate is added in a mixture of 100 ml of acetic anhydride and 100 ml of acetic acid. Instead of 300 ml of acetic anhydride, a mixture of 150 ml of acetic anhydride and 150 ml of acetic acid is used. After the reaction, acetic acid and acetic anhydride are distilled off. This is followed by heating the residue under reflux with acetic acid. After evaporation of the acetic acid, the product obtained is extracted with ether. The extract is dried and the ether is evaporated. 10.6 g of a liquid product remained, which consists of 47.3% by weight of decanoic acid. Gas chromatographic analysis showed the presence of γ-lactone and γ-acetoxy acid. Although the amount of decanoic acid obtained is higher than that in example 1, this example shows that this process is not suitable for industrial use.

Example 8 (comparative example)

Example 2 is repeated, but MnO₂ is used as catalyst. Although the reaction was carried out in 100% acetic acid, the reaction rate was so low that only 6.7 g of product were obtained after 70 hours. The weight fraction of decanoic acid in this product was only 21.4%.

Claims (5)

1. A process for the preparation of alkanecarboxylic acids having 4 to 24 C atoms by reacting olefins with a carbonyl compound which has at least one hydrogen atom on the α-carbon atom in the presence of a trivalent manganese compound, characterized in that a carbonyl compound which consists of at least 60% by weight of acetic anhydride is used and the concentration of the trivalent manganese compound is kept at 10 ^-3 to 10 ^-10 mol per liter and the concentration of the olefin is kept at less than 0.1 mol per liter during the reaction. 2. Process according to claim 1, characterized in that at least 95% by weight of the carbonyl compound is used in the form of acetic anhydride. 3. Process according to claim 1, characterized in that the concentration of the trivalent manganese compound is kept at 10 ^-5 to 10 ^-7 moles per liter. 4. Process according to claim 1, characterized in that the concentration of the olefin is kept at 10 ^-4 to 10 ^-5 moles per liter. 5. Process according to claim 1, characterized in that manganese acetate is used as the trivalent manganese compound.