DE2749754A1 - Di:alkyl carbonate esters prodn. by ester interchange - of di:methyl carbonate and alcohol, using (cyclo)paraffin as entrainer for methanol - Google Patents

Abstract

In the prodn. of carbonate esters (I) of the formula O=C(-O-R)2 (in which R is an organic radical with at least 2C) by ester interchange of dimethyl carbonate (II) with an alcohol R-OH, the MeOH formed is removed azeotropically with a 5-8C (cyclo)paraffin. The process is economical and does not necessitate the use of special conditions. (I) are intermediates for the prodn. of pharmaceuticals and plant protection agents.

Description

Process for the production of carbonic acid esters

The present invention relates to a new process for the preparation of carbonic acid esters of the general formula I. in which R denotes an organic radical with at least 2 carbon atoms.

From Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd edition, Volume 4 (1964), page 386, and Ullmann's Enzyklopadie der Technischen Chemie 3rd edition, Volume 9, page 775 f it is known that the ester I, e.g. diethyl carbonate, from phosgene and to prepare the corresponding alcohols R-OH. This procedure is for technical syntheses, however, very problematic, since working with larger amounts of the highly toxic phosgene requires extensive safety precautions, and there also the hydrogen chloride produced in this way the use of expensive corrosion-resistant Apparatus conditional. In addition, the hydrogen chloride can be used in an economical way can only be recovered in the context of large-scale syntheses. Because this requirement not present in the production of organic specialty chemicals such as the esters I, the hydrogen chloride must be made harmless, which is at least high additional Causes costs.

Another way to get to the compounds I consists in the transesterification of ethylene carbonate, which in turn is easily accessible from ethylene oxide and carbon dioxide, with the alcohol R-OH: Both transesterification products, the carbonic acid ester and the glycol, are not very volatile and therefore cannot be removed from the equilibrium by distillation. As a result, the transesterification only takes place up to equilibrium, and the work-up of the resulting four-component mixture also presents technical difficulties. Dimethyl carbonate is technically easily accessible by this method, since it can be removed from the equilibrium azeotropically with methanol. The azeotrope in turn can then in various ways, for example

be broken down into its components by extractive distillation.

Other alcohols than methanol, however, form with their carbonic acid ester no azeotrope.

Finally, it is known that the carbonic acid esters I can be prepared by transesterification of other carbonic acid esters, for example diethyl carbonate (US Pat. No. 2,514,354) or diphenyl carbonate (Ullmann cit. P. 776): / OR1 OR O = C + 2R OH, β O = CX + R'OH OR 'OR R' = remainder from group R, but + R This transesterification cannot be carried out satisfactorily with dimethyl carbonate (R '= -CH3). If the methanol formed is distilled off in order to shift the equilibrium in the desired direction, then, as can be seen from the preceding explanations, part of the dimethyl carbonate in the form of its azeotrope with methanol is also removed at the same time.

The invention was therefore based on the object of the carbonic acid ester I to make it more economically accessible than before.

It has been found that carbonic acid esters of the general formula I in which R denotes an organic radical with at least 2 carbon atoms, is obtained very advantageously by transesterification of dimethyl carbonate with an alcohol R-OII if the methanol formed is removed azeotropically with a C5-C8 paraffin or cycloparaffin.

Among the defined paraffins and cycloparaffins are Cyclohexane and the unbranched paraffins are preferred, and especially that n-hexane. The azeotropes have the following composition at normal pressure: entrainer Composition of the azeotrope in wt .-% entrainer methanol dimethyl carbonate Cyclopentane 81 S 19 d O n-hexane 67% 33% O n-heptane 43 »45% 7 # n-octane 21 M 64% 15% cyclohexane 62% 37% 1% The azeotropes separate after their condensation into a methanol-rich and a lighter paraffin-rich phase. The latter Phase is fed back directly to the transesterification. So you only need the amount of entrainer that is lost in the methanol-rich phase, and aligns it expediently so that per mole of methanol in the reaction mixture to be distilled constantly 1 to 30 mol of the entrainer are present. The methanol-rich phase, which still contains hexane and a little dimethyl carbonate can be used directly for production of dimethyl carbonate used by transesterification of ethylene carbonate or on conventional Way to be broken down into their components.

In the transesterification of dimethyl carbonate with a higher alcohol the formation of the mixed carbonates cannot be completely avoided. About their share to keep the reaction product as low as possible, it is advisable to use more of to use the alcohol - about a 0.1-3 molar excess - than the stoichiometric Implementation corresponds. After the reaction has ended, a mixture of the entrainer is obtained, the ester 1 and the mixed carbonic acid ester from the alcohol ROH and methanol. This mixture can be broken down into its components by simple distillation.

If one disregards the inventive use of the entrainer, the procedure has no procedural peculiarities. In general it is sufficient, at normal pressure and the boiling temperatures of the azeotropes (approx. 40 - 800C) to work, but in the case of higher alcohols R-OH can increase the Reaction rate higher temperatures - about up to 2000C - and corresponding We recommend higher pressures - up to about 10 bar.

As usual, transesterification is carried out through the use of transesterification catalysts such as potassium hydroxide, sodium methylate or basic or acidic ion exchangers performed.

As expected, the success of the procedure is of the nature of the Alcohols are basically not dependent on ROH. Only the reaction speeds differ from case to case, the known rule being that the rate of reaction decreases with increasing molecular weight of the alcohol. It is therefore superfluous that different classes of alcohols or individual alcohols must be listed separately.

Only the carbonic acid esters of ethanol and should be emphasized of alkyl alcohol, since these compounds are used as intermediates for the manufacture of pharmaceuticals and pesticides are of particular importance.

Since the paraffin or cycloparaffin / methanol azeotropes with significantly Allow less effort to separate than the methanol / dimethyl carbonate azeotropes the advantages of the method according to the invention are obvious.

Example 1 Preparation of diethyl carbonate A solution of 450 g (5th Mol) dimethyl carbonate, 552 g (12 mol) methanol, 600 g (approx. 7 mol) cyclohexane and 3 g of potassium hydroxide was refluxed for 1 hour at 60.degree. C. during which the azeotrope that forms from 37% by weight of methanol, 62% by weight of cyclohexane and 1% by weight Dimethyl carbonate was continuously distilled off. The condensate divided into one each Phase of methanol / dimethyl carbonate and cyclohexane, which is continuously back into the Transesterification reaction was returned.

After the transesterification had ended, the reaction mixture was fractionated. The diethyl carbonate fell in 70% strength and the ethylmethyl carbonate in 19% strength Yield to. The mixed ester was fed to another reaction batch, and the dimethyl carbonate / methanol phase was in the synthesis stage of manufacture recycled from dimethyl carbonate from ethylene carbonate and methanol.

Example 2 Preparation of diallyl carbonate To that given in Example 1 Way, but with n-hexane as an entrainer, the diallyl carbonate was in one yield won by 58%. In addition, 314% allyl methyl carbonate was obtained.

Claims (1)

A process for the preparation of carbonic acid esters of the general formula I in which the radicals R denote organic radicals with at least 2 carbon atoms, by transesterification of dimethyl carbonate with an alcohol R-OH, characterized in that the methanol formed is removed azeotropically with a C5-C8 paraffin or cycloparaffin.