FR2833260A1 - Preparation of esters by microwave heating of a heterogeneous mixture of a solid acid and a liquid alcohol - Google Patents

Abstract

Preparation of an ester comprising stages of (a) mixing an acid and an alcohol and (b) heating the mixture by means of a microwave field to evaporate the water formed in the esterification. The mixture is heterogeneous, the acid and alcohol being, respectively, in solid and liquid phases. An Independent claim is also included for the ester obtained by the process.

Description

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 The invention relates to a process for the preparation of an ester comprising at least the following steps: a) mixing at least one acid and at least one alcohol, b) heating said mixture using a microparticle field. waves so as to evaporate the water formed by the esterification reaction of said acid with said alcohol.

 Japanese Patent JP 7330667 discloses such a method.

 The use of microwave energy allows a more homogeneous and faster heating of the reaction mixture than allowed by conventional heating means, that is to say other than microwaves. Advantageously, microwave heating thus makes it possible to limit breaks in the molecules of certain reagents causing undesirable by-products.

 It also makes it possible to effectively remove the water resulting from the synthesis reaction of the ester. This avoids the use of solvents, such as toluene or xylene, to carry out azeotropic distillation at the end of the reaction.

 In addition, the elimination of water is carried out as soon as it is formed, which advantageously makes it possible to favorably shift the equilibrium of the ester synthesis reaction, and thus to avoid the use of catalysts such as Broensted acids. No purification step to remove these catalysts is therefore necessary.

 Certain purification steps may, however, remain indispensable. In particular, when the acid is not in the liquid phase under the conditions of the reaction, it is usual to dissolve it by adding a solvent to the reaction mixture. This solvent must then be removed at the end of the ester preparation process.

 The object of the present invention is to provide a process for preparing esters without resorting to solvents, so as to limit the purification steps at the end of the reaction.

 This object is achieved by means of a process for preparing an ester comprising at least the following steps: a) mixing at least one acid and at least one alcohol, b) heating said mixture by means of a microwaves field so as to evaporate the water formed by the esterification reaction of said acid with said alcohol,

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procédé remarquable en ce que ledit mélange est hétérogène, lesdits acide et alcool étant respectivement en phases solide et liquide.

a remarkable process in that said mixture is heterogeneous, said acid and alcohol being respectively in solid and liquid phases.

It is found that, under microwave heating, the heterogeneity of the mixture does not impair the reaction between the acid and the alcohol. Advantageously, the heterogeneity of the mixture makes it possible to eliminate the use of a solvent to dissolve the acid, and thus to eliminate the purification step required at the end of the reaction to eliminate this residual solvent.

The present invention also relates to an ester obtained by a process according to the invention.

The microwaves that can be used for the synthesis of the invention are electromagnetic waves having frequencies between 30 and 300,000 MHz. In France, for industrial applications, only certain frequencies are authorized by legislation, in particular frequencies in the range of 40 to 50 MHz and frequencies of 896, 915 and 2450 MHz. These specific frequencies are in no way limiting.

The alcohol used in the esterification is preferably a C7 to C36 saturated alcohol, for example - a linear alcohol selected from n-heptanol, n-octanol, n-nonanol, n-decanol, nundecanol, n-dodecanol, n-tetradecanol, n-hexadecanol, n-octadecanol, neicosanol, n-docosanol; a branched alcohol chosen from 2-butyl-octanol, 2-butyl-decanol, 2-hexyl-octanol, 2-hexyl-decanol, 2-octyl-decanol, 2-hexyl-dodecanol, 2-octyl-dodecanol, 2 -decyltetradecanol, 2-dodecyl-hexadecanol, 2-tetradecyloctadecanol, 2-tetradecylicosanol, 2-hexadecyloctadecanol, 2-hexadecyl-eicosanol; - or a mixture of these alcohols.

According to the invention, the acid in solid form is introduced into a tank, for example made of quartz, and mixed with the alcohol.

The heterogeneous mixture of solid acid and liquid alcohol is stirred and heated by means of energy supplied by a microwave generator. The generator can for example emit through the lid of the tank.

The generator can also emit through a pipe connected to the lid and the bottom of the tank, and in which is inserted a circulation pump.

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 When the circulation pump is running, the mixture contained in the tank circulates in a loop in said pipe and is heated during its passage in front of the generator by the microwave fields emitted by the latter.

 The heated mixture reacts by producing the ester and water. The power of the generator is determined so as to immediately vaporize the produced water. The water vapor formed is discharged from the tank via a circulation of air.

 Dry air is injected into the tank. This air is charged with water vapor and then discharged outside the tank.

 The air flow is determined to prevent any accumulation of water in the tank.

 The evaporation of the water and its elimination advantageously make it possible not to resort to catalysts.

 Without the invention being limited by this theory, it seems that the solid acids do not substantially absorb the energy of the electromagnetic waves emitted by the microwave transmitter. This energy is mostly absorbed by alcohol and then converted into heat energy. Even if the heat energy is transmitted, at least in part to acids, the energy received by the molecules of the solid acid is insufficient to break them down into unwanted by-products or reactive to form undesirable products. The combination of heating by means of microwaves and heterogeneity of the reaction mixture thus allows a maximum limitation of the content of undesirable products in the final mixture.

 As will be seen in more detail in the following description, the present invention is particularly suitable for the preparation of esters from 2-pyrrolidone-5-carboxylic acid and / or glutamic acid in solid phase. The processes according to the present invention described below use these two acids, as non-limiting examples.

 The esters of 2-pyrrolidone-S-carboxylic acids are known and used in the cosmetics industry, for example for the preparation of lipsticks, and in the pharmaceutical industry.

 According to the prior art, the synthesis of esters by reaction of 2-pyrrolidone-S-carboxylic acid (also called pyroglutamic acid), abbreviated as APC, comprises the following steps.

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Cette synthèse s'accompagne de la synthèse de produits secondaires indésirables tels que le glutamyl-2-pyrrolidone-5-carboxylique,

b) Purification de l'APC par élimination du ou des produits indésirables. Pour opérer cette purification, on peut par exemple cristalliser l'APC et le séparer des produits indésirables restés en solution dans l'eau. c) Solubilisation de l'APC cristallisé au moyen d'un solvant. d) Ajout d'un alcool et d'un catalyseur dans la solution d'APC et chauffage conventionnel de la solution résultante. La synthèse de l'ester de l'APC s'effectue alors selon la réaction d'estérification réversible roi suivante :

où R désigne le squelette fondamental d'un alcool. Elle s'accompagne d'une synthèse d'autres produits indésirables, comme l'ester du glutamyl-2pyrrolidone 5 carboxylique et le succinimide, de formules [1] et [2], respectivement. a) Synthesis of APC by heating glutamic acid, abbreviated to AG, in the solid phase. The glutamic acid is melted at a temperature of about 180 ° C. and cyclizes by loss of water according to the following reaction Ro:

This synthesis is accompanied by the synthesis of undesirable by-products such as glutamyl-2-pyrrolidone-5-carboxylic acid,

b) Purification of APC by removal of unwanted product (s). To carry out this purification, it is possible, for example, to crystallize the APC and to separate it from unwanted products remaining in solution in water. c) Solubilization of the APC crystallized by means of a solvent. d) Addition of an alcohol and a catalyst in the APC solution and conventional heating of the resulting solution. The synthesis of the APC ester is then carried out according to the following reversible reversible esterification reaction:

where R denotes the basic skeleton of an alcohol. It is accompanied by a synthesis of other undesirable products, such as 2-glutamyl-2-pyrrolidone carboxylic acid ester and succinimide, of formulas [1] and [2], respectively.

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e) Purification du produit obtenu par élimination du catalyseur et du solvant.
Selon un premier mode de réalisation de l'invention, l'APC purifié à l'étape b) est séché, et l'étape c) est supprimée. L'APC cristallisé séché est dispersé dans l'alcool, de préférence un alcool aliphatique saturé de 7 à 36 atomes de carbone, sans ajout de solvant ni de catalyseur. Le mélange hétérogène est agité et chauffé au moyen d'énergie micro-ondes jusqu'à une température comprise entre 100 et 200OC, de préférence entre 130 et 180 C.

e) Purification of the product obtained by removal of the catalyst and the solvent.
According to a first embodiment of the invention, the purified APC in step b) is dried, and step c) is suppressed. The dried crystallized PCA is dispersed in the alcohol, preferably a saturated aliphatic alcohol of 7 to 36 carbon atoms, without addition of solvent or catalyst. The heterogeneous mixture is stirred and heated with microwave energy up to a temperature between 100 and 200OC, preferably between 130 and 180 C.

 The heating lasts between 2 and 6 hours, preferably between 3 and 5 hours, whereas the methods using so-called conventional heating means require synthesis times of about 12 to 24 hours. A reduced synthesis time advantageously makes it possible to reduce the manufacturing costs of the ester and to limit the formation of unwanted by-products.

 In addition, the energy provided by the microwave field is mainly absorbed by the alcohol and the formed ester in the liquid phase. Being in solid phase, the APC absorbs only very little energy. Advantageously according to the invention, this substantially avoids any decomposition of the APC, which contributes to obtaining a final product substantially free of undesirable by-products such as the ester of glutamyl-2-pyrrolidone carboxylic acid and succinimide .

 The process according to the invention thus makes it possible to minimize the formation of by-products during the esterification reaction.

 The crystallized APC reacts with the alcohol by forming liquid phase ester molecules and water molecules. As the ester is synthesized, the APC crystals disappear and the mixture clears. By determining the molar ratio of the acid to the alcohol between 0.5 and 2.5, preferably between 1 and 2, and more preferably equal to 1.5, the amount of acid or alcohol not reacted and to be eliminated at the end of synthesis.

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 Advantageously, the water captures the microwave energy and is vaporized as soon as it is formed, which facilitates its elimination and therefore the reaction Ri in the direction of the synthesis of the ester. The yield of the reaction is greater than 95%.

 Advantageously, the process according to the invention makes it possible to carry out the synthesis of the ester in a rapid manner, substantially without the production of undesirable side products, without catalyst or solvent.

 In a very advantageous variant of the embodiment described above, solid phase glutamic acid, generally in the form of a powder, is added to an alcohol, preferably a saturated aliphatic alcohol of 7 to 36 carbon atoms. carbon, more preferably auric alcohol, without addition of solvent or catalyst.

 Preferably, the molar ratio of the acid to the alcohol is between 0.5 and 2.5, preferably between 1 and 2, and more preferably is equal to 1.5.

 The heterogeneous mixture is stirred and heated with microwave energy up to a temperature between 100 and 200 C, preferably between 130 and 180 C. Preferably, the heating is maintained for a period of between 2 and 6 hours, preferably between 3 and 5 hours.

 Surprisingly, the reaction of the acid with the alcohol provides APC ester, substantially without the production of undesired by-products.

 Without limiting the invention, it can be assumed that the synthesis of this ester results from the following reactions:

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où R désigne le squelette fondamental d'un alcool.

where R denotes the basic skeleton of an alcohol.

The precise mechanism by which these reactions are carried out, however, is not fully understood. The yield of the reaction is greater than or equal to 94%.

The process according to this variant is particularly advantageous since it makes it possible to carry out, simultaneously and in a single reactor, the cyclization Ra of glutamic acid to 2-pyrrolidone-5-carboxylic acid, and the esterification R 1 from the 2-pyrrolidone-5-carboxylic acid, and, according to a second synthetic route, the R3 reactions of esterification of glutamic acid, followed by R4 cyclization of the ester of the glutamic acid obtained.

Advantageously, the reactions R 1, R 1. R3 and R4 are substantially total, the produced water being removed by evaporation and the circulation of air.

In the examples which follow, provided by way of illustration and without limitation, the compositions of the reaction products are given in percentages by weight.

The device for implementing the invention used at the laboratory scale is a Synthewave 1000 (Prolabo) having a useful volume of 800 ml for a microwave power of 800 Watts. In the examples below, a similar device was used, the vessel of which has a volume of 63 liters and is equipped with a generator

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 operating at a frequency of 2450 MHz, with a power of 6000 Watts. The air flow rate is between 10 and 100 l / min, preferably between 30 and 50 l / min.

 Examples 1 to 10 relate to ester preparations according to the invention, by esterification of solid 2-pyrrolidone-5-carboxylic acid with a saturated aliphatic alcohol. This esterification can be carried out in a temperature range between 100 and 200 ° C., preferably between 130 and 180 ° C., for a period of 1 to 5 hours, preferably 2 to 4 hours.

EXAMPLE 1 Preparation of n-heptyl 2-pyrrolidone-5-carboxylate
19.3 kg of 2-pyrrolidone-5-carboxylic acid and 11.6 kg of n-heptanol are introduced into the reactor. The mixture is subjected, with stirring, to the microwave field for a period of 3 hours at a temperature of 1500C. Throughout the reaction, the water formed is continuously removed from the reaction medium by a circulation of air (40 l / min).

 At the end of the 3 hours of synthesis, the mixture is cooled and then washed three times with water to remove the excess of highly water-soluble APC. The product is then decanted and the liquid phase separated. This liquid phase is allowed to cool. It solidifies during cooling. The solid obtained is then dried under vacuum for 72 hours at 30 C.

 The resulting product has the following composition: 97.8% ester, 1% heptanol, 0.8% APC and 0.3% glutamic acid.

EXAMPLE 2 Preparation of n-octyl 2-pyrrolidone-5-carboxylate
19.3 kg of APC and 13.0 kg of n-octanol are introduced into the reactor. By following the same protocol as that described in Example 1, a product having the following composition is obtained: ester 97.3%, octanol: 1.2%, APC: 1.1% and glutamic acid: 0.4% .

EXAMPLE 3 Preparation of n-nonyl 2-pyrrolidone-5-carboxylate
19.3 kg of APC and 14.4 kg of n-nonanol are introduced into the reactor. By following the same protocol as that described in Example 1, a product having the following composition is obtained: ester 97.1%, nonanol: 1.6%, APC: 0.9% and glutamic acid: 0.4% .

 EXAMPLE 4 Preparation of n-decyl 2-pyrrolidone-5-carboxylate

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19, 3 kg d'APC et 15, 8 kg de n-décanol sont introduits dans le réacteur. En suivant le même protocole que celui décrit dans l'exemple 1, on obtient un produit ayant la composition suivante : ester 96, 9%, décanol 2, 1 %, APC : 0, 7% et acide glutamique : 0, 3%.

19.3 kg of APC and 15.8 kg of n-decanol are introduced into the reactor. Following the same protocol as that described in Example 1, a product having the following composition is obtained: ester 96, 9%, decanol 2, 1%, APC: 0.7% and glutamic acid: 0.3%.

EXAMPLE 5 Preparation of n-undecyl 2-pyrrolidone-s-carboxylate 19.3 kg of APC and 17.2 kg of n-undecanol are introduced into the reactor. Following the same protocol as that described in Example 1, a product having the following composition is obtained: ester 96, 4%, undecanol: 2.5%, APC 1.3% and glutamic acid: 0.4%.

EXAMPLE 6 Preparation of n-dodecyl 2-pyrrolydone-5-carboxylate 19.3 kg of APC and 18.6 kg of n-dodecanol are introduced into the reactor. Following the same protocol as that described in Example 1, a product having the following composition is obtained: ester 95, 8%, dodecanol: 2, 3%, APC 1, 0% and glutamic acid: 0.3%.

EXAMPLE 7 Preparation of n-tetradecyl 2-pyrrolidone-5-carboxylate 19.3 kg of APC and 21.4 kg of n-tetradecanol are introduced into the reactor. By following the same protocol as that described in Example 1, a product having the following composition is obtained: ester 95, 6%, tetradecanol: 2.8%, APC: 1.1% and glutamic acid: 0.5% .

EXAMPLE 8 Preparation of n-hexadecyl 2-pyrrolidone-5-carboxylate 19.3 kg of APC and 24.2 kg of n-hexadecanol are introduced into the reactor.

Following the same protocol as that described in Example 1, a product having the following composition is obtained: ester 95, 7%, hexadecanol: 3.0%, APC: 0.9% and glutamic acid: 0.4% .

EXAMPLE 9 Preparation of n-octadecyl 2-pyrrolidone-s-carboxylate 19.3 kg of APC and 27.0 kg of n-octadecanol are introduced into the reactor. By following the same protocol as that described in Example 1, a product having the following composition is obtained: ester 95, 3%, octadecanol: 3.3%, APC: 0.9% and glutamic acid: 0.5% .

EXAMPLE 10 Preparation of 2-octyl-dodecyl 2-pyrrolidone-s-carboxylate

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19.3 kg of APC and 29.8 kg of 2-octyl-dodecanol are introduced into the reactor.

 According to the same protocol as that described in Example 1, a product having the following composition is obtained: ester 95, 1%, 2-octyl-dodecanol: 3.5%, APC: 1.1% and glutamic acid: 0 , 3%.

 Examples 11 to 20 relate to the preparation of esters, according to the invention, by heterogeneous esterification of glutamic acid with different saturated aliphatic alcohols. The temperature may be between 120 and 2000C, preferably between 140 and 180 C, for a period of 2 to 6 hours, preferably 3 to 5 hours.

EXAMPLE 11 Preparation of n-heptyl 2-pyrrolidone-5-carboxylate
22.1 kg of APC and 11.6 kg of n-heptanol are introduced into the reactor. The mixture subjected, with stirring, to the microwaves field for a period of 3 hours at a temperature of 150 ° C. Throughout the reaction, the water formed is continuously removed from the reaction medium by a circulation of air ( 40 I / min).

 At the end of the 3 hours of synthesis, the product obtained is cooled and then washed twice with water to remove excess glutamic acid and APC, the APC being highly soluble in water. The product is then decanted and the solid phase separated and then dried under vacuum for 72 hours at 30 ° C.

 The resulting product has the following composition: 97.0% ester, 1.2% heptanol, 0.8% APC and 1.0% glutamic acid.

EXAMPLE 12 Preparation of n-octyl 2-pyrrolidone-5-carboxylate
22.1 kg of APC and 13.0 kg of n-octanol are introduced into the reactor.

 According to the same protocol as that described in Example 1, a product having the following composition is obtained: ester: 97.1%, octanol: 1.3%, APC: 0.7% and glutamic acid: 0.9% .

EXAMPLE 13 Preparation of 2-pyrrolidone-5-carboxylate of n-nonyl
22.1 kg of APC and 14.4 kg of n-nonanol are introduced into the reactor.

 According to the same protocol as that described in Example 1, a product having the following composition is obtained: ester 95.9%, nonanol: 1.8%, APC: 1.2% and glutamic acid: 1.1%.

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EXAMPLE 14 Preparation of n-decyl 2-pyrrolidone-5-carboxylate
22.1 kg of APC and 15.8 kg of n-decanol are introduced into the reactor.

 According to the same protocol as that described in Example 1, a product having the following composition is obtained: ester 96.0%, decanate 2.3%, APC: 1.0% and glutamic acid: 0.7%.

EXAMPLE 15 Preparation of n-undecyl 2-pyrrolidone-5-carboxylate
22.1 kg of APC and 17.2 kg of n-undecanol are introduced into the reactor.

 According to the same protocol as that described in Example 1, a product having the following composition is obtained: ester 96.0%, undecanol: 2.5%, APC: 0.8% and glutamic acid: 0.7%.

EXAMPLE 16 Preparation of n-dodecyl 2-pyrrolidone-5-carboxylate
22.1 kg of APC and 18.6 kg of n-dodecanol are introduced into the reactor.

 According to the same protocol as that described in Example 1, a product having the following composition is obtained: ester 95.3%, dodecanol: 2.8%, APC: 0.9% and glutamic acid: 1.0%.

EXAMPLE 17 Preparation of n-tetradecyl 2-pyrrolidone-5-carboxylate
22.1 kg of APC and 21.4 kg of n-tetradecanol are introduced into the reactor.

 According to the same protocol as that described in Example 1, a product having the following composition is obtained: 95.3% ester, tetradecanol: 3.1%, APC: 0.8% and glutamic acid: 0.8%.

EXAMPLE 18 Preparation of 2-pyrrolidone-5-carboxylate of n-hexadecyl
22.1 kg of APC and 24.2 kg of n-hexadecanol are introduced into the reactor.

 According to the same protocol as that described in Example 1, a product having the following composition is obtained: ester 94.7%, hexadecanol: 3.3%, APC: 1.2% and glutamic acid: 0.8%.

EXAMPLE 19 Preparation of n-octadecyl 2-pyrrolidone-5-carboxylate
22.1 kg of APC and 27.0 kg of n-octadecanol are introduced into the reactor.

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 According to the same protocol as that described in Example 1, a product having the following composition is obtained: ester 94.8%, octadecanol: 3.6%, APC: 0.7% and glutamic acid: 0.9%.

EXAMPLE 20 Preparation of 2-octyl-dodecyl pyrrolidone-5-carboxylate
22.1 kg of APC and 29.8 kg of 2-octyl-dodecanol are introduced into the reactor.

 According to the same protocol as that described in Example 1, a product having the following composition is obtained: 94% ester, 2-octyl-dodecanol: 3.9%, APC: 1.1% and glutamic acid: 1.0 %.

TABLE 1

<Tb>
<tb> A <SEP> from <SEP> of APC <SEP> A <SEP> from <SEP> of glutamic acid <SEP>
<tb> Ester <SEP> synthesized <SEP> Example <SEP>% <SEP> Ester <SEP> Example <SEP>% <SEP> Ester
<tb> pyrrol <SEP> idone-5-carboxylate
<tb> of ..
<Tb>

.. <SEP> n-heptyl <SEP> 1 <SEP> 97, <SEP> 8% <SEP> 11 <SEP> 97%
<tb> .. <SEP> n-octyl <SEP> 2 <SEP> 97, <SEP> 3% <SEP> 12 <SEP> 97, <SEP> 1%
<tb> .. <SEP> n-nonyl <SEP> 3 <SEP> 97, <SEP> 1% <SEP> 13 <SEP> 95.9%
<tb> .. <SEP> n-Decyl <SEP> 4 <SEP> 96.9% <SEP> 14 <SEP> 96%
<tb> .. <SEP> n-undecylate <SEP> 5 <SEP> 96, <SEP> 4% <SEP> 15 <SEP> 96%
<tb> .. <SEP> n-dodecyl <SEP> 6 <SEP> 95, <SEP> 8% <SEP> 16 <SEP> 95, <SEP> 3%
<tb> .. <SEP> n-tetradecyl <SEP> 7 <SEP> 95, <SEP> 6% <SEP> 17 <SEP> 95, <SEP> 3%
<tb> .. <SEP> n-hexa-decyl <SEP> 8 <SEP> 95, <SEP> 7% <SEP> 18 <SEP> 94, <SEP> 7%
<tb> .. <SEP> n-octadecyl <SEP> 9 <SEP> 95, <SEP> 3% <SEP> 19 <SEP> 94, <SEP> 8%
<tb> .. <SEP> n-octyl-dodecyl <SEP> 10 <SEP> 95, <SEP> 1 <SEP>% <SEP> 20 <SEP> 94%
<Tb>

Table 1 summarizes the results obtained for the examples described above.

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 It appears that the process for preparing esters according to the present invention allows a very high conversion of the reagents and provides a product of very good quality, always having more than 90% of ester.

 Synthesis from APC provides a product always containing 95% or more of esters.

 Synthesis from glutamic acid provides a product always containing 94% or more of esters.

 Advantageously, the process according to the invention thus makes it possible to efficiently and simultaneously carry out in the same reactor the cyclization of glutamic acid and the esterification of the APC resulting from this cyclization.

 APC removed in the wash water can be crystallized according to techniques known to those skilled in the art, and then re-used. Advantageously, the method according to the invention therefore does not generate, or little, waste.

 In addition, the method according to the invention has the advantage of requiring the use of neither solvent nor catalyst, which improves the quality of the product obtained and further reduces the amount of waste produced.

 Table 2 makes it possible to compare the results obtained, for the preparation of 2-pyrrolidone-5-carboxylate of n-dodecyl, by means of a method with conventional heating, using toluene and p-toluenesulfonic acid, and according to two methods according to the invention, from APC and glutamic acid, respectively.

The figures are given as a percentage by mass of the final product.

TABLE 2

<Tb>
<tb> Ester <SEP> Alcohol <SEP> APC <SEP> Acid <SEP> Toluene
<tb> residual <SEP> glutamic
<tb> Synthesis <SEP> according to <SEP> the <SEP> technique <SEP> earlier, <SEP> in
<tb> presence <SEP> of <SEP> toluene <SEP> and <SEP> of acid <SEP> p-tuumen-q, <SEP>
<tb> sulfonic
<tb> Synthesis <SEP> according to <SEP> the invention <SEP> to <SEP> depart <SEP> from APC <SEP> 95.8 <SEP> 2.5 <SEP> 1.3 <SEP> 0 , 4 <SEP> 0, <SEP> 0
<tb> Synthesis <SEP> according to <SEP> the invention <SEP> to <SEP> starting <SEP> of acid <SEP> 95.3 <SEP> 2.8 <SEP> 0.9 <SEP> 1 , 0 <SEP> 0.0
<tb> glutamic
<Tb>

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The present invention is not limited to the embodiments described and shown provided as illustrative and non-limiting examples.

Claims (15)

 1. A process for preparing an ester comprising at least the following steps: a) mixture of at least one acid and at least one alcohol, b) heating said mixture by means of a microwaves field of in order to evaporate the water formed by the esterification reaction of said acid with said alcohol, characterized in that said mixture is heterogeneous, said acid and alcohol being respectively in solid and liquid phases. 2. Process according to claim 1, characterized in that no solvent is added to said mixture. 3. Process according to any one of claims 1 and 2, characterized in that no catalyst is added to said mixture. 4. Method according to any one of the preceding claims, characterized in that the frequency of said microwaves is between 30 and 300 000 MHz. 5. Method according to any one of the preceding claims, characterized in that said alcohol is a saturated aliphatic alcohol of 7 to 36 carbon atoms. 6. Process according to any one of the preceding claims, characterized in that said alcohol is a linear alcohol chosen from n-heptanol, n-octanol, n-nonanol, n-decanol, n-undecanol, n-dodecanol, n tetradecanol, n-hexadecanol, n-octadecanol, n-eicosanol, n-docosanol; a branched alcohol selected from 2-butyloctanol, 2-butyl-decanol, 2-hexyloctanol, 2-hexyl-decanol, 2-octyl-decanol, 2-hexyl-

 dodecanol, 2-octyl-dodecanol, 2-decyl-tetradecanol, 2-dodecyl-hexadecanol, 2-tetradecyl-octadecanol, 2-tetradecyl-eicosanol, 2-hexadecyl-octadecanol, 2-hexadecyl-eicosanol; or a mixture of these alcohols. 7. Method according to any one of the preceding claims, characterized in that said acid is 2-pyrrolidone-5-carboxylic acid or glutamic acid. <Desc / Clms Page number 16>  8. The method of claim 7, characterized in that said heating brings the temperature of said mixture between 100 and 200 C. 9. The method of claim 8, characterized in that said heating brings the temperature of said mixture between 130 and 180 C. 10. Process according to any one of claims 7 to 9, characterized in that the molar ratio of said acid to said alcohol is between 0.5 and 2.5.  11. The method of claim 10, characterized in that the molar ratio of said acid to said alcohol is between 1 and 2. 12. The method of claim 11, characterized in that the molar ratio of said acid to said alcohol is 1.5. 13. Method according to any one of claims 7 to 12, characterized in that said heating lasts between 2 and 6 hours. 14. The method of claim 13, characterized in that said heating lasts between 3 and 5 hours. 15. Ester obtained by a process according to any one of the preceding claims.