Process for the production of fluorocarboxylic acid anhydrides

Details Process for the production of fluorocarboxylic acid anhydrides Process for the production of fluorocarboxylic acid anhydrides

2000-03-08

2001-06-19

Killos, Paul J. (Department: 1623)

Organic compounds -- part of the class 532-570 series

Organic compounds

Carboxylic acid anhydrides

Reexamination Certificate

active

06248923

ABSTRACT:

INTRODUCTION
This invention pertains to a novel process for the production of certain fluoro-substituted carboxylic acid anhydrides which are symmetrical, i.e., wherein the two carboxylic acid acyl groups of the anhydrides are the same. More specifically, this invention pertains to a process for the preparation of symmetrical fluoro-substituted anhydrides from mixed or unsymmetrical anhydrides, i.e., wherein the two carboxylic acid acyl groups of the anhydrides are different, by subjecting an unsymmetrical anhydride to reactive distillation to produce a lower boiling product comprising a symmetrical fluorocarboxylic acid anhydride and a higher boiling product comprising a second symmetrical anhydride. In accordance with a preferred embodiment, the present invention pertains to a process wherein ketene and trifluoroacetic acid (TFA) are reacted to produce a mixed anhydride, acetyl trifluoroacetate (Ac-TFA), which then is subjected to reactive distillation to produce a vapor product comprising TFM and a liquid product comprising Ac
2
O.
BACKGROUND OF THE INVENTION
Various processes for the preparation of carboxylic acid anhydrides are known. For example, acetic anhydride, the most common anhydride, may be prepared from acetic acid by the steps of (1) cracking or pyrolyzing acetic acid to produce ketene and (2) reacting the ketene with acetic acid to produce acetic anhydride. Higher molecular weight carboxylic acid anhydrides, e.g., substituted anhydrides and/or anhydrides which contain more than 4 carbon atoms, typically are prepared by contacting the corresponding carboxylic acid with acetic anhydride, e.g., butyric anhydride may be prepared by contacting butyric acid with acetic anhydride.
Trifluoroacetic anhydride (TFAA) is a strong dehydrating agent and has a broad range of chemical reactivity including the activation of carboxylic acids as mixed anhydrides (J. M. Tedder Chem. Rev. 1955, 55, 787-827). TFAA is a useful chemical in the synthesis of polymers and fine chemicals. However, it is expensive and, thus, more efficient methods for its synthesis are desirable. Because TFAA is a very reactive anhydride, a strong desiccant is required for its preparation. It was first prepared by Swarts in 1922 (
Bull. Sci. Acad. Roy. Belg.
1922, 8, 343-70) by the dehydration of TFA using phosphorus pentoxide. This method is convenient for small-scale TFAA synthesis but too inefficient for large-scale production. Phosphorus pentoxide is a water-sensitive solid that is difficult to work with on a large scale. Its cost and the expenses associated with the large amount of phosphate-containing waste it generates are strong disadvantages. In addition, minimizing manufacturing costs by minimizing waste is highly desirable.
The use of sulfur trioxide as the desiccant is an improvement over phosphorus pentoxide in this respect. TFM can be produced by the reaction of trifluoroacetyl chloride and sodium trifluoroacetate with the coproduction of sodium chloride which is less difficult to dispose of than is phosphoric acid or sulfuric acid resulting from the phosphorus pentoxide and sulfur trioxide processes. However, the cost of producing trifluoroacetyl chloride limits the feasibility of this method.
In 1954, E. J. Bourne and coworkers,
J. Chem. Soc.
1954, 2006-12, showed that at equilibrium the reaction of acetic anhydride and TFA to produce Ac-TFA and the reaction of Ac-TFA and TFA to produce TFM are not favorable. Nonetheless, the production of TFAA by reacting Ac
2
O with TFA is described in U.S. Pat. No. 4,595,541 which discloses a process for the preparation of TFAA by contacting TFA with the anhydride of acetic or an &agr;-halogenated carboxylic acid. Thus, contacting TFA (8 molar equivalents) with acetic, mono-, di- or tri-chloroacetic anhydrides produces TFAA in yields of 36, 59, 67 and 74%, respectively, and the conclusion that anhydride-based processes using a-chlorinated acetic anhydrides are preferred for the synthesis of TFAA. A process that produces TFAA in high yields from inexpensive raw materials and generates little or no waste which presents disposal problems, e.g., only water or marketable byproducts, is thus highly desirable. The present invention provides such a process.
BRIEF SUMMARY OF THE INVENTION
We have developed a process for the production of two symmetrical carboxylic acid anhydrides, i.e., anhydrides composed of two identical, carboxylic acid acyl groups, by first forming mixed or unsymmetrical anhydrides, i.e., anhydrides composed of two different carboxylic acid acyl groups, and then subjecting the unsymmetrical anhydrides to reactive distillation to produce a lower boiling product comprising a first symmetrical fluorocarboxylic acid anhydride and a higher boiling product comprising a second symmetrical anhydride.
One embodiment of our invention, therefore, concerns a process for the production of a first, symmetrical, fluorocarboxylic acid anhydride and acetic anhydride as a second symmetrical, carboxylic acid anhydride which comprises contacting ketene and a fluorocarboxylic acid having 2 to 4 carbon atoms to produce an acetyl mixed anhydride and subjecting the acetyl mixed anhydride to reactive distillation to produce a lower boiling product comprising the symmetrical fluorocarboxylic acid anhydride and a higher boiling product comprising acetic anhydride (Ac
2
O), provided that (i) the boiling point of the symmetrical fluorocarboxylic anhydride is lower than the boiling point of the fluorocarboxylic acid and (ii) the boiling point of the symmetrical fluorocarboxylic anhydride is lower than the boiling point of acetic anhydride.
A second and especially preferred embodiment of the present invention concerns a process for the production of TFAA using ketene and TFA as the raw materials which is suitable for continuous operation to produce commercial quantities of TFAA. This second embodiment concerning the production of TFAA comprises contacting ketene and TFA to produce a mixed anhydride, Ac-TFA, which then is subjected to reactive distillation to produce a highly volatile product comprising TFAA and a liquid product comprising Ac
2
O. In contrast to the known TFAA syntheses described above, the only byproduct of our novel process is acetic anhydride which represents an economic advantage rather than a burden.
A third embodiment of the invention pertains to a process for the coproduction of trifluoroacetic anhydride (TFAA) and a carboxylic anhydride having the formula [R—C(O)]
2
O which comprises the steps of: preparing a mixed anhydride having the formula R—C(O)—O—(O)-CF
3
and subjecting the mixed anhydride to reactive distillation wherein the mixed anhydride disproportionates to produce a lower boiling product comprising TFM and a higher boiling product comprising an anhydride having the formula [R—C(O)]
2
O; wherein R is a hydrocarbyl group containing up to about 6 carbon atoms.
DETAILED DESCRIPTION
The process of the present invention engages the disproportionation of a mixed anhydride derived from a carboxylic acid (R—C(O)OH) and a fluorinated carboxylic acid (R
F
—C(O)OH) to yield the corresponding symmetric anhydrides.
This disproportionation is carried out via a novel reactive distillation. Reactive distillations that convert one symmetric anhydride to another are well known. For instance, acetic anhydride (Ac
2
O) can be used to convert propionic acid (PrOH) to its anhydride (Pr
2
O) in a reactive distillation. In order for such a process to operate in an ideal and continuous fashion the two products, AcOH and Pr
2
O must be the lowest and highest boiling constituents of the mixture equilibrating in the length of the distillation column.
For the process of the invention proposed herein a reactant mixed anhydride is disproportionated to yield two symmetric anhydrides. As above, in order for such a process to operate in an ideal fashion the mixed anhydride must have a boiling point intermediate between the two symmetric anhydrides, as shown below for acetyl trifluoroacetate (Ac-TFA).
This is in general the case for mixed anhydr

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