Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acid esters
Reexamination Certificate
2000-08-07
2001-05-22
McKane, Joseph K. (Department: 1626)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acid esters
Reexamination Certificate
active
06235924
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the preparation of benzoic acid esters. More particularly, this invention relates to a continuous batch process for preparing esters from mono- or dihydric alcohols and benzoic acid that may contain at least trace amounts of at least one of the three isomeric phthalic acids or phthalic anhydride. The resultant benzoic acid esters are at least 99 percent pure and substantially free of phthalic acid esters.
2. Description of the Prior Art
Methods for preparing esters, including those of aromatic acids such as benzoic and the three isomeric phthalic acids are well known. In accordance with one of these methods the carboxylic acid and the alcohol are typically reacted in the presence of a suitable catalyst. Because this type of esterification is usually an equilibrium reaction, to favor formation of the desired ester a stoichiometric excess of the alcohol reactant is typically used and the water formed as a by-product is continuously removed. Strong acids, stannates, and organic titanates are typical catalysts for esterification reactions.
If the boiling point of the final ester is sufficiently low, it can be isolated from the reaction mixture by distillation.
The distillation of esters prepared from 1) aromatic acids such as benzoic and phthalic acids and 2) alcohols containing less than 4 carbon atoms is known. For example, Japanese laid open application 60/94939, published on May 28, 1985 describes a method for purifying esters by treating the initial ester with a solid alkali or a concentrated alkaline solution followed by distillation of the resultant mixture. No conditions for the distillation are reported. The only esters exemplified are mono- and dimethyl phthalate.
Fractional distillation of dimethyl terephthalate at a temperature of 185-7° C. under a pressure of 50 torr is described by P.Zernov et al in Khim. Volokna (1975) (4) 27-8.
Distillation of ethyl 2,3,4,5-tetrafluorobenzoate is described in Japanese laid open application 11/130,734, laid open on May 18, 1999. The ester is prepared from the corresponding acid and alcohol in the presence of a sulfonic acid catalyst, which is subsequently recycled.
Steam distillation of esters prepared by reacting phthalic acid and an alcohol containing from 5 to 12 carbon atoms is described in Russian patent No. 763,322, published on Sep. 15, 1980. Steam distillation yields a mixture of the desired ester together with water, which must be removed as part of the purification procedure. Because removal of the last traces of water from an organic compound is difficult, expensive and time consuming, steam distillation is less than satisfactory when the objective is isolation of a high purity, substantially anhydrous ester directly from a reaction mixture.
The failure of the prior art to teach distillation of esters derived from aromatic acids and alcohols containing more than about 6 carbon atoms in the absence of steam suggests that conventional distillation of these esters is not feasible, particularly on a commercial scale, even under reduced pressures.
Commercially available grades of benzoic acid typically contains at least trace amounts of at least one of the three isomeric phthalic acids and/or phthalic anhydride. One source for these impurities is the presence of one or more of the three isomeric xylenes in the toluene that is oxidized in accordance with one of the more commonly used commercial processes for preparing benzoic acid. Zone refining and distillation have been used to separate benzoic acid from these impurities, however these methods are not cost efficient.
The toxicity to humans of esters derived from phthalic acid and alcohols containing 8 carbon atoms has been investigated by government regulatory agencies. Use of these esters as plasticizers in toys for children under 3 is banned in several European countries.
In the United States, for certain end use applications of the corresponding benzoic acid esters, for example as plasticizers for vinyl polymers such as polyvinyl chloride used as food grade packaging materials, the esters must contain no more than 0.1 percent by weight of the corresponding phthalic acid esters in order to avoid the reporting requirements of federal and state regulatory agencies.
One objective of this invention is to provide a semi-continuous process for preparing substantially pure esters from benzoic acid and either monohydric alcohols containing from 6 to 12 carbon atoms or dihydric alcohols containing from 2 to 8 carbon atoms.
SUMMARY OF THE INVENTION
This invention provides a method for preparing esters derived from at least one alcohol and benzoic acid, said method comprising conducting a first reaction, at least one intermediate reaction, and a final reaction, wherein
the method for conducting said first reaction comprises:
forming a first reaction mixture comprising
(1) a mixture of carboxylic acids consisting essentially of benzoic acid and at least trace amounts of at least one compound selected from the group consisting of the three isomeric phthalic acids and phthalic anhydride; and
(2) at least one alcohol selected from the group consisting of monohydric alcohols containing from 6 to 12 carbon atoms and dihydric alcohols containing from 2 to 8 carbon atoms, wherein the total number of moles of alcohol-bonded hydroxyl groups is equivalent to at least 1.02 times the number of moles of all carboxylic acids and carboxylic acid anhydrides present in said first reaction mixture;
heating said first reaction mixture at the boiling point sufficiently to remove substantially all of the water generated as a by-product of the resultant reaction while maintaining a stoichiometric excess of said alcohol in said reaction mixture, and continuing said heating until substantially all of the carboxylic acids in said reaction mixture have reacted;
distilling from said first reaction mixture under subatmospheric pressure substantially all unreacted alcohol and reaction products exhibiting a lower boiling point than said esters;
distilling from said first reaction mixture under subatmospheric pressure substantially all benzoic acid esters and substantially none of any phthalic acid esters to form a first reaction residue;
the method for conducting said intermediate reaction comprises:
forming an intermediate reaction mixture comprising
(1) a mixture of carboxylic acids consisting essentially of benzoic acid and at least trace amounts of at least one compound selected from the group consisting of the three isomeric phthalic acids and phthalic anhydride;
(2) at least one alcohol selected from the group consisting of monohydric alcohols containing from 6 to 12 carbon atoms and dihydric alcohols containing from 2 to 8 carbon atoms, wherein the total number of moles of all alcohol-bonded hydroxyl groups is equivalent to at least 1.02 times the number of moles of all carboxylic acids and carboxylic acid anhydrides present in said intermediate reaction mixture;
(3) at least a portion of a residue selected from the group consisting of said first reaction residue and the residue from a preceding intermediate reaction;
heating said intermediate reaction mixture sufficiently to remove substantially all of the water generated as a by-product of the resultant reaction while maintaining a
stoichiometric excess of said alcohol in said intermediate reaction mixture, and continuing said heating until substantially all of the carboxylic acids in said intermediate reaction mixture have reacted;
distilling from said intermediate reaction mixture under subatmospheric pressure substantially all unreacted alcohol and reaction products exhibiting a lower boiling point than said esters;
distilling from said intermediate reaction mixture under subatmospheric pressure substantially all benzoic acid esters and substantially none of any phthalic acid esters to form an intermediate reaction residue; and
the method for conducting said final reaction comprises
forming a final reaction mixture comprising
(1) a mixture of carboxylic acids consisting ess
Luxem Franz Josef
McConnell Wesley Wayne
Stanhope Bruce Edward
McKane Joseph K.
Murray Joseph
Spector Robert
Velsicol Chemical Corporation
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