Method of producing dicarboxylic acids suitable for synthesis of polymers or polyamides

DETAILED DESCRIPTION OF THE INVENTION The method according to said independent claims constitutes a departure from the prior art and its success is surprising in several regards.
The use of the carboxylic acids of a medium carbon chain length as defined in the claims, such as those corresponding to the chain length of the carboxylic acids obtained by oxidative ozonolysis of fatty acids, as recrystallization solvents and preferably also as wash solvents would represent an almost inconceivable measure to those skilled in the art.
Thus, those skilled in the art could not in principle expect recrystallization, regardless of the solvents used, to lead rapidly and unproblematically to products of the desired purity, because the properties of the impurities to be separated are too similar to those of the desired compound to be produced, and furthermore because, according to the previous findings and experience of the those skilled in the art, long-chain fatty acid derivatives tend to become included in the compounds crystallizing out.
Therefore, experience indicated that recrystallization would have to be repeated several times, which would unavoidably lead to great losses of yield.
Thus, the procedure followed in the prior art has been to first remove by distillation the reaction solvent, usually in the form of the medium-length monocarboxylic acid (often pelargonic acid) which is formed in the reaction anyway, to simplify and thus facilitate the separation problem to be solved.
This procedure is all the more self-evident as the pelargonic acid which is generally used as the reaction solvent is a monocarboxylic acid of the fatty acid type which has negative effects even in small amounts in most of the applications of dicarboxylic acids and in particular in synthesis of polymers.
Then the dicarboxylic acid is purified by distillation, whereby residues of short-chain monocarboxylic acids and especially also pelargonic acid can be removed reliably.
It would have to be surprising to those skilled in the art that adding pelargonic acid in particular or a carboxylic acid similar to it, in addition to the pelargonic acid not removed from the reaction end solution of oxidative ozonolysis, could simplify the separation problem and permit successful recrystallization