Sauron
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Pyridine Dicarboxylic Acids
These are interesting compounds often bearing jawbreaker trivial names that profs are fond of sprinkling into exam questions.
2,3 pyridinedicarboxylic acid, quinolinic acid, corresponds to phthalic acid and can be prepared by KMnO4 oxidation of quinoline, or of 2,3-lutidine
(dimethylpyridine).
3,4-pyridinecarboxylic acid, cinchomeronic acid, is prepared by similar oxidation of isoquinoline.
2,5-pyridinedicarboxylic acid, isocinchomeronic acid, is according to Merck Index prepared by liquid phase oxidation of 5-ethyl-2-picoline, aka
aldehydildine, MEP, a synthetic alkylpyridine produced from paraldehyde and ammonia at high pressure and temperature. However, on looking up the
Merck references, we find that it is only an unisolable intermediate, since under the reaction conditions the 2-position decarboxylates, leaving
nicotinic acid.
However, Org.Syn. teacher that the potassium salts of aromatic dicarboxylic acids thermally rearrange at 400-450 C; thus isophthalic and phthalic acid
potassium salts rearrange to terephthalic salt. This is a general reaction and indeed, 2.3-pyridinedicarboxylates and 3,4- K-salts as well rearrange
to 2,5-pyridinedicarboxylates.
There is a closely related reaction. Pyridine monocarboxylates, such as notably nicotinic acid, undergo disproportionation to an equilibrium mixture
of the hydrocarbon (pyridine) and the 2,5-dicarboxylate potassium salt. Yields run 30-50%.
A review is available in German. While awaiting that paper from Angew.Chem., here is the Org.Syn. monograph on this reaction applied to napthalene
dicarboxylate.
Attachment: CV5P0813.pdf (144kB)
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not_important
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This is the Henkel reaction, you omitted the cadmium salt catalyst typically used. It has been used commercially for making terephthalic acid,
before controlled production of 1,4-xylene and its oxidation took over.
When performed on dicarboxylic acid the reaction is driven by the lower solubility of the "para" form of the acid (as salt), which crystallises out
under the reaction conditions. The monocarboxylate combines that with the loss of the hydrocarbon to form dicarboxylates.
I've wondered that if performed on a mixture of predominately the free dicarboxylic acid with some of the salt, the ortho di-acid would be removed as
the anhydride and water vapour with the result of forcing the reaction to that product.
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Sauron
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Naughty of me to have delayed posting the mini-review by Raecke, which does indeed talk about the cadmium salt catalyst. Also I am sure the patents as
well.
Anyway here it is. It appeared a few years prior to the launch of the International Edition of Angew.Chem. so as far as I know there is no English
version.
PS The Org.Syn. monograph did include the cadmium chloride catalyst.
It is s shame that this procedure required temperatures 75-100 C above the top end of my SS 316 Parr autoclave. The authors used Hastalloy. Those are
painfully expensive.
[Edited on 29-12-2008 by Sauron]
Attachment: ditricarbsaure.pdf (559kB)
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Sauron
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But no need to despair.
Although the Merck Index refs from J.Chem.Soc. and Ind.Eng.Chem. turned out to be pretty useless, and I have not yet obtained the one from Monatsch.,
I did dig out my compilation of Bull.Chem.Soc.Japan and hit paydirt.
5-ethyl-2-methylpyridine is oxidized with cupric nitrate and O2. Yields are 85% or better for isocinchomeric acid (2,5-pyridinedicarboxylic acid)
which is produced as its copper salt and liberated with HCl.
[Edited on 29-12-2008 by Sauron]
Attachment: Pages from 34.pdf (585kB)
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