Sauron
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Preparation of Acyl Chlorides: TCT Redux
The well discussed procedure for facile synthesis of acyl chlorides from carboxylic acids (Tet Lett Vaol 79 3037-3040) is one that works well in many
cases but unfortunately the general procedure recommended by the authors does not lend itself to serious preparative work. That procedure did not
elaborate the isolation of the acyl chlorides but rather, various derivatives (esters, amides, etc.) prepared in situ.
The general procedure for the reaction is to dissolve 1 mol TCT in acetone along with 1 mol TEA and add 1 mol of monocarboxylic acid (or 500 mmol
dicarboxylic acid) in acetone. Stir 3 hours at room temperature or till no more TCT remains in solution. The instructions then call for stripping off
the acetone and extracting the remants (TEA, dichlorocyanuric acid, and any unreacted acid, plus acyl chloride) with CCl4.
Acetone boils at 56 C. Acetyl chloride, if one is chlorinating AcOH, boils at 51 C so will come over with the acetone. Duh?
If oxalic acid is substrate oxalyl chloride boils at 62 C. Still way too close for a clean seperation.
In the case of acetic acid, I think a higher boiling ketone would solve the problem, then the acetyl chloride could be stripped off rather than the
solvent.
In the oxalic acid instance, a polar solvent is required. Acetonitrile solvates TCT almost as well as acetone and boils 5 degrees lower at 51 C,
allowing an 11-12 C spread between it and oxalyl chloride. Not as wide a delta as I would prefer but better than a mere 6 C. THF solvates TCT very
well and is polar enough for oxalic acid. There's a 20 C difference in bp. p-Dioxane, same qualities and a 40 C spread. But costlier.
Any suggestions? I think the procedure needs to be adjusted for the particular acyl chloride product.
I am in need of better solubility information for anhydrous oxalic acid (not the dihydrate.)
Incidentally, for those who want to dry their own, oxalic acid dihydrate can be dried by azeotropic distillation of the water from benzene or toluene.
Oxalic acid is insoluble in these hydrocarbons. Using a reflux condenser and a Dean-Stark trap, the theoretical amount of water is readily removed and
when no more water comes over, just filter off the anhydrous acid on a Buchner frit - it is only very slowly hygroscopic.
Oxalic acid is slightly soluble in ether. TCT is reasonably soluble in ether, so maybe this reaction can run in Et2O which can then be stripped from
the oxalyl chloride, then the oxalyl chloride distilled off leaving TEA, any unreacted oxalic acid and lots of dichlorocyanuric acid. The delta in bp
between Et2) (mid 30s I think) and oxalyl chloride (62 C) is significant while TEA boils at 81 C.
[Edited on 6-9-2008 by Sauron]
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Klute
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I know that anhydrous oxalic acid is very soluble in alcohols like Meoh, IPA, EtOH, etc, somewhat soluble in ether (~2g/100mL). I should think it has
appreciable solubility in acetonitrile.
You might find further info on the solubilities in this article:
Oxalic Acid
Hiroyuki Sawada and Toru Murakami
Kirk‑Othmer Encyclopedia of Chemical Technology
DOI: 10.1002/0471238961.1524011219012301.a01
Link
Abstract:
The properties of anhydrous oxalic acid and oxalic acid dihydrate are discussed, tabulating their physical and thermochemical constants. The reactions
of oxalic acid are also included.
Five manufacturing processes which are employed for commercial production are illustrated: oxidation of carbohydrates, ethylene glycol process,
propylene process, dialkyl oxalate process, and sodium formate process. The newest of the five, the dialkyl oxalate process (from UBE) is discussed in
depth. Advances are covered, including a new synthetic route from CO2.
Economic aspects of oxalic acid are described, outlining the regional features in the world market. Shipment, storage, and handling are included,
covering disposal methods. Health and safety factors and toxicology are also presented.
Uses of oxalic acid are described, including separation/recovery of rare-earth elements, metal treatment, bleaching agents, textile treatment, leather
tanning, marble polishing, millet jelly production, etc. Derivatives, such as salts/complexes and organic derivatives, are also presented. Vol. 17,
pp. 882–902, 91 refs to December 1993.
Keywords: Oxalic acid; Dihydrate; Ethylene glycol process; Carbohydrates; Propylene; Diallyl oxalate; Metal treatments; Rare earths; Separations;
Bleaching agents; Textile treatments; Salts; Complexes; Organic derivatives
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Sauron
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I have two editions of Kirk Othmer. I will take a look. However Ullmann's and Merck Index were very unrewarding on this point. A try at googling did
not help.
Acetonitrile is only 11 C away from oxalyl chloride in bp.
The choices are Et2O, THF, or p-Dioxane. The further it is from 62 C the happier I will be.
Alcohols will not do as they will react with TCT, as well as with oxalyl chloride.
MEK (methyl ethyl ketone) boils at 80 C and ought to solvate both oxalic acid and TCT about as well as acetone. It is a lot more available and cheaper
than THF (bp 81 C) Diethyl ketone boils just over 100 C and so can replace the toxic dioxane if a 40 degree delta in bp's makes for a more
convenient and clean seperation. I will have to look up the price. Acetophenone is another possibility as is cyclohexanone.
[Edited on 7-9-2008 by Sauron]
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Sauron
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3-pentanone (diethyl ketone) is rather expensive. However 2-pentanone (methyl propyl ketone) is a lot more reasonable and also boils over 100 C.
[If $100 for 2.5 liters is deemed still too costly, diethyl ether can be prepared from propionic acid by slowly percolating its vapors over manganous
oxide on pumice at 320-340 C according to Vogel, or same catalyst on clay shards at 440450 C according to Org.Syn. A tube furnace is ideal.
Assymmetrical ketones like methyl n-propyl ketone can be prepared by same method using a mixture of glacial acetic acid and n-butyric acid instead of
propionic acid. In this case a small amount of acetone and 4-heptanone are also produced but are easily removed by fractionation.
This is handy for those who are queer for making their own solvents but I'd rather look for an off the shelf (not necessarily OTC) economical solvent
for oxalic acid and TCT that is inert to the acyl chloride product.
[Edited on 7-9-2008 by Sauron]
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