Sciencemadness Discussion Board

Elemental Chloorination Catalyzed by Quaternary Amine

Sauron - 7-3-2010 at 00:22

I ran across a US patent from mid 1970s assigned to Dow, which teaches the use of various pyridines, bur especially tetraalkylammonium halides as catalyst for the chlorination of acyl chlorides such as acetyl chloride to more highly chlorinated compounds such as trichloroacetyl chloride.

Patent is attached

A Russian paper cited therein reported that pyridine accelerated such chlorination.

The patent claims that 3-alkylpyridine, 5-alkylpyridines and especialy 3,5-dialkylpyridines were more effective than pyridine itself, and tetraalkylammonium chlorides were most effective.

My supposition is that the amine increases the solubility of the Cle in the neat acyl chloride. If so I wonder at the scope of this catalysis.Might it extend to alpha hydrogens of acids, esters, amides?

Attachment: US3751461.pdf (180kB)
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[Edited on 8-3-2010 by Sauron]

not_important - 8-3-2010 at 00:53

Well, acetyl halides and esters are somewhat reactive to alpha halogenation on their own, free acids not so much; true? No mechanism suggested, I'm not seeing how the substances might overcome the problems with having the -OH. Unusubstituted amides would be xpected to have N-hologenation as a competing reaction.

I wish they'd included data on the uncatalysed rates as well, to get a better idea on how catalytic this is.

Sauron - 8-3-2010 at 04:58

In another US patent to Hoechst (as assignee) in 1997, GAC is the catalyst for chlorination of acyl chlorides and aldehydes to TCAC. Again Cl2 is used and no UV employed.

Thus acetyl chloride or acetaldehyde are chlorinated to trichloroacetyl chloride.

The GAC provides a vastly increased reaction surface.

Yes we know that acids chlorinate more slowly than do acyl halides. But, here are two catalysts, one homogeneous PTC and the other heterogeneous, that produce higher reaction rates with acyl chlorides and aldehydes.

In the Russian journal pyridine hydrochloride greatly increased chlorination rate. In the patent half life well defined, dropped from 150 minutes with pyridine to 100 minutes with Pr4N.HCl

So even if the rate increase for pyridine viz. no catalyst is assumed small, the rate increase with Pr4N.Hcl is highly significant.

Attachment: US5659078.pdf (102kB)
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[Edited on 8-3-2010 by Sauron]