A Convenient Preparation of Carboxylic Acid Anhydrides Using a “Supported” Phosphorus Pentoxide Reagent

References

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• Phosphorus pentoxide on an inert mineral support, supplied by E. Merck as “SICAPENT” with indicator (Cat. No. 543).
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• Earlier studies (vacation projects by Hiss H. Moore and Miss D. A. Kaiser at the University of Natal, Pietermaritzburg) also demonstrated applicability to other aliphatic acids.
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• IR analysis of the isolated anhydrides typically indicated the presence of trace quantities of acid precursors, evidenced by residual carbonyl absorption at ca. 1700 cm−1
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• Reactions conducted in DME (purified, prior to use, by passage through activated alumina) appear to be complicated by the formation, in each case, of a contaminant presumed to be the corresponding methyl ester (ca. 10% conversion) from IR and NHR analysis of isolated crude anhydride.
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• The usual precautions should be observed when handling the highly reactive and finely powdered phosphorus pentoxide reagent.
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• In other reactions, solvent volumes as low as 60 ml were used.
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• Under these conditions, solutions in benzene and DME were maintained at gentle reflux.
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• In some cases DME was substituted for Et20 since the latter solvent appeared to precipitate material from solution.
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