What is the mechanism by which phosphorus pentoxide ($\ce{P4O10}$) and a carboxylic acid form an anhydride? I can't find it anywhere online. Does $\ce{COOH}$ attack twice (I'm assuming it's the nucleophile) because there ends up being two carbonyls in the product? What leaves?

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    $\begingroup$ What would the carboxylic acid attack twice? What bonds do you need to make and break to convert two acids to an anhydride? What is the (theoretically at least) byproduct of anhydride formation? $\endgroup$
    – jerepierre
    Jun 18, 2015 at 12:39
  • $\begingroup$ Would it attack P? Wiki says "when combined with a carboxylic acid, the result is the corresponding anhydride: P4O10 + RCO2H --> P4O9(OH)2 + [RC(O)]2O. I cannot find the mechanism anywhere for this equation. $\endgroup$ Jun 18, 2015 at 13:15
  • $\begingroup$ I highly suspect that this reaction does not have a known mechanism, and $\ce{P4O10 + 2\ RCO2H -> P4O9(OH)2 + [RC(O)]2O}$ is just a formal reaction meant to balance atoms. $\endgroup$ Jun 18, 2015 at 21:37
  • $\begingroup$ David Klein (in O.C. As A Second Language) says that the mechanism is rarely shown in texts so he invites the reader to ponder it (but does not show it.) I cannot figure it out. And can't find it anywhere! Should I email David Klein? Lol $\endgroup$ Jun 19, 2015 at 2:25