Reaction of carboxylic acid with phosphorus trichloride

Question

The reaction of carboxylic acid $\ce{RCOOH}$ with $\ce{PCl5}$/ $\ce{SOCl2}$ / $\ce{PCl3}$ yields an acyl chloride $\ce{RCOCl}$. With $\ce{PCl5}$ and $\ce{SOCl2}$ the reaction is quite simple utilizing one mole reactants and yielding products.

But in case of $\ce{PCl3}$ the balanced reaction is as follows: $$\ce{3RCOOH + PCl3 -> 3RCOCl + H3PO3}$$

First I thought the reaction would proceed with $\ce{PCl2−}$ attacking the oxygen from which the oxygen-hydrogen bond cleavage happened but then I realized that it would form product $\ce{POCl}$ which is not a real compound I suppose so I am stuck how would the reaction proceed.

So what is the mechanism for this reaction?

Answer 1

A plausible mechamism of this reaction is discussed in a paper by Xiao et. al¹ where they tried to react benzoic acid with $\ce{PCl3}$ to get acyl chloride. They noted the following mechanism:

Although the detailed mechanism was not clear, a possible mechanism is proposed [...] the first step should be a reaction of the acid with $\ce{PCl3}$, probably via a four-center transition state A, with carbonyl oxygen attacks the electron-deficient $\ce{P(Cl)}$ while $\ce{Cl}$ attacking the electronically deficient carbonyl carbon. Formation of a $\ce{Cl-C}$ bond and $\ce{P-O}$ bond (A’), further $\ce{C-O}$ bond cleavage and hydrogen transfer took place to give the products B and C. Product C then reacts further with the acid to give the corresponding chlorides and $\ce{HP(O)(OH)2}$

This product C will again react with carboxylic acid two times so as to replace all the chlorine atoms and ultimately forming phosphorous acid (actually phosphorous acid technically exist as $\ce{HPO(OH)2}$ called phosphonic acid) So, the final reaction would be three moles of carboxylic acid reacting with $\ce{PCl3}$ to form three moles of acyl chloride and phosphonic acid thus fully utilizing the three chlorine atoms in $\ce{PCl3}$. The major advantage of this reaction is that there is only one side product i.e phosphonic acid which is non-toxic and easily removed by filtration whereas reaction with $\ce{PCl5/SOCl2}$ gives two side products making the reaction more volatile.

Reference

1. Atom-efficient chlorination of benzoic acids with PCl3 generating acyl chlorides, Jing Xiao and Li-Biao Han, Journal of Chemical Research, 2019, Vol. 43(5-6) 205–210 (PDF)