# Reaction of carboxylic acid with phosphorus trichloride

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 PCl2− attacking the oxygen from which the oxygen-hydrogen bond cleavage happened but then I realized that it would form product OPCl 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?

• What do you think it is? Extrapolate from the PCl5/SOCl2 reactions and give us your view. Aug 1 '20 at 8:34
• @Waylander , 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 $\ce{OPCl}$ which is not a real compound i suppose so i am stuck how would the reaction proceed. Aug 1 '20 at 8:58
• The mechanism is same for $\ce{PCl3}$ as it is with $\ce{PCl5}$ ans $\ce{SOCl2}$. Only the side products are different. Aug 1 '20 at 9:03
• Edit your question to include this. It shows you've made an effort and the question will not get closed as homework Aug 1 '20 at 9:03
• @NilayGhosh if the mechanism is the the same so why are are using 3 moles in this case? Aug 1 '20 at 9:08

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.