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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?

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  • $\begingroup$ What do you think it is? Extrapolate from the PCl5/SOCl2 reactions and give us your view. $\endgroup$
    – Waylander
    Aug 1, 2020 at 8:34
  • $\begingroup$ @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. $\endgroup$
    – idontknow700
    Aug 1, 2020 at 8:58
  • $\begingroup$ The mechanism is same for $\ce{PCl3}$ as it is with $\ce{PCl5}$ ans $\ce{SOCl2}$. Only the side products are different. $\endgroup$
    – Nilay Ghosh
    Aug 1, 2020 at 9:03
  • $\begingroup$ Edit your question to include this. It shows you've made an effort and the question will not get closed as homework $\endgroup$
    – Waylander
    Aug 1, 2020 at 9:03
  • $\begingroup$ @NilayGhosh if the mechanism is the the same so why are are using 3 moles in this case? $\endgroup$
    – idontknow700
    Aug 1, 2020 at 9:08

1 Answer 1

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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:

enter image description here

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)
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  • $\begingroup$ The structure of H3PO3 has a hydrogen bonded to the central phosphorus. How can we account for this hydride transfer? It is actually the deviation from the standard reaction of PCl5 or SOCl5 with alcohols or carboxylic acids. $\endgroup$
    – Solid - NMR
    Aug 2, 2020 at 17:03
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    $\begingroup$ @ADITYAROYCHOWDHURY I am not sure if I am interpreting your comment correctly. 1. Are you sure there is a hydrogen bond in H3PO3, IIRC, hydrogen bond only occurs with oxygen, nitrogen and fluorine. 2. H3PO3 is the minor tautomer. The major one is actually HPO(OH)2 called phosphonic acid and this form exist almost all the time. As you can see, this acid is subject to tautomerization involving hydrogen transfer between the oxygen and phosphorus atoms. My organic chem is rusty so if you are having doubts with my answer, feel free to post a new answer sharing your idea. $\endgroup$
    – Nilay Ghosh
    Aug 3, 2020 at 4:10
  • $\begingroup$ @ Thank you I was asking about the tautomerism part. Not hydrogen bonding. $\endgroup$
    – Solid - NMR
    Aug 3, 2020 at 10:20

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