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With the help of HVZ reaction, we can get alpha-chloro or alpha-bromo carboxylic acid. enter image description here

But can we make alpha-iodocarboxylic acid with the help of HVZ reaction? I could not find any reference of HVZ reaction forming alpha-iodocarboxylic acid.

Though $\ce{PI3}$ is much less stable than $\ce{PBr3}$ and $\ce{PCl3}$, it has a decent shelf life of few months and is actually widely available reagent. But still, $\ce{PBr3}$ and $\ce{PCl3}$ is seemed to be used in HVZ reaction. Why not $\ce{PI3}$? Is the reaction thermodynamically unfavourable? Is so, what is the reason?

If $\ce{PI3}$ is used instead of $\ce{PBr3}$ or $\ce{PCl3}$, will the reaction proceed to form alpha-iodocarboxylic acid? If so, why?

Update (I was visiting some of my older questions and encountered this question and thought of providing a little more information)

I found this information from a handbook:

Carboxylic acid can react with iodine without any intermediate enolate ion to produce alpha-iodocarboxylic acid. alpha-iodocarboxylic acid chlorides can be produced when hexanoic acid reacts with iodine and thionyl chloride at 85°C to give 80% yield of 2-iodohexanoyl chloride. Similarly, butanoic acid reacts with chlorosulfonic acid and iodine to give 94% of 2-iodobutanoyl chloride. These examples are nothing more than iodine analog of Hell-Volhard-Zelinsky reaction.

  1. Why are these reaction considered as iodine analog of HVZ reaction? Doesn't HVZ reactions strictly uses phosphorus halides?
  2. If these examples are iodine analog of HVZ reaction, why are they producing 2-iodoacyl chloride? They should be producing 2-iodocarboxylic acid.
  3. If it is producing 2-iodoacyl chloride, how to get rid of the chloride atom to get 2-iodocarboxylic acid?

In this site, it is mentioned that only iodine cannot simply iodinate carboxyl acid due to thermodynamic reasons and alternative and noble ways were discovered to overcome this problem. The reactions mentioned were with carboxylic acid dianions, acyl chlorides with iodine in thionyl chloride and iodination with iodine in the presence of cupric salts. There is no mention of phosphorus triiodide where I later found that with phosphorus diiodide, $\ce{P2I4}$, it can produce 1-iodopropanoic acid and not 2-iodopropanoic acid). Is it considered HVZ reaction?

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    $\begingroup$ Hello @SafdarFaisal. Thank you for placing the bounty. If you consider the main question, it is "Can we use PI3 in HVZ reaction instead of PCl3 and PBr3 to yield alpha-iodocarboxylic acid?" The other questions are side question resulting from the update I did when I was doing self-research for the question. They are all related questions which are linked to my main question. (Answerer may opt to not answer all questions). $\endgroup$
    – Nilay Ghosh
    Apr 19, 2021 at 5:03
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    $\begingroup$ Ketones differ from their reactivity from carboxylic acids and the HVZ is about the later substrate. This in mind, Denis and Krief published 1981TetLet22 a paper already stating in the abstract «$\ce{PI3}$ and $\ce{P2I4}$ clearly reduce $\alpha$-bromo and $\alpha$-iodo ketones», testing on multiple substrates (in DCM, 25 C, 1-8h, 51-91% yield), stating «The methods proposed can be advantageously compared to the ones already described.» Their substrates were obtained via epoxide opening (TMS-iodide) and oxdidation ($\ce{CrO3/H+}$). $\endgroup$
    – Buttonwood
    Apr 21, 2021 at 10:24
  • $\begingroup$ I doubt if there is enough research done on $PX_3$ or $PX_5$ reagents. Your question is still unanswered properly inspite of being heavily upvoted (my doubt too +1) $\endgroup$
    – D13G
    Sep 16 at 10:49

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