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I am trying to find out how $\ce{POBr3}$ reacts with $\ce{-COOH}$ group (benzoic acid to be specific). I found that it performs an elimination reaction with alcohols to give alkenes. I have also seen that keto group was converted to $\ce{-Cl}$ using $\ce{POCl3}.$

But I don't think that $\ce{POCl3}$ will eliminate $\ce{-COOH}$ group — that seems like nonsense. Eliminating using $\ce{-OH}$ from $\ce{-COOH}$ would give me a carbon with five bonds. Exchanging the $\ce{=O}$ from $\ce{-COOH}$ would give me something I have never seen and it doesn't even look good…

So I am sort of out of ideas. I'd appreciate any hint what to do.

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    $\begingroup$ I've not see this done this way but my gut feeling is that you would get the acyl bromide. Anyone got any references or other thoughts? $\endgroup$
    – Waylander
    Oct 20 '19 at 22:32
  • $\begingroup$ I don't know about POBr3 but PBr3 has been used to brominate carboxylic acid to produce alpha-bromocarboxylic acid. This is the base for HVZ reaction. I cannot say if this happens for benzoic acid since it can be brominated using Br2/FeBr3. $\endgroup$
    – Nilay Ghosh
    Oct 21 '19 at 5:33
  • $\begingroup$ SciFinder does not uncover any published reactions that use a carboxylic acid and $\ce{POBr3}$ or $\ce{POCl3}$. Thus, it seems we will have to speculate about the answer. $\endgroup$
    – Jan
    Oct 31 '19 at 4:43
  • $\begingroup$ @Waylander you are correct :) thank you for your help $\endgroup$
    – Geek
    Nov 2 '19 at 21:55
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So the correct answer (according to my teacher) is that R-COBr is generated from the reaction of POBr3 with R-COOH (benzoic acid)

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Well actually @Waylander and @Nilay Ghosh are both correct in a way. To convert carboxylic acids into acyl chloride, $\ce{PCl5}$ is often used as a chlorinating agent , which yields an acyl chloride and $\ce{POCl3}$ (and $\ce{HCl}$) as a by-product.

This $\ce{POBr3}$ (or likewise $\ce{POCl3}$) is essentially a dehydrating agent and is thus used to dehydrate alcohols to alkenes(1)

POBr3 promoted dehydration via E2 elimination

or brominate(2) (in some cases) to form alkyl bromides. Deoxy bromination using POBr3

But this dehydrating capability might also be used on carboxylic acids to convert them into their anhydrides just like $\ce{P4O10}$, though there's no such special mention of the viability of the reaction progress.

But since you're really asking, I think conversion of the acid into its anhydride could be a possible outcome in presence of a dehydrating agent $\ce{POBr3}$.

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