# Phosphoryl bromide reaction with benzoic acid

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.

• 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? – Waylander Oct 20 '19 at 22:32
• 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. – Nilay Ghosh Oct 21 '19 at 5:33
• 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. – Jan Oct 31 '19 at 4:43
• @Waylander you are correct :) thank you for your help – Geek Nov 2 '19 at 21:55

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)
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 $$\ce{POBr3}$$.