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Why do we need PBr to help enolize a carboxylic acid. My textbook doesn't give a reason for this at all. Does it have to do something with the acidity of carboxylic acid. Thanks.

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Think about the pKa of the carboxylic acid protons compared to the pKa of the enolisable proton (hint: the acid is more acidic).

Once the carboxylic acid is turned into the carboxylate, enolisation is not possible, as the carbonyl cannot simultaneously conjugate with the carbon and the oxygen.

The use of phosphorus tribromide is known as the Hell-Vollhard-Zelinski reaction, it allows carboxylic acids to be directly brominated at the alpha position without initial protection of the carboxylic acid via in situ formation of the acyl bromide which hydrolyses upon workup.

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  • $\begingroup$ Second paragraph isn't ok, there's no problem with conjugation. You'd need to get dianion first, though. $\endgroup$ – Mithoron Apr 23 '17 at 19:20
  • $\begingroup$ They don't form a linearly conjugated system. At any one time the carbonyl can only conjugate with the oxygen or the carbon, not both. I'm not saying its impossible, the carbon just becomes significantly less acidic and hence significantly less enolisable (hence why nobody ever really uses a carboxylic acid to enolise). $\endgroup$ – NotEvans. Apr 23 '17 at 19:45
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-OH group in carboxylic acids have a much better resonance than an acid chloride due to difference in the size of halogen atoms vs carbon; this difference even increases with Br.
This lack of resonance from LP of Br onto carbonyl group in acid chloride makes Enolization possible, which is very difficult to be in -COOH.

Using PBr3 to convert an RCOOH in RCOBr brings in Enolization, which can give Substitution with Br-Br.

Mechanism: HVZ

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