When tribromophosphine is used to activate an OH to enable its substitution, the leaving group is listed by PubChem as "phosphorodibromidous acid" and doesn't seem to really exist from a quick Google search. Seeing as using SOCl2 for the same purposes results in the evolution of SO2 and HCl, I assume that this compound also breaks down into something more stable like a salt or a gas, but I can't figure out what. 
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2$\begingroup$ It reacts further, I think. $\endgroup$– MithoronCommented May 29 at 16:41
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$\begingroup$ I also think that it reacts with the remaining alcohol $\endgroup$– WaylanderCommented May 29 at 17:17
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$\begingroup$ The three bromine atoms from PBr3 react with 3 alcohol molecules and produce three times one alkyl bromide, plus one H3PO3. $\endgroup$– MauriceCommented May 29 at 19:51
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2 Answers
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Primary and secondary alcohols reach with phosphorus tribromide to yield alkyl bromide
$$\ce{3ROH (1^\circ or 2^\circ) + PBr3 -> 3RBr + H3PO3}$$
The reaction is simply an SN2 reaction and doesn't involve any carbocation formation. The reaction mechanism can be divided into two parts:
- Conversion of the $\ce{OH-}$ group to a better leaving group
First $\ce{PBr3}$ will target the $\ce{-OH}$ group displacing a bromide ion and forming a protonated alkyl dibromophosphite:
- Nucleophilic attack by $\ce{Br-}$
The bromide ion will displace the $\ce{-HOPBr2}$ part of the protonated species which is a good leaving group due to electronegative atoms bonded to the phosphorus:
Finally you have $\ce{HOPBr2}$ as an intermediate species which reacts with remaining 2 moles of alcohol, so the net result is conversion of 3 mol of alcohol to one mol of alkyl bromide. Since the reaction is a simple substitution, $\ce{PBr3}$ is a preferred reagent (than $\ce{HBr}$) to transform alcohol to alkyl bromide
But the reaction of alcohol and $\ce{PCl3}$ moves quite differently. See: Reaction of alcohols with PCl5 and PCl3
Reference:
- Organic Chemistry, 9th Ed by T. W. Graham Solomons and Craig B. Fryhle, John Wiley & Sons, 2008
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Your question is:
What does phosphorodibromidous acid do in solution?
The answer is it might not exist in this reaction procedure. According to detailed experimental procedure given in Ref.1, the following steps are suggested for bromination of optically active organic alcohols: $$\ce{R-OH + PBr3 -> ROPBr2 + HBr} \tag1$$ $$\ce{R-OH + ROPBr2 -> (RO)2PBr + HBr} \tag2$$ $$\ce{R-OH + (RO)2PBr -> (RO)3P + HBr} \tag3$$ $$\ce{HBr + (RO)3P ->[fast] (RO)2PHO + RBr} \tag4$$ $$\ce{HBr + (RO)2PHO ->[slow] (RO)PH(OH)O + RBr} \tag5$$ $$\ce{HBr + (RO)PH(OH)O ->[slow] H3PO3 + RBr} \tag6$$
Accordingly,
The general procedure involves treatment of a solution of the chiral alcohol and twofold molar excess of pyridine in ether at $\pu{-25 ^\circ C}$ with phosphorus tribromide over a $1$-$\pu{h}$ period followed by stirring for $\pu{1-2 days}$ at $\pu{4 ^\circ C}$ to complete cleavage of the intermediate phosphite ester (Ref.1).
Therefore, it is safe to assume that phosphorodibromidous acid may not exist in solution after the reaction is over. If any remains (due to use of excess $\ce{PBr3}$ in reaction, all including excess $\ce{PBr3}$ will be decomposed to $\ce{H3PO3}$ during the workup procedure.
References:
- Robert O. Hutchins, Divakar Masilamani, and Cynthia A. Maryanoff, "Convenient synthesis of labile optically active secondary alkyl bromides from chiral alcohols," J. Org. Chem. 1976, 41(6), 1071–1073 (https://doi.org/10.1021/jo00868a034)
answered May 30 at 22:17