text: Phosphorus tribromide, like PCl and PF, has both properties of a Lewis base and a Lewis acid.  For example, with a Lewis acid such as boron tribromide it forms stable 1 :1 adducts such as BrB · PBr.  At the same time PBr can react as an electrophile or Lewis acid in many of its reactions, for example with amines.
  An important reaction of PBr is with alcohols, where it replaces an OH group with a bromine atom to produce an alkyl bromide. All three bromides can be transferred.
  Several detailed procedures are available.  In some cases, triphenylphosphine/Br is superior to PBr.
  The mechanism for a primary alcohol involves formation of a phosphorous ester (to form a good leaving group), followed by an S2 substitution.
  Because of the S2 substitution step, the reaction generally works well for primary and secondary alcohols, but fails for tertiary alcohols.  If the reacting carbon centre is chiral, the reaction usually occurs with inversion of configuration at the carbon alpha to the alcohol, as is usual with an S2 reaction.
  In a similar reaction, PBr also converts carboxylic acids to acyl bromides: