Reactions
Phosphorus tribromide, like PCl3 and PF3, 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 Br3B · PBr3. At the same time PBr3 can react as an electrophile or Lewis acid in many of its reactions, for example with amines.
The most important reaction of PBr3 is with alcohols, where it replaces an OH group with a bromine atom to produce an alkyl bromide. Note that all three bromines can be transferred.
- PBr3 + 3 ROH → 3 RBr + HP(O)(OH)2
The mechanism (shown for a primary alcohol) involves initial activation of the alcohol oxygen by the electrophilic phosphorus (to form a good leaving group), followed by an SN2 substitution at the alcohol carbon.
Because of the SN2 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 alcohol alpha carbon, as is usual with an SN2 reaction.
In a similar reaction, PBr3 also converts carboxylic acids to acyl bromides.
- PBr3 + 3 RCOOH → 3 RCOBr + HP(O)(OH)2
PBr3 is a reasonably strong reducing agent, and the oxidation of PBr3 with oxygen gas is more vigorous than seen with PCl3. It gives an explosive reaction that forms P2O5 and Br2.
Read more about this topic: Phosphorus Tribromide
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