Phosphorus tribromide

Phosphorus tribromide is $\mathrm{PB}{\mathrm{r}}_3$, a molecular compound with three phosphorus-bromine bonds and a lone pair on phosphorus. It reacts with alcohol functional groups in transformations that can replace hydroxyl by bromine.

For an appropriate primary or secondary alcohol, substitution can proceed through attack by bromide on an activated carbon-oxygen bond. This connects reagent choice with both the mechanism and the stereochemistry of the product.