= Phosphorus tribromide
<Phosphorus tribromide> is $\mathrm{PBr_3}$, 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.
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