Alcohol to Alkyl Bromide

Reagents

$\mathrm{PBr}{}_3$

Mechanism: SN2

1. $\mathrm{O}$ from $\mathrm{OH}$ attacks $\mathrm{P}$, which kicks off a $\mathrm{Br}{}^{-}$
2. $\mathrm{Br}{}^{-}$ back side attacks the $\alpha$ carbon, kicking off ${}_{}^{}{}_{}^{-}\mathrm{O}-\mathrm{PBr}{}_2$ and creating $\mathrm{R}-\mathrm{Br}$
3. Steps 1-2 occur 2 more times, resulting in $\mathrm{H}{}_3\mathrm{PO}{}_4$ being produced and 3 total $\mathrm{R}-\mathrm{Br}$