PBr3
Summary
PBr₃ is phosphorus tribromide, a reagent that converts primary and secondary alcohols into alkyl bromides. It works by SN2, so the carbon bearing the OH is inverted if it is a stereocenter, and carbocation rearrangements are generally avoided.
Detail
Meaning: PBr₃ replaces an alcohol OH with Br. The alcohol oxygen attacks phosphorus and displaces bromide, which turns the OH into a good leaving group (an O–PBr₂ group). The freed bromide then attacks the carbon from the back side in an SN2 step. The result is R–Br, with phosphorous acid (H₃PO₃) as a by-product. SOCl₂ does the analogous job for making alkyl chlorides.
Equation: 3 ROH + PBr₃ → 3 RBr + H₃PO₃. R is an alkyl group, and the coefficients are mole ratios, so one PBr₃ can in principle convert three alcohols.
On the MCAT: Expect a synthesis question asking for the reagent that turns an alcohol into an alkyl halide, or the product when a chiral secondary alcohol reacts. Because the step is SN2, the configuration at the stereocenter is inverted (an R alcohol gives an S bromide, if priorities stay the same). Tertiary alcohols generally work poorly because SN2 is blocked by steric hindrance.
Common confusion: Students pair PBr₃ with HBr and expect the same outcome. HBr protonates the alcohol and can proceed through a carbocation (SN1), so it can give rearrangements and racemization for secondary and tertiary alcohols. PBr₃ avoids the carbocation, so the carbon skeleton stays intact and stereochemistry is inverted in a predictable way.
Sources
- Organic Chemistry (Clayden)
- Organic Chemistry (Wade)
- Organic Chemistry (McMurry)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.