PBr3 converts alcohols to which product?

• A. Alkyl chlorides
• B. Alkyl bromides
• C. Alkenes
• D. Alcohols

Answer: (D)

PBr3 substitutes the hydroxyl group on an alcohol with bromide. The alcohol oxygen coordinates to phosphorus, turning the poor leaving group (OH) into a much better leaving group as a phosphorate ester. Then bromide ion attacks the carbon bearing the oxygen in an SN2 step, displacing the phosphite and forming the alkyl bromide. The byproduct is phosphorous acid (H3PO3). This is a classic way to transform alcohols into alkyl bromides, especially for primary and secondary alcohols; it does not produce alkyl chlorides or alkenes under these conditions, and it does not keep the molecule as an alcohol. If the alcohol is chiral, the SN2 displacement also tends to invert the stereochemistry at that carbon.