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Alcohol Reactions: Alcohol Bromination Using PBr3

Phosphorus Tribromide (PBr3) in the presence of alcohols reacts to form alkyl bromides:

Alcohol Reactions: Alcohol Bromination Using PBr3 image2.png

This reaction only occurs with primary and secondary alcohols (tertiary alcohols will not react). Additionally, if the alcohol molecule begins with a chiral center on the carbon the alcohol group is attached to, the reaction will invert the stereochemistry (SN2 reaction):

Primary Alcohols

Alcohol Reactions: Alcohol Bromination Using PBr3 image2.png

Secondary Alcohols

Alcohol Reactions: Alcohol Bromination Using PBr3 image5.png

NOT Tertiary Alcohols

Alcohol Reactions: Alcohol Bromination Using PBr3 image4.png

Stereochemistry Inversion

Alcohol Reactions: Alcohol Bromination Using PBr3 image3.png

The reaction mechanism is depicted below:

Alcohol Reactions: Alcohol Bromination Using PBr3 image1.png

In the first step, lone pair electrons from the alcohol group attack phosphorus, causing the phosphorus-bromine bond to break and give the electrons to the bromine, forming a bromide ion.

In the second step, the free, negatively charged bromide ion, attacks the backside of the intermediate structure, causing the alcohol group to leave and the bromine group to take its place in an SN2 reaction. This is also the step where stereochemistry inversion would occur as the bromide ion performs a “backside attack”.

The reagent used for this reaction is PBr3.