PBr3 (phosphorus tribromide) converts alcohols into alkyl bromides by replacing the hydroxyl group (OH) with a bromine atom. It is one of the most commonly used reagents in organic chemistry for this exact transformation, working reliably on primary and secondary alcohols. Beyond alcohol conversion, PBr3 also plays a key role in brominating carboxylic acids at specific positions.
How PBr3 Converts Alcohols to Alkyl Bromides
The OH group on an alcohol is a poor leaving group, meaning it doesn’t detach easily during a reaction. PBr3 solves this problem by activating the oxygen, turning it into something that can be displaced. The driving force behind the reaction is the formation of a strong phosphorus-oxygen bond, which makes the overall process energetically favorable.
The reaction proceeds through two successive SN2-like steps. First, the alcohol attacks the phosphorus atom, forming an intermediate where the oxygen is now bonded to phosphorus and carries a positive charge. This converts the poor OH leaving group into an excellent one. Then, a bromide ion attacks the carbon in a backside displacement, kicking off the phosphorus-containing group and installing the bromine.
Because of that backside attack, the stereochemistry at the carbon inverts. If the OH was pointing “up” relative to the other groups, the new bromine ends up pointing “down.” This inversion is a hallmark of the SN2 mechanism and is predictable every time, which makes PBr3 useful when you need a specific three-dimensional arrangement in your product.
One molecule of PBr3 can theoretically react with up to three alcohol molecules, since phosphorus has three bromine atoms available. The byproducts are HBr (hydrobromic acid) and phosphorous acid. The presence of HBr in the reaction mixture can occasionally cause complications: with secondary alcohols, carbocation intermediates may form transiently, and these can rearrange. If rearrangement occurs, you get isomeric products mixed in with the one you wanted. For primary alcohols, rearrangement is rarely an issue.
Its Role in the Hell-Volhard-Zelinsky Reaction
PBr3 does more than swap OH for bromine on simple alcohols. In the Hell-Volhard-Zelinsky (HVZ) reaction, it enables bromination at the alpha carbon of a carboxylic acid, the carbon directly next to the carbonyl group. Carboxylic acids are normally resistant to this type of bromination because they don’t form the reactive enol intermediate needed for the bromine to attack.
PBr3 gets around this by first converting the carboxylic acid into an acyl bromide. With the acidic OH proton gone and replaced by bromine, the acyl bromide can enolize much more readily. The enol form then reacts with Br2, placing a bromine on the alpha carbon. Finally, the acyl bromide is hydrolyzed back to a carboxylic acid, yielding an alpha-bromo carboxylic acid as the final product.
Only a catalytic amount of PBr3 is needed for this reaction because it gets regenerated during the process. In some textbook problems, you’ll see just bromine and red phosphorus listed as reagents instead of PBr3 directly. The PBr3 forms in the reaction flask from the combination of those two starting materials: three equivalents of Br2 react with two equivalents of phosphorus to produce two equivalents of PBr3.
Why PBr3 Instead of Other Reagents
You might wonder why not just use HBr to convert an alcohol to an alkyl bromide. HBr works, but it has drawbacks. With secondary and tertiary alcohols, HBr tends to proceed through an SN1 mechanism, which means carbocations form freely and rearrangements are common. The reaction conditions are also harsher, typically requiring strong acid and heat.
PBr3 offers milder conditions and a more controlled SN2 pathway, particularly for primary and secondary alcohols. The stereochemical outcome is predictable (inversion), and rearrangement products are minimized. It pairs naturally with thionyl chloride (SOCl2), which does the analogous job of converting alcohols to alkyl chlorides. Together, these two reagents cover the most common alcohol-to-alkyl-halide conversions in synthetic chemistry.
PBr3 is not well suited for tertiary alcohols. Tertiary carbons are too sterically crowded for the SN2 backside attack to occur efficiently, so other methods are used for those substrates.
Physical Properties and Handling
Phosphorus tribromide is a colorless to slightly yellow fuming liquid with a sharp, penetrating odor. It boils at about 173°C and is significantly denser than water, with a density near 2.85 g/cm³, meaning it sinks if it contacts water. The fuming behavior comes from its reactivity with moisture in the air: PBr3 reacts vigorously with water, releasing HBr gas and producing phosphorous acid. This makes it corrosive and requires careful handling under dry conditions. Reactions using PBr3 are typically run in dry solvents like chloroform or THF to avoid unwanted hydrolysis.