Phosphorus tribromide is a clear colourless liquid. It is a covalent compound of phosphorus and bromine with the formula PBr₃. It fumes in air due to decomposition by water and it has a penetrating odour. It is widely used in the laboratory for the conversion of alcohols to alkyl bromides.

Chemical properties

Phosphorus tribromide, like PCl₃ and PF₃ , has both properties of a Lewis base and a Lewis acid. For example, with a Lewis acid such as boron tribromide it can form stable 1:1 adducts such as Br₃B⁻−⁺PBr₃. At the same time it can react as an electrophile or Lewis acid in many of its reactions.

The most important reaction of PBr₃ is with alcohols, where it replaces an OH group with a bromine atom to produce an alkyl bromide. Note that all three bromines can be transferred.

PBr₃ + 3 R-OH → 3 RBr + HP(O)(OH)₂

The mechanism (shown for a primary alcohol) involves initial activation of the alcohol oxygen by the electrophilic phosphorus (to form a good leaving group), followed by an S_N2 substitution at the alcohol carbon.

Because of the S_N2 substitution step, the reaction generally works well for primary and secondary alcohols, but fails for tertiary alcohols. If the reacting carbon centre is chiral, the reaction usually occurs with inversion of configuration at the alcohol carbon, as is usual with an S_N2 reaction.

In a similar reaction, PBr₃ also converts carboxylic acids to acyl bromides.

PBr₃ + 3 RCOOH → 3 RCOBr + HP(O)(OH)₂

PBr₃ is a reasonably strong reducing agent, and the oxidation of PBr₃ with oxygen gas is more vigorous than seen with PCl₃. It gives an explosive reaction that forms P₂O₅ and Br₂.

Preparation

PBr₃ is made by the reaction of elemental phosphorus with bromine, using PBr₃ itself as the solvent (white phosphorus is soluble in PBr₃). An excess of phosphorus is used in order to prevent formation of PBr₅.

P₄ + 6 Br₂ → 4 PBr₃

Uses

The main use for phosphorus tribromide is for conversion of primary or secondary [[alcohol]s to alkyl bromides^[7], as described above. PBr₃ usually gives higher yields than hydrobromic acid, and it avoids problems of carbocation rearrangement- for example even neopentyl bromide can be made from the alcohol in 60% yield^[6].

Another use for PBr₃ is as a catalyst for the α-bromination of carboxylic acids. Although acyl bromides are rarely made in comparison with acyl chlorides, they are used as intermediates in the Hell-Volhard-Zelinsky reaction ^[8]. Initially PBr₃ reacts with the carboxylic acid to form the acyl bromide, which is more reactive towards bromination. The overall process can be represented as

On a commercial scale, phosphorus tribromide is used in the manufacture of pharmaceuticals such as alprazolam, methohexital and fenoprofen .

Precautions

Highly corrosive, toxic. In reactions that produce phosphorous acid as a by-product, when working up by distillation be aware that this can decompose above about 160 °C to give phosphine which can cause explosions in contact with air.^[7]

Suppliers/Manufacturers

• Strem
• Fisher
• VWR
• Alfa
• Aldrich

References

1. N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997.
2. Handbook of Chemistry and Physics, 71st edition, CRC Press, Ann Arbor, Michigan, 1990.
3. J. March, Advanced Organic Chemistry, 4th ed., p. 723, Wiley, New York, 1992.
4. The Merck Index, 7th edition, Merck & Co, Rahway, New Jersey, USA, 1960.
5. R. R. Holmes, Journal of Inorganic and Nuclear Chemistry 12, 266-275 (1960).
6. L. G. Wade, Jr., Organic Chemistry, 6th ed., p. 477, Pearson/Prentice Hall, Upper Saddle River, New Jersey, USA, 2005.
7. George C. Harrison, H. Diehl, in Organic Syntheses Collective Volume 3, p 370, Wiley, New York, 1955.
8. L. G. Wade, Jr., Organic Chemistry, 6th ed., p. 1051, Pearson/Prentice Hall, Upper Saddle River, New Jersey, USA, 2005.