Phosphorus tribromide is a colourless liquid with the formula P Br₃. The liquid fumes in moist air due to

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

and has a penetrating odour. It is used in the laboratory for the conversion of

alcohol Alcohol may refer to: Common uses * Alcohol (chemistry), a class of compounds * Ethanol, one of several alcohols, commonly known as alcohol in everyday life ** Alcohol (drug), intoxicant found in alcoholic beverages ** Alcoholic beverage, an alco ...

s to alkyl bromides.

Preparation

PBr₃ is prepared by treating

red phosphorus Red phosphorus is an Allotropes of phosphorus, allotrope of phosphorus. It is an amorphous polymeric red solid that is stable in air. It can be easily converted from white phosphorus under light or heating. It finds applications as matches and fir ...

with

bromine Bromine is a chemical element; it has chemical symbol, symbol Br and atomic number 35. It is a volatile red-brown liquid at room temperature that evaporates readily to form a similarly coloured vapour. Its properties are intermediate between th ...

. An excess of phosphorus is used in order to prevent formation of PBr₅: :P₄ + 6 Br₂ → 4 PBr₃ Because the reaction is highly exothermic, it is often conducted in the presence of a diluent such as PBr₃. Phosphorus tribromide is also generated in situ from

and bromine.

Reactions

Phosphorus tribromide, like PCl₃ and PF₃, has both properties of a Lewis base and a

Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any ...

. For example, with a Lewis acid such as boron tribromide it forms stable 1 :1 adducts such as Br₃B · PBr₃. At the same time PBr₃ can react as an

electrophile In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively Electric charge, charged, have an ...

or Lewis acid in many of its reactions, for example with

amine In chemistry, amines (, ) are organic compounds that contain carbon-nitrogen bonds. Amines are formed when one or more hydrogen atoms in ammonia are replaced by alkyl or aryl groups. The nitrogen atom in an amine possesses a lone pair of elec ...

s. An important reaction of PBr₃ is with

alcohols In chemistry, an alcohol (), is a type of organic compound that carries at least one hydroxyl () functional group bound to a Saturated and unsaturated compounds, saturated carbon atom. Alcohols range from the simple, like methanol and ethanol ...

, where it replaces an OH group with a bromine atom to produce an alkyl bromide. All three bromides can be transferred. : Several detailed procedures are available. In some cases, triphenylphosphine/Br₂ is superior to PBr₃. The mechanism for a primary alcohol involves formation of a phosphorous ester (to form a good leaving group), followed by an S_N2 substitution. : 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 Chirality () is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek language, Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is dist ...

, the reaction usually occurs with inversion of configuration at the carbon alpha to the alcohol, as is usual with an S_N2 reaction. In a similar reaction, PBr₃ also converts

carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an Substituent, R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl ...

s to acyl bromides: :

Applications

The main use for phosphorus tribromide is for conversion of primary or secondary

s to alkyl bromides, as described above. PBr₃ usually gives higher yields than hydrobromic acid, and it avoids problems of

carbocation Carbocation is a general term for ions with a positively charged carbon atom. In the present-day definition given by the IUPAC, a carbocation is any even-electron cation with significant partial positive charge on a carbon atom. They are further ...

rearrangement- for example even neopentyl bromide can be made from the alcohol in 60% yield. Another use for PBr₃ is as a catalyst for the α-bromination of

s. Although acyl bromides are rarely made in comparison with

acyl chloride In organic chemistry, an acyl chloride (or acid chloride) is an organic compound with the functional group . Their formula is usually written , where R is a side chain. They are reactive derivatives of carboxylic acids (). A specific example o ...

s, they are used as intermediates in Hell-Volhard-Zelinsky halogenation. 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

pharmaceutical Medication (also called medicament, medicine, pharmaceutical drug, medicinal product, medicinal drug or simply drug) is a drug used to diagnose, cure, treat, or prevent disease. Drug therapy ( pharmacotherapy) is an important part of the ...

s such as

alprazolam Alprazolam, sold under the brand name Xanax among others, is a fast-acting, potent tranquilizer of moderate duration within the triazolobenzodiazepine group of chemicals called benzodiazepines. Alprazolam is most commonly prescribed in the ...

methohexital Methohexital or methohexitone (marketed under the brand names Brevital and Brietal) is a drug which is a barbiturate derivative. It is classified as short-acting, and has a rapid onset of action. It is similar in its effects to sodium thiopental ...

and

fenoprofen Fenoprofen, sold under the brand name Nalfon among others, is a nonsteroidal anti-inflammatory drug (NSAID). Fenoprofen calcium is used for symptomatic relief for rheumatoid arthritis, osteoarthritis, and mild to moderate pain. It has also been ...

. It is also a potent

fire suppression agent A fire retardant is a substance that is used to slow down or stop the spread of fire or reduce its intensity. This is commonly accomplished by chemical reactions that reduce the flammability of fuels or delay their combustion. Fire retardants m ...

marketed under the name PhostrEx. Phosphorus tribromide is used for doping in

microelectronics Microelectronics is a subfield of electronics. As the name suggests, microelectronics relates to the study and manufacture (or microfabrication) of very small electronic designs and components. Usually, but not always, this means micrometre ...

Precautions

PBr₃ evolves corrosive HBr, which is toxic, and reacts violently with water and alcohols. :PBr₃ + 3 H₂O → H₃PO₃ + 3 HBr In reactions that produce

phosphorous acid Phosphorous acid (or phosphonic acid) is the Compound (chemistry), compound described by the chemical formula, formula . It is diprotic (readily ionizes two protons), not triprotic as might be suggested by its formula. Phosphorous acid is an in ...

as a by-product, when working up by distillation be aware that this can decompose above about 160 °C to give

phosphine Phosphine (IUPAC name: phosphane) is a colorless, flammable, highly toxic compound with the chemical formula , classed as a pnictogen hydride. Pure phosphine is odorless, but technical grade samples have a highly unpleasant odor like rotting ...

which can cause explosions in contact with air.

Preparation

Reactions

Applications

Precautions

References

Further reading