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 alcohols to alkyl bromides.

Preparation

PBr₃ is prepared by treating

red phosphorus Elemental phosphorus can exist in several allotropes, the most common of which are white and red solids. Solid violet and black allotropes are also known. Gaseous phosphorus exists as diphosphorus and atomic phosphorus. White phosphorus White ...

with

bromine Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table (halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simila ...

. An excess of phosphorus is used in order to prevent formation of PBr₅: :2 P + 3 Br₂ → 2 PBr₃ Because the reaction is highly exothermic, it is often conducted in the presence of a diluent such as PBr₃.

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 sp ...

. For example, with a Lewis acid such as

boron tribromide Boron tribromide, BBr3, is a colorless, fuming liquid compound containing boron and bromine. Commercial samples usually are amber to red/brown, due to weak bromine contamination. It is decomposed by water and alcohols. Chemical properties Boron ...

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 charged, have an atom that carrie ...

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

amine In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituen ...

s. The most 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 carbon atom. The term ''alcohol'' originally referred to the primary alcohol ethanol (ethyl alcohol), which is ...

, where it replaces an OH group with a bromine atom to produce an

alkyl bromide Organobromine compounds, also called organobromides, are organic compounds that contain carbon Chemical bond, bonded to bromine. The most pervasive is the naturally produced bromomethane. One prominent application of synthetic organobromine comp ...

. All three bromides can be transferred. :PBr₃ + 3 ROH → 3 RBr + HP(O)(OH)₂ The mechanism (shown for a primary alcohol) involves formation of a phosphorus 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 (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable from ...

, the reaction usually occurs with inversion of configuration at the alcohol

alpha carbon In the nomenclature of organic chemistry, a locant is a term to indicate the position of a functional group or substituent within a molecule. Numeric locants The International Union of Pure and Applied Chemistry (IUPAC) recommends the us ...

, 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 R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...

s to

acyl bromide In organic chemistry, an acyl halide (also known as an acid halide) is a chemical compound derived from an oxoacid by replacing a hydroxyl group () with a halide group (, where X is a halogen). If the acid is a carboxylic acid (), the compoun ...

s. :PBr₃ + 3 RCOOH → 3 RCOBr + HP(O)(OH)₂

Applications

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

s, as described above. PBr₃ usually gives higher yields than

hydrobromic acid Hydrobromic acid is a strong acid formed by dissolving the diatomic molecule hydrogen bromide (HBr) in water. "Constant boiling" hydrobromic acid is an aqueous solution that distills at and contains 47.6% HBr by mass, which is 8.77 mol/L. ...

, and it avoids problems of

carbocation A carbocation is an ion with a positively charged carbon atom. Among the simplest examples are the methenium , methanium and vinyl cations. Occasionally, carbocations that bear more than one positively charged carbon atom are also encountere ...

rearrangement- for example even

neopentyl Pentyl is a five-carbon alkyl group or substituent with chemical formula -C5H11. It is the substituent form of the alkane pentane. In older literature, the common non-systematic name amyl was often used for the pentyl group. Conversely, the name ...

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 chlorides, 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 A medication (also called medicament, medicine, pharmaceutical drug, 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 medical field and re ...

s such as

alprazolam Alprazolam, sold under the brand name Xanax, among others, is a fast-acting, potent tranquilizer of medium duration in the triazolobenzodiazepine (TBZD) class, which are benzodiazepines (BZDs) fused with a triazole ring. It is most commonly u ...

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 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 used to treat postoperative pain. Fenoprofen i ...

. 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 PhostrEx is a fire suppression agent developed for use in aviation applications to replace halon, a greenhouse gas (GHG). It was developed by Eclipse Aviation for use aboard their Eclipse 500 very light jets as an engine fire suppression system, ...

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 (singular)) is the compound described by the formula H3PO3. This acid is diprotic (readily ionizes two protons), not triprotic as might be suggested by this formula. Phosphorous acid is an intermediate in th ...

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