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| Names | |
|---|---|
| IUPAC name Phosphorus tribromide | |
| Other names phosphorus(III) bromide, phosphorous bromide, tribromophosphine | |
| Identifiers | |
3D model (JSmol) | |
| ChemSpider |
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| ECHA InfoCard | 100.029.253 |
| EC Number |
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| RTECS number |
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| UNII | |
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| Properties | |
| PBr3 | |
| Molar mass | 270.69 g/mol |
| Appearance | clear, colourless liquid |
| Density | 2.852 g/cm3 |
| Melting point | −41.5 °C (−42.7 °F; 231.7 K) |
| Boiling point | 173.2 °C (343.8 °F; 446.3 K) |
| rapidhydrolysis | |
Refractive index (nD) | 1.697 |
| Viscosity | 0.001302 Pas |
| Structure | |
| trigonal pyramidal | |
| Hazards | |
| GHS labelling: | |
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| Danger | |
| H314,H335 | |
| P260,P261,P264,P271,P280,P301+P330+P331,P303+P361+P353,P304+P340,P305+P351+P338,P310,P312,P321,P363,P403+P233,P405,P501 | |
| NFPA 704 (fire diamond) | |
| Related compounds | |
Otheranions | phosphorus trifluoride phosphorus trichloride phosphorus triiodide |
Othercations | nitrogen tribromide arsenic tribromide antimony tribromide |
Related compounds | phosphorus pentabromide phosphorus oxybromide |
| Supplementary data page | |
| Phosphorus tribromide (data page) | |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Phosphorus tribromide is a colourless liquid with the formulaPBr3. The liquid fumes in moist air due tohydrolysis and has a penetrating odour. It is used in the laboratory for the conversion ofalcohols toalkyl bromides.
PBr3 is prepared by treatingred phosphorus withbromine. An excess of phosphorus is used in order to prevent formation of PBr5:[1][2]
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 fromred phosphorus and bromine.[3]
Phosphorus tribromide, likePCl3 andPF3, has both properties of aLewis base and aLewis acid. For example, with a Lewis acid such asboron tribromide it forms stable 1 :1 adducts such as Br3B · PBr3. At the same time PBr3 can react as anelectrophile or Lewis acid in many of its reactions, for example withamines.
An important reaction of PBr3 is withalcohols, where it replaces anOH group with a bromine atom to produce analkyl bromide. All three bromides can be transferred.[4]
Several detailed procedures are available.[5][6] In some cases,triphenylphosphine/Br2 is superior to PBr3.[7]
The mechanism for a primary alcohol involves formation of a phosphorous ester (to form a good leaving group), followed by anSN2 substitution.
Because of the SN2 substitution step, the reaction generally works well forprimary and secondary alcohols, but fails for tertiary alcohols. If the reacting carbon centre ischiral, the reaction usually occurs withinversion of configuration at the carbon alpha to the alcohol, as is usual with an SN2 reaction.
In a similar reaction, PBr3 also convertscarboxylic acids toacyl bromides:[8]
The main use for phosphorus tribromide is for conversion of primary or secondaryalcohols toalkyl bromides,[9] as describedabove. PBr3 usually gives higher yields thanhydrobromic acid, and it avoids problems ofcarbocation rearrangement- for example evenneopentyl bromide can be made from the alcohol in 60% yield.[10]
Another use for PBr3 is as a catalyst for the α-bromination ofcarboxylic acids. Although acyl bromides are rarely made in comparison withacyl chlorides, they are used as intermediates inHell-Volhard-Zelinsky halogenation.[11]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 ofpharmaceuticals such asalprazolam,methohexital andfenoprofen. It is also a potentfire suppression agent marketed under the namePhostrEx.
Phosphorus tribromide is used for doping inmicroelectronics.[12]
PBr3 evolves corrosiveHBr, which is toxic, and reacts violently with water and alcohols.
In reactions that producephosphorous acid as a by-product, when working up by distillation be aware that this can decompose above about 160 °C to givephosphine which can cause explosions in contact with air.[9]
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