Phosphorus tribromide
| Phosphorus tribromide | |
|---|---|
 |
|
| General | |
| Systematic name | phosphorus tribromide |
| Other names | phosphorus(III)
bromide phosphorous bromide tribromophosphine |
| Molecular formula | PBr3 |
| Molar mass | 270.70 g/mol |
| Appearance | clear, colourless liquid |
| CAS number | [7789-60-8] |
| Properties | |
| Density and phase | 2.852 g/cm3, liquid |
| Solubility in water | rapid hydrolysis |
| in methanol | decomposes |
| in
acetone, chloroform, diethyl ether |
soluble |
| Melting point | -41.5 °C (231.7 K) |
| Boiling point | 173.2 °C (446.4 K) |
| Viscosity | ? c P at ? °C |
| Structure | |
| Molecular shape | trigonal bipyramidal |
| Dipole moment | ? D |
| Hazards | |
| MSDS | External MSDS |
| Main hazards | corrosive, toxic,
reactive with water and alcohols |
| NFPA 704 |
   |
| Flash point | ? °C |
| R/S statement | R: 14, 34,
37 S: 25, 45 |
| RTECS number | TH4460000 |
| Supplementary data page | |
| Structure & properties | n, εr, etc. |
| Thermodynamic data | Phase
behaviour Solid, liquid, gas |
| Spectral data | UV, IR, NMR, MS |
| Related compounds | |
| Other anions |
phosphorus
trifluoride phosphorus trichloride phosphorus triiodide |
| Other cations |
nitrogen
tribromide arsenic tribromide antimony tribromide |
| Related compounds |
phosphorus
pentabromide phosphorus oxybromide |
| Except where noted otherwise, data are given
for materials in their standard state (at 25°C, 100 kPa) Infobox disclaimer and references |
|
Phosphorus tribromide is a clear colourless liquid. It is a covalent compound of phosphorus and bromine with the formula P Br3. 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. The phosphorus atom has an NMR chemical shift of 227 ppm (downfield of H3PO4).
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 ROH → 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 SN2 substitution at the alcohol carbon.
Because of the SN2 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 SN2 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 alcohols 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, reacts violently with water and alcohols. Wear gloves and goggles and work in a fume cupboard (hood), use an apron and face shield with larger amounts. Keep away from strong oxidising agents.
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