Phosphorus tribromide

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Phosphorus tribromide
Phosphorus tribromide
Phosphorus tribromide
Phosphorus tribromide
Identifiers
CAS number 7789-60-8 YesY
PubChem 24614
ChemSpider 23016 YesY
EC number 232-178-2
RTECS number TH4460000
Jmol-3D images Image 1
Properties
Molecular formula PBr3
Molar mass 270.69 g/mol
Appearance clear, colourless liquid
Density 2.852 g/cm3
Melting point -41.5 °C (231.7 K)
Boiling point 173.2 °C (446.4 K)
Solubility in water rapid hydrolysis
Refractive index (nD) 1.697
Viscosity 0.001302 Pas
Structure
Molecular shape trigonal pyramidal
Hazards
MSDS External MSDS
EU Index 015-103-00-6
EU classification Corrosive C
R-phrases R14, R34, R37
S-phrases (S1/2), S26, S45
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g., phosphorus Special hazard W: Reacts with water in an unusual or dangerous manner. E.g., cesium, sodiumNFPA 704 four-colored diamond
Related compounds
Other anions phosphorus trifluoride
phosphorus trichloride
phosphorus triiodide
Other cations nitrogen tribromide
arsenic tribromide
antimony tribromide
Related compounds phosphorus pentabromide
phosphorus oxybromide
Supplementary data page
Structure and
properties
n, εr, etc.
Thermodynamic
data
Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Phosphorus tribromide is a colourless liquid with the formula PBr3. It is a colourless liquid that fumes in moist air due to hydrolysis and has a penetrating odour. It is used in the laboratory for the conversion of alcohols to alkyl bromides.

Preparation[edit]

PBr3 is prepared by treating red phosphorus with bromine. An excess of phosphorus is used in order to prevent formation of PBr5:[1][2]

P4 + 6 Br2 → 4 PBr3

Because the reaction is highly exothermic, it is often conducted in the presence of a diluent such as PBr3.

Reactions[edit]

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 forms stable 1 :1 adducts such as Br3B · PBr3. At the same time PBr3 can react as an electrophile or Lewis acid in many of its reactions, for example with amines.

The most important reaction of PBr3 is with alcohols, where it replaces an OH group with a bromine atom to produce an alkyl bromide. 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 SN2 substitution.

PBr3 alcohol rxn.jpg

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 alpha 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


Applications[edit]

The main use for phosphorus tribromide is for conversion of primary or secondary alcohols to alkyl bromides,[3] 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.[4]

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 Hell-Volhard-Zelinsky halogenation.[5] 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

PBr3 HVZ rxn.png

On a commercial scale, phosphorus tribromide is used in the manufacture of pharmaceuticals such as alprazolam, methohexital and fenoprofen. It is also a potent fire suppression agent marketed under the name PhostrEx.

Precautions[edit]

PBr3 evolves corrosive HBr, is toxic, and reacts violently with water and alcohols.

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.[3]

References[edit]

  1. ^ J. F. Gay, R. N. Maxson "Phosphorus(III) Bromide" Inorganic Syntheses, 1947, vol. 2, 147ff. doi:10.1002/9780470132333.ch43
  2. ^ Burton, T. M.; Degerping, E. F. (1940). "The Preparation of Acetyl Bromide". Journal of the American Chemical Society 62 (1): 227. doi:10.1021/ja01858a502. 
  3. ^ a b Harrison, G. C.; Diehl, H. (1955), "β-Ethoxyethyl Bromide", Org. Synth. ; Coll. Vol. 3: 370 
  4. ^ Wade, L. G. Jr. (2005). Organic Chemistry (6th ed.). Upper Saddle River, NJ, USA: Pearson/Prentice Hall. p. 477. 
  5. ^ Wade, L. G. Jr. (2005). Organic Chemistry (6th ed.). Upper Saddle River, NJ, USA: Pearson/Prentice Hall. p. 1051. 

Further reading[edit]