Phosphorus tribromide

Phosphorus tribromide

Names
IUPAC name Phosphorus tribromide
Other names phosphorus(III) bromide, phosphorous bromide, tribromophosphine
Identifiers
CAS Number	7789-60-8 Y
3D model (Jmol)	Interactive image
ChemSpider	23016 Y
ECHA InfoCard	100.029.253
EC Number	232-178-2
PubChem	24614
RTECS number	TH4460000
InChI InChI=1S/Br3P/c1-4(2)3 Y Key: IPNPIHIZVLFAFP-UHFFFAOYSA-N Y
SMILES BrP(Br)Br
Properties
Chemical formula	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)
Solubility in water	rapid hydrolysis
Refractive index (nD)	1.697
Viscosity	0.001302 Pas
Structure
Molecular shape	trigonal pyramidal
Hazards
Safety data sheet	See: data page
EU classification (DSD)	C
R-phrases	R14, R34, R37
S-phrases	(S1/2), S26, S45
NFPA 704	0 3 2 W
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	Refractive index (n), Dielectric constant (εr), etc.
Thermodynamic data	Phase behaviour solid–liquid–gas
Spectral data	UV, IR, NMR, MS
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Y  (what is YN ?)
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.

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

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

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.

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, the reaction usually occurs with inversion of configuration at the alcohol alpha carbon, as is usual with an S_N2 reaction.