Phosphorus tribromide is a colourless liquid with the formula PBr3. It fumes in air due to hydrolysis and has a penetrating odour. It is widely used in the laboratory for the conversion of alcohols to alkyl bromides.
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 alpha carbon, as is usual with an SN2 reaction.
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]. 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. It is also a potent fire suppression agent marketed under the name PhostrEx
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.
PROTECTION OF HYDROXY GROUPS AS TRIMETHYLSILYL ETHERS USING 1,1,1,3,3,3-HEXAMETHYLDISILAZANE (HMDS) CATALYZED BY POLY(4-VINYLPYRIDINIUM TRIBROMIDE)
May 01, 2010; A very efficient procedure for the protection of alcohols and phenols is presented. The mixture of...
US Patent Issued to Dow Global Technologies on Feb. 12 for "Process for Brominating Unsaturated Organic Compounds Using a Quaternary Phosphonium Tribromide as the Brominating Agent" (Michigan Inventor)
Feb 13, 2013; ALEXANDRIA, Va., Feb. 13 -- United States Patent no. 8,372,921, issued on Feb. 12, was assigned to Dow Global Technologies LLC...
Development of potential manufacturing routes for substituted thiophenes - Preparation of halogenated 2-thiophenecarboxylic acid derivatives as building blocks for a new family of 2,6-dihaloaryl 1,2,4-triazole insecticides.(Full Research Paper)
Sep 04, 2007; Authors: John W Hull, Jr (corresponding author) ; Duane R Romer ; David E Podhorez ; Mezzie L Ash ; Christine H Brady...