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Phosphorus tribromide

2007 Schools Wikipedia Selection. Related subjects: Chemical compounds

                              Phosphorus tribromide
                   Phosphorus tribromide Phosphorus tribromide
                                     General
          Systematic name                                  phosphorus tribromide
              Other names                    phosphorus(III) bromide
                                                             phosphorous bromide
                                                               tribromophosphine
        Molecular formula                                                 PBr[3]
               Molar mass                                           270.70 g/mol
               Appearance                               clear, colourless liquid
               CAS number                                            [7789-60-8]
                                    Properties
        Density and phase                                   2.852 g/cm^3, 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 Image:nfpa_h3.png Image:nfpa_f0.png Image:nfpa_r2.png
              Flash point                                                   ? °C
            R/S statement                                          R: 14, 34, 37
                                                                       S: 25, 45
             RTECS number                                              TH4460000
                             Supplementary data page
   Structure & properties                                          n, ε[r], etc.
       Thermodynamic data                                        Phase behaviour
                                                              Solid, liquid, gas
                      NMR                           δ227 (downfield of H[3]PO[4]
                                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 colourless liquid with the formula PBr[3].
   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.

Chemical properties

   Phosphorus tribromide, like PCl[3] and PF[3], 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 Br[3]B-PBr[3]. At
   the same time PBr[3] can react as an electrophile or Lewis acid in many
   of its reactions, for example with amines.

   The most important reaction of PBr[3] 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.

          PBr[3] + 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 S[N]2 substitution at the alcohol
   carbon.

   Because of the S[N]2 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 S[N]2 reaction.

   In a similar reaction, PBr[3] also converts carboxylic acids to acyl
   bromides.

          PBr[3] + 3 RCOOH → 3 RCOBr + HP(O)(OH)[2]

   PBr[3] is a reasonably strong reducing agent, and the oxidation of
   PBr[3] with oxygen gas is more vigorous than seen with PCl[3]. It gives
   an explosive reaction that forms P[2]O[5] and Br[2].

Preparation

   PBr[3] is by treating phosphorus with bromine, using PBr[3] itself as
   the solvent (white phosphorus is soluble in PBr[3]). An excess of
   phosphorus is used in order to prevent formation of PBr[5].

          P[4] + 6 Br[2] → 4 PBr[3]

Uses

   The main use for phosphorus tribromide is for conversion of primary or
   secondary alcohols to alkyl bromides^, as described above. PBr[3]
   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^.

   Another use for PBr[3] 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 PBr[3] 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.

Precautions

   PBr[3] 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.^
   Retrieved from " http://en.wikipedia.org/wiki/Phosphorus_tribromide"
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