phosphorus tribromide

PBr₃ is prepared by treating red phosphorus with bromine. An excess of phosphorus is used in order to prevent formation of PBr₅:{{cite book | doi = 10.1002/9780470132333.ch43 | chapter = Phosphorus(III) Bromide | title = Inorganic Syntheses | date = 1946 | last1 = Gay | first1 = J. F. | last2 = Maxson | first2 = R. N. | last3 = Kleinberg | first3 = J. | last4 = Haan | first4 = R. E. | volume = 2 | pages = 147–151 | isbn = 978-0-470-13161-9 }}{{ cite journal | journal = Journal of the American Chemical Society | title = The Preparation of Acetyl Bromide |author1=Burton, T. M. |author2=Degerping, E. F. | year = 1940 | volume = 62 | issue = 1 | pages = 227 | doi = 10.1021/ja01858a502 | bibcode = 1940JAChS..62..227B }}

:P₄ + 6 Br₂ → 4 PBr₃

Because the reaction is highly exothermic, it is often conducted in the presence of a diluent such as PBr₃. Phosphorus tribromide is also generated in situ from red phosphorus and bromine.{{cite journal |doi=10.15227/orgsyn.014.0042|title=Glycerol α,γ-Dibromohydrin |journal=Organic Syntheses |year=1934 |volume=14 |page=42|author=Géza Braun }}

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.

An 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.{{cite journal |doi=10.15227/orgsyn.013.0020|title=Isobutyl Bromide |journal=Organic Syntheses |year=1933 |volume=13 |page=20|author=C. R. Noller, R. Dinsmore }}

:{{chem2|PBr3 + 3 (CH3)2CHCH2OH → 3 (CH3)2CHCH2Br + HP(O)(OH)2}}

Several detailed procedures are available.{{cite journal |doi=10.15227/orgsyn.023.0032|title=β-Ethoxyethyl Bromide |author=George C. Harrison, Harvey Diehl|journal=Organic Syntheses |year=1943 |volume=23 |page=32 }}{{cite journal |doi=10.15227/orgsyn.017.0073|title=Pentaerythrityl Bromide and Iodide |author=H. B. Schurink|journal=Organic Syntheses |year=1937 |volume=17 |page=73 }} In some cases, triphenylphosphine/Br₂ is superior to PBr₃.{{cite journal |doi=10.15227/orgsyn.048.0051|title=Cinnamyl Bromide |author=John P. Schaefer, J. G. Higgins, P. K. Shenoy|journal=Organic Syntheses |year=1968 |volume=48 |page=51 }}

The mechanism for a primary alcohol involves formation of a phosphorous ester (to form a good leaving group), followed by an S_N2 substitution.

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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 carbon alpha to the alcohol, as is usual with an S_N2 reaction.

In a similar reaction, PBr₃ also converts carboxylic acids to acyl bromides:{{cite journal |doi=10.15227/orgsyn.033.0029|title=Dimethylketene |journal=Organic Syntheses |year=1953 |volume=33 |page=29|author=C. W. Smith, D. G. Norton }}

:{{chem2|PBr3 + 3 RCO2H → 3 RCOBr + HP(O)(OH)2}}

The main use for phosphorus tribromide is for conversion of primary or secondary alcohols to alkyl bromides,{{ OrgSynth | title = β-Ethoxyethyl Bromide | collvol = 3 | collvolpages = 370 | prep = cv3p0370 | author = Harrison, G. C. | author2 = Diehl, H. | year = 1955 }} 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.{{ cite book | author = Wade, L. G. Jr. | title = Organic Chemistry | edition = 6th | page = 477 | publisher = Pearson/Prentice Hall | location = Upper Saddle River, NJ, USA | year = 2005 }}

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.{{ cite book | author = Wade, L. G. Jr. | title = Organic Chemistry | edition = 6th | page = 1051 | publisher = Pearson/Prentice Hall | location = Upper Saddle River, NJ, USA | year = 2005 }}

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

:350px

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.

Phosphorus tribromide is used for doping in microelectronics.{{cite journal | last1=Knoell | first1=R. V. | last2=Murarka | first2=S. P. | title=Epitaxial growth induced by phosphorus tribromide doping of polycrystalline silicon films on silicon | journal=Journal of Applied Physics | publisher=AIP Publishing | volume=57 | issue=4 | date=1985-02-15 | issn=0021-8979 | doi=10.1063/1.334533 | pages=1322–1327| bibcode=1985JAP....57.1322K }}

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

:PBr₃ + 3 H₂O → H₃PO₃ + 3 HBr

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.

{{Reflist}}

• {{ Greenwood&Earnshaw2nd }}
• {{ cite book | editor = Lide, D. R. | title = Handbook of Chemistry and Physics | url = https://archive.org/details/crchandbookofche00lide | url-access = registration | edition = 71st | publisher = CRC Press | location = Ann Arbor, MI | year = 1990 | isbn = 978-0849304712 }}
• {{ cite book | author = March, J. | title = Advanced Organic Chemistry | url = https://archive.org/details/advancedorganicc00marc_339 | url-access = limited | edition = 4th | page = [https://archive.org/details/advancedorganicc00marc_339/page/n739 723] | publisher = Wiley | location = New York | year = 1992 | isbn = 978-0471601807 }}
• {{ cite book | editor = Stecher, P. G. | title = The Merck Index | edition = 7th | publisher = Merck & Co | location = Rahway, NJ, USA | year = 1960 }}
• {{ cite journal | author = Holmes, R. R. | title = An Examination of the Basic Nature of the Trihalides of Phosphorus, Arsenic and Antimony | journal = Journal of Inorganic and Nuclear Chemistry | year = 1960 | volume = 12 | issue = 3–4 | pages = 266–275 | doi = 10.1016/0022-1902(60)80372-7 }}

{{Phosphorus compounds}}

{{bromine compounds}}

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Category:Phosphorus bromides

Category:Reagents for organic chemistry

Category:Inorganic solvents

Category:Fire suppression agents

Category:Phosphorus(III) compounds