Phosphorus trihalides

All 4 trihalides are volatile reactive compounds which feature pyramidal molecules. The fluoride is best made by the action of CaF2, Znp2 or Asp3 on PCI3, but the others are formed by direct halogenation of the element. PP3 is colourless, odourless and does not fume in air, but is very hazardous due to the formation of a complex with blood haemoglobin (cf. 

PCI5 Off-white tetragonal crystals 167 160 (subl) See text  

PBrj Reddish-yellow rhombohedral crystals 100 (d) 106 (d) See text  

It is about as toxic as COCI2. The similarity of PF3 and CO as ligands was first noted by J. Chatt and many complexes with transition elements are now known, e.g. [Ni(CO) (PF3)4- ] in = 0-4), [PdfPFsla], [Pt(PF3)4], [CoH(PF3)4], [C02(M-PF2)2(PF3)6], etc. Such complexes can be prepared by ligand replacement reactions, by fluorination of PCI3 complexes, by direct reaction of PF3 with metal salts or even by direct reaction of PF3 with metals at elevated temperatures and pressures. 

unlike the other trihalides of phosphorus, hydrolyses only slowly with water, the products being phosphorous acid and HF PF3 + 3H2O H3PO3 + 3HF. 

Tornieporth-Oetting and T. Klapotke, J. Chem. Soc., Chem. Commun., 132-3 (1990). 

The phosphorus trihalides are generally very reactive compounds which can cause both acute and chronic poisoning. They are strong electron donors by virtue of the lone pair electrons on the P atom. All are colourless or nearly so, except the red brown triiodide (Table 4.20). 

Well-Characterised Phosphorus Halide Ions and Molecules 

The trivalent halides, PX3, are well known (4.200). All exist as pyramidal molecules with X/P/X angles of about 100°. Bond lengths in the trifluoride are abnormally short and this may indicate that, unlike other trihalides, some n bonding may be present. Similar pyramidal molecules are formed by the As, Sb and Bi trihalides and physical properties are listed in Tables 4.21 and 4.22. 

Each trihalide may be made (although not necessarily most conveniently) by direct union of the elements. They will easily undergo atmospheric oxidation to the oxyhalide, add sulphur to form the thiohalide, hydrolyse under acid conditions to form phosphorous acid, readily add more halogen to form the pentahalide, and form addition complexes with various metals. 

Phosphorus trifluoride, PF3, is a colourless gas, odourless in toxic concentrations, which bums in air in the presence of nitric oxide as a catalyst. It is best made by fluorination of the chloride with arsenic or zinc fluorides, or potassium fluoride dissolved in liquid SO2. It may also be made by the action of HF on red phosphorus at elevated tanperatures (4.201), or by reacting copper phosphide with lead fluoride (4.202). 

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Reaction with phosphorus trihalides (Section 4 13) Phosphorus trichloride and phosphorus tribromide convert alcohols to alkyl halides... 

Alcohol Phosphorus trihalide Alkyl halide Phosphorous acid... 

Section 19 16 Halogenation at the a carbon atom of carboxylic acids can be accom plished by the Hell-Volhard-Zehnsky reaction An acid is treated with chlorine or bromine m the presence of a catalytic quantity of phospho rus or a phosphorus trihalide... 

Hell-Volhard-Zelinsky reaction (Section 19.16) The phosphorus trihalide-catalyzed a halogenation of a carboxylic acid ... 

Paralleling the various phosphorus trihalides are numerous pseudohalides and mixed pseudohalide-halides of which the various isocyanates and isothiocyanates are perhaps the best known. Most are volatile liquids, e.g. 

The formulated mechanism is supported by the finding that no halogen from the phosphorus trihalide is transferred to the a-carbon of the carboxylic acid. For instance, the reaction of a carboxylic acid with phosphorus tribromide and chlorine yields exclusively an a-chlorinated carboxylic acid. In addition, carboxylic acid derivatives that enolize easily—e.g. acyl halides and anhydrides—do react without a catalyst present. 

The preparation of a-iodocarboxylic acids is of particular interest, since iodide is a better leaving group as is chloride or bromide. A similar a-iodination with a phosphorus trihalide as catalyst is not known. However the iodination can be achieved in the presence of chlorosulfonic acid mechanistically the intermediate formation of a ketene 10 by dehydration of the carboxylic acid is assumed ... 

When the phosphorus compound is a phosphorus trihalide or phosphoryi (or... 

Although 1,3,2-diazaphospholenium cations are usually prepared from neutral NHPs or 1,3,2-diazaphospholes via Lewis-acid induced substituent abstraction or A-alkylation, respectively (cf. Sect. 3.1.2), the group of Cowley was the first to describe a direct conversion of a-diimines into cationic heterocycles by means of a reaction that can be described as capture of a P(I) cation by diazabutadiene via [4+1] cycloaddition [31] (Scheme 4). The P(I) moiety is either generated by reduction of phosphorus trihalides with tin dichloride in the presence of the diimine [31] or, even more simply, by spontaneous disproportionation of phosphorus triiodide in the presence of the diimine [32], The reaction is of particular value as it provides a straightforward access to annulated heterocyclic ring systems. Thus, the tricyclic structure of 11 is readily assembled by addition of a P(I) moiety to an acenaphthene-diimine [31], and the pyrido-annulated cationic NHP 12 is generated by action of appropriate... 

Because of the reactive covalent bonds to halogen atoms, all of the trihalides of the group VA elements hydrolyze in water. It is found that the rates decrease in the order P > As > Sb > Bi, which agrees with the decrease in covalent bond character that results from the increase in metallic character of the central atoms. Not all of the trihalides react in the same way. The phosphorus trihalides react according to the equation... 

As in the case of phosphorus trihalides, the phosphorus atom in trialkyl phosphites will undergo addition reactions in which oxygen, sulfur, or selenium is added. The latter two react as elements, but a suitable source of oxygen is hydrogen peroxide. 

Reasonable yields (55-85%) of dialkylphosphinic acids have been reported from a complex reaction system beginning with red phosphorus, iodoalkane, and elemental iodine.26 Presumably, the phosphorus trihalide is generated as an intermediate. The reaction is worked up with a nitrous acid system from aqueous HCl/NaN02. 

The reaction is performed most simply by the addition of the propargylic alcohol to a solution of the phosphorus halide. Rearrangement of the phosphorus ester proceeds at ambient temperature or with mild heating. When phosphorus trihalides are used, the product can be isolated as the phosphonic dichloride.168169 Aqueous workup provides the phosphonic acid.162 In most instances, however, a dialkyl phosphorochloridite with only a single halogen on phosphorus available for reaction with alcohol has been used.165 170 174... 

Elder, R.C., Florian, L.R., Kennedy, E.R., and Macomber, R.S., Phosphorus containing products from the reaction of propargyl alcohols with phosphorus trihalides. II. The crystal and molecular structure of 2-hydroxy-3,5-di-ferf-butyl-l,2-oxaphosphol-3-ene 2-oxide, /. Org. Chem., 38, 4177, 1973. 

Butadiene, isoprene, chloroprene and 2,3-dimethyl-l,3-butadiene add phosphorus trihalides to form 3-phospholene 1,1,1-dihalides, e.g. 297 from isoprene and phosphorus... 

Reaction with phosphorus trihalides Alcohols are converted to alkyl bromides by reaction with phosphorus tribromide (Refer Unit 10, Class XII). 

White phosphorus reacts spontaneously with halogens at ordinary temperatures forming phosphorus trihalides. However, in excess halogen the product is phosphorus pentahahde ... 

Cyclizations with perfluoroacylating agents seem to be quite general for the synthesis of 5(2i7)-oxazolones with aromatic substituents directly bonded to the heterocyclic ring. For example, perfluoroacylation of a solution of an arylgly-cine containing a phosphorus trihalide affords 4-aryl-2-(perfluoroalkyl)-5(2//)-oxazolones (Table 7.1, Fig. 12) Similar results were obtained when amino nitriles were used as starting materials. ... 

The reagents most frequently used to reduce the A-oxides to the parent heterocycle are trivalent phosphorus compounds. These include the phosphorus trihalides, usually in chloroform or ethyl acetate, triphenylphosphine, and phosphite esters such as triethyl phosphite. Substituents which are easily reduced are not affected, although nitro groups... 

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