Phosphoric Trihalides

The phosphoryl fluorides are a source of many inorganic and organic derivatives.16,17 The easy cleavage of a phosphorus-chlorine or phosphorus-bromine bond is responsible for the synthetic utility of the compounds. 

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The first part of the mechanism includes the conversion of the carboxylic acid functionality to the acyl halide by the phosphorous trihalide. The acyl halide easily tautomerizes to the corresponding enol in the presence of a catalytic... 

P3o-idine-I-oxides are comparatively resistant to reduction because of resonance stabilization by the aromatic system. Typical reagents that have been used for the formation of pyridones and pyridinols are Raney Nickel in methanol, palladium-on-charcoal, phosphorous trichloride, or phosphorus oxychloride in ethyl acetate. The N-oxides of pyridoxine, pyridoxal, and pyridoxamine have been deoxygenated catalytically. 4-Alkoxy-3-halopyri-dine-1-oxides are A-deoxygenated by phosphorous trichloride in chloroform. 2-Amino-3-pyridinol can be prepared ffom2-nitro-3-pyridinol-l-oxide (X1I450) in acetic acid by treatment with iron and mercuric chloride and then with zinc. 2-Halo-3-pyridinols can be prepared from XII-450 by treatment with phosphorous trihalides in chlorofiMm ... 

Different reagents such as HX and PX3 may be used to prepare alkyl halides from primary and secondary alcohols. However, because elimination reactions predominate when tertiary alcohols are treated with phosphorous trihalides, preparing tertiary alkyl halides from tertiary alcohols proceeds with good yields only if concentrated hydrogen halides, HX, are used. The reaction of 2-methyl-2-butanol with hydrochloric acid to produce 2-chloro-2-methylbutane (Eq. 14.17) illustrates this transformation. 

Imination of Phosphites and Phenylphosphonous Dihalides. TfNs reacts with (PhOlsP and (C6Hi30)3P quantitatively to 7V-triflyl-phosphorimidates (eq 2). Similarly, PhPX2 (X = F, Cl, or Br) afford the corresponding phosphonimidic dihalides, whereas phosphorous trihalides do not react. 

Alcohol Phosphorus trihalide Alkyl halide Phosphorous acid... 

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

When looking for other molecules with lone electron-pairs that perhaps will combine these electrons with d electrons of platinum metals, there are, first of all, the trihalides of the nitrogen-phosphor group of elements. Complexes formed from chlorides of the platinum... 

The unsaturated character of these trialkyl esters was shown by the ease with which they were attacked by nitric acid, but still more clearly by the formation, with evolution of heat, of stable crystalline addition compounds when they were mixed with cuprous halides. Thus CuC1.P(OC2H5)3 was described as consisting of colourless crystals melting at 190° to 192° C. and soluble in organic solvents.4 This property they share with phosphine, alkylphosphines and phosphorus trihalides. The phosphoric esters were quite indifferent to cuprous halides. Nor were such addition compounds formed either by phosphorous acid itself or by the dialkyl esters, which may show that the latter compounds have the unsymmetrical formula. Phosphorous acid probably exists in both forms, but first as P(OH)3, i.e. when produced from PC13 and Ha0.5 This may be transformed into the unsym-metrieal form through an addition compound HC1.P(0H)3,4 and probably also exists in the form of complex molecules, such as... 

The halides and oxyhalides of some nonmetals hydrolyze (react with water) to produce two acids a (binary) hydrohalic acid and a (ternary) oxyacid of the nonmetal. Phosphorus trihalides react with water to produce the corresponding hydrohalic acids and phosphorous acid, a weak diprotic acid, whereas phosphorus pentahalides give phosphoric acid and the corresponding hydrohalic acid. 

Table 5 Some physical properties and NMR chemical shifts S of phosphoric and thiophosphoric trihalides and mixed halides ...

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. 

The interaction of phosphorns trihalides with alcohols involves a series of consecntive and concnr-rent reactions. In the case of phosphorus trichloride, these can be represented by Eqnations 6.306 through 6.309 in which triesters are first formed, followed by stepwise dealkylation to phosphorous acid. 

The most widespread method of producing complete esters of phosphorous acid, in which R = R2 = R3, is the treatment of alcohols or alkylated phenols, taken in small excesses (0.1-0.5 mole) with phosphorus trihalide (most often trichloride) at a temperature of 150-200°C [70, 158, 285, 294] ... 

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