Phosphorus trichloride, combination with chlorine

When phosphorus (P4) combines with chlorine, phospho-ms trichloride is formed. 

H. A. Taylor f made a similar experiment with phosphorus trichloride vapour and chlorine, which in glass vessels combine almost instantaneously to give the solid pentachloride. In this instance the combination still took place with immeasurable rapidity in a vessel of paraffin wax. This combination, unlike that of ethylene and bromine, appears to be a homogeneous reaction, since when the trichloride and chlorine are mixed there is a delay of a few seconds due to supersaturation, and then the solid is formed as a cloud which settles slowly to the bottom of the vessel. Some solid formation also takes place on the walls. 

Chlorine readily combines with many nonmetals. Reaction with sulfur yields sulfur dichloride, SCI2 and with phosphorus the products are phosphorus trichloride, PCI3 and phosphorus pentachloride, PCI5. 

In the area of trivalent phosphorus acid chemistry some very unusual reactions have been reported from combinations of 4-dimethylamino-pyridine (DMAP) with phosphorus trichloride. In one case a phosphide and free chlorine are claimed to be formed The extreme nucleofugicity of a tervalent phosphorus atom bearing two DMAP groups also stimulates an unprecedented Arbusov reaction of a tervalent derivative. Another surprise is that, although the reaction is certainly acid-catalysed, the mechanism of substitution at phosphoramidites does not involve either P- or N-protonated species. A final surprise in this area is the preparation, by Russian chemists, of some tervalent phosphorus compounds which apparently contain P-OH groups. 

When two nonmetals combine with each other, they form binary covalent compounds. In such reactions, the oxidation state of the element with the more positive oxidation state is often variable, depending on reaction conditions. For example, phosphorus (Group 5A) combines with a limited amount of chlorine to form phosphorus trichloride, in which phosphorus exhibits the +3 oxidation state. 

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