4_4 Phosphorus: White, Red and Black Allotropes, PCl₃ and PCl₅

Phosphorus, the second element of group 15, gets its name from the Greek for "light-bearer", because white phosphorus glows in the dark as it slowly oxidizes in air. Unlike nitrogen, it never exists as small multiply bonded molecules like

\mathrm{N_2}

. Its larger atoms prefer to form three single bonds, which gives phosphorus several solid forms with very different reactivities. Its chlorides are important reagents in both inorganic and organic chemistry.

Occurrence

Phosphorus is too reactive to occur free. Almost all of it is found as phosphate minerals, mainly phosphate rock (impure calcium phosphate,

\mathrm{Ca_3(PO_4)_2}

) and apatite,

\mathrm{Ca_5(PO_4)_3F}

. Life depends on it. Calcium phosphate forms bones and teeth, phosphate groups link the units of DNA and RNA, and ATP carries energy in every cell. Bone ash, about 80% calcium phosphate, was once an important source.

The Allotropes of Phosphorus

Phosphorus has several solid allotropes. The three most important are white, red and black phosphorus.

White phosphorus is a waxy, poisonous, yellowish-white solid made of separate

\mathrm{P_4}

molecules. Each molecule is a tetrahedron of four phosphorus atoms, with every atom bonded to the other three. The P–P–P bond angles are only 60°, much smaller than the angle the bonds would naturally take. This ring strain stores energy and makes the molecule very reactive. White phosphorus catches fire in air at about 30 °C, so it is stored under water. Its molecules are held together only by weak forces, so it melts at 44 °C and dissolves in non-polar solvents such as carbon disulphide.

Red phosphorus forms when white phosphorus is heated to about 250 °C without air, often with a trace of iodine as a catalyst. One bond in each

\mathrm{P_4}

tetrahedron breaks, and the units link into long chains. Red phosphorus is a polymeric solid, much less strained, so it does not ignite in air below about 240 °C. It is far less toxic and insoluble in carbon disulphide, and it is used on the striking strip of safety matches. Black phosphorus, made by heating red phosphorus under very high pressure, has puckered sheets of atoms. It is the most stable and least reactive form, and it conducts electricity.

Comparing the allotropes

Property	White	Red	Black
Structure	Separate P₄ tetrahedra	Chains of linked P₄ units	Layered sheets
Reactivity in air	Ignites near 30 °C	Ignites above about 240 °C	Very low
Toxicity	Very poisonous	Low	Low
Soluble in CS₂	Yes	No	No

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Phosphorus Trichloride (PCl₃)

Phosphorus trichloride is made by passing dry chlorine over heated white phosphorus in an inert atmosphere such as carbon dioxide. Excess chlorine must be avoided, or the pentachloride forms. It is a colourless, fuming liquid that boils at 76 °C. Its molecule is trigonal pyramidal, with a lone pair on phosphorus.

P12P3P1P23.ClCl.ClCl.ClCl.ClCl.ClCl.ClCl>>ClP(Cl)Cl.ClP(Cl)Cl.ClP(Cl)Cl.ClP(Cl)Cl

• Water hydrolyses it to phosphorous acid and HCl, which is why it fumes in moist air:

  \mathrm{PCl_3 + 3H_2O \rightarrow H_3PO_3 + 3HCl}

  .
• It combines with more chlorine to give

  \mathrm{PCl_5}

  , and with oxygen to give phosphoryl chloride,

  \mathrm{POCl_3}

  .
• In organic chemistry it replaces OH groups with Cl, turning alcohols into chloroalkanes and carboxylic acids into acid chlorides.

CC(=O)O.CC(=O)O.CC(=O)O.ClP(Cl)Cl>>CC(=O)Cl.CC(=O)Cl.CC(=O)Cl.[H]P(=O)(O)O

Phosphorus Pentachloride (PCl₅)

Passing excess dry chlorine through phosphorus trichloride gives phosphorus pentachloride, a pale yellowish-white solid that sublimes at about 160 °C. As a gas, the molecule is trigonal bipyramidal, with five chlorines around phosphorus. In the solid, it exists as ions,

\mathrm{[PCl_4]^+[PCl_6]^-}

. Nitrogen has no

\mathrm{NCl_5}

equivalent, because its small atom cannot fit five neighbours.

\mathrm{PCl_3(l) + Cl_2(g) \rightleftharpoons PCl_5(s)}

Heating shifts this equilibrium back to

\mathrm{PCl_3}

and chlorine. Water hydrolyses

\mathrm{PCl_5}

in two stages, first to

\mathrm{POCl_3}

and then to phosphoric acid:

\mathrm{PCl_5 + 4H_2O \rightarrow H_3PO_4 + 5HCl}

. Like the trichloride, it is a chlorinating agent. It also turns some metals into their chlorides.

Tips for remembering

• Strained tetrahedron, reactive phosphorus: the 60° bond angles in P₄ are badly strained, so white phosphorus bursts into flame in air, while red phosphorus has opened chains and is much safer.
• PCl₃ + Cl₂ ⇌ PCl₅: the trichloride takes up two more chlorines, and heating drives them off again.
• Both chlorides replace OH with Cl: they turn alcohols into alkyl chlorides and carboxylic acids into acid chlorides.