Allotropes Of Phosphorus: Structure and Properties

Allotropes of phosphorus

Phosphorus has many allotropes, which includes white phosphorus, red phosphorus and black phosphorus.

Structure Of Phosphorus

Phosphorus exists as tetrahedral P4 molecules in the liquid and gas phases. At very high temperatures, P4 dissociates into P2. At approximately 1800 °C, this dissociation reaches 50 per cent.

Different Allotropes of Phosphorus

White Phosphorus – The Most Reactive Allotropes of Phosphorus

White phosphorus, which exists in two modifications, α-P4 (cubic), and β-P4 (hexagonal), the most common. Condensation of gaseous or liquid phosphorus, both of which contain tetrahedral P4 molecules, gives primarily the α form, which slowly converts to the β form above –76.9° C.

It is a waxy solid consisting of tetrahedral P4 molecules.

White phosphorus

White phosphorus is the most reactive allotropes of phosphorus and bursts into flame in the air to yield P4O10.

White phosphorus (P4) is readily accessible industrially from phosphate minerals, hence the high demand for phosphate rock.

During oxidation in air, α-P4 emits yellow-green light, an example of phosphorescence known since antiquity; white phosphorus is commonly stored underwater to slow its oxidation.

White phosphorus is chlorinated or oxygenated to afford P(III) and P(V) molecules (for example, PCl3, PCl5, POCl3). These are starting materials for high-demand phosphorus compounds (for example, phosphoric acid and phosphines). A vigorous research area motivated by sustainability concerns is to directly incorporate phosphorus atoms from white phosphorus into these desired compounds without the need to synthesize chlorinated or oxygenated intermediates. In this regard, “P4 activation” is being actively pursued.

Structure and Bonding In White Phosphorus

White phosphorus in the liquid and solid forms consists of tetrahedral P4 molecules; in the vapour below 800°, where measurable dissociation to P2 occurs, the element is also as P4 molecules. The P—P distances are 2.21 Angstrom; the P—P—P angles, of course, are 60°. The low angle indicates considerable strain, and the strain energy has been estimated to be about 96 kJ. This means that the total energy of the six P—P bonds in the molecule is that much smaller than would be the total energy of six P—P bonds of the same length formed by phosphorus atoms with normal bond angles. Thus the structure of the molecule is consistent with its high reactivity.

It is most likely that pure 3p orbitals are involved, even though the bonds are bent, since hybridization such as pd2, which would give 60° angles, would require rather large promotion energy. MO calculations also indicate that d-orbital participation is negligible.

Red phosphorus

Red phosphorus can be obtained by heating white phosphorus at 300°C in an inert atmosphere for several days. It is normally obtained as an amorphous solid, but crystalline materials can be prepared that have very complex three-dimensional network structures. 

Red phosphorus

 The colour and other physical properties depend on the method of preparation. Commercial red phosphorus is amorphous with pyramidal phosphorus linked in a random network.

Unlike white phosphorus, red phosphorus does not ignite readily in air. When phosphorus is heated under high pressure, a series of phases of black phosphorus is formed, the thermodynamically most stable form below 550°C. One of the phases consists of puckered layers composed of pyramidal three-coordinate P atoms.

In contrast to the usual practice of choosing the most stable ambient phase of an element as the reference phase for thermodynamic calculations, white phosphorus is adopted because it is more accessible and better characterized than the other forms.

Black phosphorus – The Least Reactive Allotropes of Phosphorus

Black phosphorus is the least reactive allotropes of phosphorus and the most thermodynamically stable form, obtained by heating white phosphorus at very high pressures. It converts to other forms at still higher pressures.

Black phosphorus

 

Black phosphorus is flaky, with a metallic, graphite-like appearance. It is obtained by heating white P either under very high pressure or at 220 to 370° for 8 days in the presence of mercury as a catalyst and with a seed of black P. The structure consists of double layers, each P atom being bound to three neighbours. The closest P—P distances within each double layer are 2.23 Angstrom (i.e., normal single-bond distances). The entire structure consists of a stacking of these double layers with the shortest P—P distance between layers at 3.59 Angstrom.

The main forms of phosphorus show considerable difference in chemical reactivity; white is by far the most reactive and black the least. White P is stored under water because it inflames in air, whereas the red and black are stable in the air; indeed, black P can be ignited only with difficulty.

White P is soluble in organic solvents such as CS2 and benzene. In the P4 molecules, there should be a lone pair directed outward from each P atom; thus the molecule might be expected to have donor ability. Somewhat unstable complexes, e.g., RhCl(PR3)2(P4) have been reported.

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