The P-Block Elements (Phosphorus And Its Compounds)

Phosphorus — Allotropic Forms

Phosphorus exhibits allotropy, existing in several different forms in the same physical state due to different arrangements of its atoms.

Allotropic Forms Of Phosphorus

The most important allotropes of phosphorus are:

1. White Phosphorus ($P_4$):
• Structure: Consists of discrete tetrahedral molecules of $P_4$. In this molecule, each phosphorus atom is bonded to three other phosphorus atoms, forming a triangular pyramid. The P-P-P bond angles are 60°.
• Properties:
• Waxy, white solid.
• Spontaneously ignites in moist air (pyrophoric).
• Highly toxic.
• Soluble in carbon disulfide ($CS_2$) but insoluble in water.
• Chemically very reactive due to the strained $60^\circ$ bond angles in the $P_4$ tetrahedron.
• Preparation: Prepared by heating white phosphorus ($P_4$) in an inert atmosphere.
• Storage: Stored under water to prevent contact with air.
2. Red Phosphorus:
• Structure: A polymer of $P_4$ tetrahedra linked together in chains. It can be considered an association of $P_4$ units.
• Properties:
• Violet or red color.
• Less reactive than white phosphorus.
• Does not ignite spontaneously in air.
• Insoluble in carbon disulfide.
• Non-toxic.
• Chemically less reactive than white phosphorus because the P-P bonds in the polymeric structure are less strained.
• Preparation: Prepared by heating white phosphorus to 573 K in the presence of iodine or some other catalyst.
• Storage: Can be stored in air.
• Uses: Used in the manufacture of safety matches and pesticides.
3. Black Phosphorus:
• Structure: The most stable allotrope. Has a layered structure similar to graphite, with puckered hexagonal rings.
• Properties: Opaque, brittle, greyish-black solid with a metallic luster. It is a semiconductor. Less reactive than red phosphorus.
• Preparation: Prepared by heating white phosphorus at 803 K under high pressure.
• Uses: Used in the manufacture of semiconductors and transistors.

Interconversion: White phosphorus can be converted to red phosphorus by heating in the presence of a catalyst. Red phosphorus converts to black phosphorus under high pressure.

Phosphine

Phosphine ($PH_3$) is the simplest hydride of phosphorus, analogous to ammonia ($NH_3$).

Preparation

1. Laboratory Preparation:

• From White Phosphorus: Prepared by heating white phosphorus with concentrated sodium hydroxide solution. This reaction produces phosphine and sodium hypophosphite.

$P_4(s) + 3NaOH(aq) + 3H_2O(l) \rightarrow PH_3(g) + 3NaH_2PO_2(aq)$

• From Phosphides: Prepared by treating metal phosphides (like calcium phosphide, $Ca_3P_2$) with water or dilute acids. This method often produces phosphine contaminated with diphosphine ($P_2H_4$), which is spontaneously combustible.

$Ca_3P_2(s) + 6H_2O(l) \rightarrow 3Ca(OH)_2(aq) + 2PH_3(g)$

$2Ca_3P_2(s) + 6H_2SO_4(dilute) \rightarrow 2Ca_3(PO_4)_2(s) + 6H_2(g) + P_2H_4$

• Purification: Pure phosphine can be obtained by reacting $Ca_3P_2$ with $KOH$ solution or by passing the product through a solution of $KOH$ to remove the spontaneously combustible diphosphine.

2. Industrial Production:

• Produced by heating white phosphorus with solid alkali metal hydroxides.
• Also produced during the production of calcium phosphide from lime and coke in an electric furnace.

Properties

Physical Properties:

• Colorless gas.
• Has a foul smell of decaying fish.
• Highly toxic.
• Liquefies at 185.4 K (-87.7°C) and solidifies at 87.1 K (-186.1°C).
• Slightly soluble in water.
• Solutions in water are weakly basic due to the formation of $H_3PO_3$ or related species upon reaction.

Chemical Properties:

1. Reducing Nature: Phosphine is a strong reducing agent, especially in the presence of oxidizing agents.

• Reacts with $O_2$ or $Cl_2$ vigorously and can ignite spontaneously in moist air due to the presence of traces of $P_2H_4$.
• Reduces solutions of $CuSO_4$, $AgNO_3$, $HgCl_2$ to form metal phosphides.

$8AgNO_3(aq) + 4PH_3(g) \rightarrow Ag_3P(s) + H_3PO_4(aq) + 8NO_3^-(aq) + 8H^+(aq)$ (simplified)

$3CuSO_4(aq) + 2PH_3(g) \rightarrow Cu_3P_2(s) + 3H_2SO_4(aq)$

2. Basic Nature:

• Acts as a Lewis base, donating its lone pair of electrons to form phosphonium salts (similar to ammonium salts) with strong acids.

$PH_3 + HCl \rightarrow [PH_4]^+Cl^-$ (Phosphonium chloride)

• It is a much weaker base than ammonia because the lone pair of electrons is in the larger $3p$ orbital and is less available for protonation.

3. Reaction with Metals: Forms phosphides with some metals.

Phosphorus Halides

Phosphorus reacts with halogens to form halides, most commonly showing $+3$ and $+5$ oxidation states.

Phosphorus Trichloride ($PCl_3$)

Preparation:

• By direct reaction of white phosphorus with excess chlorine gas.

$P_4(s) + 6Cl_2(g) \rightarrow 4PCl_3(l)$

Properties:

• Colorless, fuming liquid with pungent irritating smell.
• Reacts violently with water (hydrolyzes) to form phosphorous acid ($H_3PO_3$) and hydrochloric acid ($HCl$).

$PCl_3(l) + 3H_2O(l) \rightarrow H_3PO_3(aq) + 3HCl(aq)$

• It is a Lewis base, can donate its lone pair of electrons.
• Acts as a reducing agent.
• Used in the synthesis of other phosphorus compounds, including organophosphorus compounds and other halides.

Phosphorus Pentachloride ($PCl_5$)

Preparation:

• By the action of excess dry chlorine gas on phosphorus ($PCl_3$ is formed first, then reacts further with $Cl_2$).

$PCl_3(l) + Cl_2(g) \rightarrow PCl_5(s)$

• By direct combination of phosphorus vapor and excess chlorine gas.

Properties:

• White crystalline solid.
• Sublimes easily at 430 K.
• Reacts vigorously with water (hydrolyzes) in a stepwise manner to form first phosphoryl chloride ($POCl_3$) and then phosphoric acid ($H_3PO_4$).

$PCl_5(s) + H_2O(l) \rightarrow POCl_3(l) + 2HCl(g)$

$POCl_3(l) + 3H_2O(l) \rightarrow H_3PO_4(aq) + 3HCl(aq)$

• Acts as a Lewis acid.
• It is a strong chlorinating agent and an oxidizing agent.
• In the solid state, it exists as an ionic compound $[PCl_4]^+[PCl_6]^-$
• In the vapor phase, it exists as a covalent molecule $PCl_5$.
• It decomposes on heating to $PCl_3$ and $Cl_2$.

$PCl_5(s) \rightleftharpoons PCl_3(l) + Cl_2(g)$

Uses:

• Used as a chlorinating agent.
• Used in organic synthesis.

Oxo-halides: Phosphorus also forms oxo-halides like phosphorus oxychloride ($POCl_3$) and phosphorus oxyfluoride ($POF_3$).

Oxoacids Of Phosphorus

Phosphorus forms several important oxoacids, in which phosphorus exhibits positive oxidation states (+1, +3, +5).

Key Oxoacids:

1. Orthophosphorous Acid ($H_3PO_3$):
• Structure: Phosphorus is in $+3$ oxidation state. It has a tetrahedral structure with one P-H bond, making it dibasic.

Structure: $HP(O)(OH)_2$. The hydrogen atom attached to phosphorus is not acidic.

• Preparation: By hydrolysis of $PCl_3$.

$PCl_3 + 3H_2O \rightarrow H_3PO_3 + 3HCl$

• Properties: Reducing agent.
2. Orthophosphoric Acid ($H_3PO_4$):
• Structure: Phosphorus is in $+5$ oxidation state. Tetrahedral structure with three P-OH bonds, making it tribasic.

Structure: $P(O)(OH)_3$.

• Preparation:
• From $P_4O_{10}$ and water: $P_4O_{10} + 6H_2O \rightarrow 4H_3PO_4$
• From white phosphorus via oxidation with nitric acid.
• Industrial production from phosphate rocks via reaction with sulfuric acid or by thermal process.
• Properties: Colorless crystalline solid, readily soluble in water, forms three series of salts (phosphates, hydrogen phosphates, dihydrogen phosphates).
• Uses: Fertilizers, food industry (acidulant), rustproofing, detergents.
3. Hypophosphorous Acid ($H_3PO_2$):
• Structure: Phosphorus is in $+1$ oxidation state. Tetrahedral structure with two P-H bonds and one P-OH bond, making it monobasic.

Structure: $P(O)H_2(OH)$.

• Preparation: By hydrolysis of $P_4$ with boiling $Ba(OH)_2$ solution.

$P_4 + 3Ba(OH)_2 + 6H_2O \rightarrow 3Ba(H_2PO_2)_2 + 2PH_3$

Followed by reaction with $H_2SO_4$: $Ba(H_2PO_2)_2 + H_2SO_4 \rightarrow BaSO_4(s) + 2H_3PO_2(aq)$

• Properties: Strong reducing agent.
4. Pyrophosphorous Acid ($H_4P_2O_5$): Derived from pyrophosphorous acid.
5. Pyrophosphoric Acid ($H_4P_2O_7$): A condensation product of two orthophosphoric acid molecules.
6. Metaphosphoric Acid ($HPO_3$): A polymeric form, ($HPO_3$)$_n$.

Acidity: Phosphoric acid ($H_3PO_4$) is tribasic, dissociating in three steps with decreasing $K_a$ values.

Structure of Oxoacids: All oxoacids contain P-H bonds (except phosphoric acid), and the number of P-H bonds determines the basicity of the acid (number of acidic $H$ atoms attached to $O$).