Important Chemical Compounds of Phosphorus

Introduction to the Major Compounds of Phosphorus

Phosphorus, a p-block element, forms a variety of industrially and biologically important compounds. Its versatility stems from its ability to exist in multiple oxidation states (from -3 to +5) and form stable bonds with elements like oxygen, hydrogen, and halogens. For high school examinations, phosphine (PH₃), phosphorus halides (e.g., PCl₅), and its oxoacids (e.g., H₃PO₄, H₃PO₃) are particularly important.

Phosphine (PH₃)

Phosphine is a colorless, highly poisonous gas with a characteristic smell of rotten fish. It is analogous to ammonia (NH₃) in terms of formula but is a much weaker base.

Chemical Formula

PH₃

Common Name

Phosphine

Laboratory Preparation

1. From Calcium Phosphide (Ca₃P₂) with Water or Dilute Acid: Calcium phosphide reacts with water or dilute hydrochloric acid to produce phosphine. This method often yields impure phosphine, which ignites spontaneously in air due to the presence of P₂H₄ (diphosphine) vapours.

   • Equation with Water: Ca₃P₂ (s) + 6H₂O (l) → 3Ca(OH)₂ (aq) + 2PH₃ (g)

   • Equation with Dilute Hydrochloric Acid: Ca₃P₂ (s) + 6HCl (aq) → 3CaCl₂ (aq) + 2PH₃ (g)

2. From White Phosphorus (P₄) with Sodium Hydroxide: Pure phosphine can be prepared by heating white phosphorus with a concentrated solution of sodium hydroxide in an inert atmosphere (e.g., CO₂). This is a disproportionation reaction.

   • Equation: P₄ (s) + 3NaOH (aq) + 3H₂O (l) → PH₃ (g) + 3NaH₂PO₂ (aq) (Sodium Hypophosphite)

Properties

• Colorless gas, highly poisonous.
• Sparingly soluble in water.
• Weakly basic; reacts with acids to form phosphonium compounds (e.g., PH₄I). PH₃ (g) + HI (g) → PH₄I (s)
• Burns in air to give metaphosphoric acid. 2PH₃ (g) + 4O₂ (g) → P₂O₅ (s) + 3H₂O (l) (P₂O₅ then reacts with water to form HPO₃) 2PH₃ (g) + 4O₂ (g) → 2HPO₃ (aq) + 2H₂O (l) (More accurately, 2PH₃ + 4O₂ → P₂O₅ + 3H₂O. P₂O₅ + H₂O → 2HPO₃) Or simpler combustion: 2PH₃ (g) + 3O₂ (g) → P₂O₃ (s) + 3H₂O (l) (if limited oxygen)

Phosphorus Pentachloride (PCl₅)

Phosphorus pentachloride is a pale yellow, deliquescent solid, commonly used as a chlorinating agent in organic chemistry.

Chemical Formula

PCl₅

Preparation Process

1. From White Phosphorus (P₄) and Excess Chlorine (Cl₂): When white phosphorus reacts with an excess of dry chlorine gas, phosphorus pentachloride is formed.

   • Equation: P₄ (s) + 10Cl₂ (g) → 4PCl₅ (s)

2. From Phosphorus Trichloride (PCl₃) and Chlorine (Cl₂): Phosphorus trichloride reacts readily with chlorine to form phosphorus pentachloride.

   • Equation: PCl₃ (l) + Cl₂ (g) → PCl₅ (s)

Properties

• Pale yellow to yellowish-white crystalline solid.
• Deliquescent (absorbs moisture from the air).
• Sublimes on heating.
• Undergoes thermal dissociation into PCl₃ and Cl₂.

Exam-Relevant Reactions

1. Hydrolysis (Reaction with Water): PCl₅ reacts with water differently depending on the temperature.

   • Cold water (partial hydrolysis): PCl₅ (s) + H₂O (l) → POCl₃ (l) + 2HCl (g) (Phosphoryl chloride or Phosphorus oxychloride)

   • Hot water (complete hydrolysis): PCl₅ (s) + 4H₂O (l) → H₃PO₄ (aq) + 5HCl (aq) (Orthophosphoric acid)

2. Reaction with Organic Compounds (as a Chlorinating Agent):

   • With Alcohols (e.g., Ethanol): C₂H₅OH (l) + PCl₅ (s) → C₂H₅Cl (l) + POCl₃ (l) + HCl (g) (Chloroethane)

   • With Carboxylic Acids (e.g., Acetic Acid): CH₃COOH (l) + PCl₅ (s) → CH₃COCl (l) + POCl₃ (l) + HCl (g) (Acetyl chloride)

3. Thermal Decomposition: On heating, PCl₅ dissociates into phosphorus trichloride and chlorine gas. This reaction is reversible.

   • Equation: PCl₅ (s) ⇌ PCl₃ (l) + Cl₂ (g)

Oxoacids of Phosphorus

Phosphorus forms a number of oxoacids, where phosphorus is centrally bonded to oxygen atoms, some of which may be protonated (–OH groups). The key features are the oxidation state of P, the number of P–OH bonds (determining basicity), and the presence of P=O and P–H bonds.

Orthophosphoric Acid (H₃PO₄)

• Chemical Formula: H₃PO₄
• Oxidation State of P: +5
• Basicity: Tribasic (contains three P–OH bonds)
• Structure: Tetrahedral phosphorus with one P=O and three P–OH bonds.

      O
      ||
    HO—P—OH
      |
      OH

• Preparation: By heating phosphorus pentoxide (P₄O₁₀) with hot water: P₄O₁₀ (s) + 6H₂O (l) → 4H₃PO₄ (aq) Also prepared commercially by reacting phosphate rock with sulfuric acid.

Phosphorous Acid (H₃PO₃)

• Chemical Formula: H₃PO₃
• Oxidation State of P: +3
• Basicity: Dibasic (contains two P–OH bonds and one P–H bond)
• Structure: Tetrahedral phosphorus with one P=O, two P–OH bonds, and one P–H bond. The P–H bond is responsible for its reducing properties.

      O
      ||
    HO—P—H
      |
      OH

• Preparation: By dissolving phosphorus trioxide (P₄O₆) in cold water or by hydrolysis of phosphorus trichloride (PCl₃). PCl₃ (l) + 3H₂O (l) → H₃PO₃ (aq) + 3HCl (aq)

Comparative Properties of Important Phosphorus Compounds

Property	Phosphine (PH₃)	Phosphorus Pentachloride (PCl₅)	Orthophosphoric Acid (H₃PO₄)	Phosphorous Acid (H₃PO₃)
Formula	PH₃	PCl₅	H₃PO₄	H₃PO₃
Physical State	Colorless gas	Pale yellow solid	Colorless viscous liquid	Colorless crystalline solid
Smell	Rotten fish	Pungent, irritating	Odorless	Garlic-like (pure), Odorless (solution)
Nature	Weakly basic	Lewis acid, Chlorinating agent	Strong acid (tribasic)	Medium strong acid (dibasic), Reducing agent
Oxidation State of P	-3	+5	+5	+3
Hybridization of P	sp³	sp³d (gas/liquid), sp³d² & sp³ (solid ion: [PCl₄]⁺[PCl₆]⁻)	sp³	sp³
Key Bonds	3 P–H	5 P–Cl	1 P=O, 3 P–OH	1 P=O, 2 P–OH, 1 P–H
Reactivity	Ignites spontaneously (impure), Forms PH₄⁺	Hydrolyzes, Chlorinating agent, Thermal dissociation	Forms phosphate esters and salts	Reducing agent, Forms phosphite salts