Oxoacids of Phosphorus

Oxoacids of Phosphorus: Comprehensive Notes

Target Exams: JAM, JEE, NEET, GATE, CSIR-NET, and other competitive exams. These notes cover classification, structures, preparation, properties, reactions, and important comparisons. Focus on key reactions, acidity trends, and oxidizing/reducing behavior for exam-oriented preparation.

Introduction

Oxoacids of phosphorus are compounds containing phosphorus, oxygen, and hydrogen, where at least one hydrogen is attached to oxygen (acidic hydrogen). Phosphorus forms a variety of oxoacids due to its ability to expand its octet and form P-O-P linkages. The oxidation states of P in these acids range from +1 to +5.

Key Features:

• General formula: H_nPO_m or dimeric forms like H₄P₂O_n.
• They exhibit tautomerism (e.g., phosphorous acid exists as HPO(OH)₂).
• Acidity depends on the number of -OH groups; more -OH means stronger acid.
• Many act as reducing or oxidizing agents based on P oxidation state.
• Important for exams: Structures (tetrahedral/octahedral), disproportionation reactions, and salt formation.

Exam Tip: Remember the number of acidic hydrogens equals the number of P-OH bonds, not P-H bonds (which are non-acidic).

Classification of Oxoacids

Based on structure and oxidation state:

Oxoacid	Formula	Oxidation State of P	Basicity (No. of Acidic H)	Nature
Hypophosphorous acid	H3PO2	+1	1 (Monobasic)	Reducing
Phosphorous acid	H3PO3	+3	2 (Dibasic)	Reducing
Orthophosphoric acid	H3PO4	+5	3 (Tribasic)	Weakly oxidizing
Pyrophosphoric acid	H4P2O7	+5	4 (Tetrabasic)	Weakly oxidizing
Hypophosphoric acid	H4P2O6	+4	4 (Tetrabasic)	Reducing
Metaphosphoric acid	(HPO3)n	+5	1 per unit (Monobasic for n=1)	Weakly oxidizing
Peroxomonophosphoric acid	H3PO5	+5	2 (Dibasic)	Strong oxidizing
Peroxodiphosphoric acid	H4P2O8	+5	4 (Tetrabasic)	Strong oxidizing

Exam Tip: Acidity order: H₃PO₂ (pka 1.3) > H₃PO₃ (pka 1.5) > H₃PO₄ (pka 2.1) (due to decreasing % of acidic H and inductive effect).
For GATE/CSIR: Focus on pKa values and tautomerism.

Detailed Notes on Major Oxoacids

1. Hypophosphorous Acid (H₃PO₂)

Structure: Tetrahedral; P has one =O, two -OH (but tautomerizes to H-P(=O)(OH)₂ with one acidic H and two P-H bonds).

• Actual form: Monobasic due to one P-OH.

Preparation:

1. From white P:
   P₄ + 3NaOH + 2H₂O → PH₃ + 3NaH₂PO₂.
2. From PCl₃:
   PCl₃ + 3H₂O → H₃PO₂ + 3HCl.

Properties:

• Colorless, oily liquid; soluble in water.
• Strong reducing agent: Reduces AgNO₃ to Ag, HgCl₂ to Hg, etc.
• Disproportionates on heating: 4H₃PO₂ → PH₃ + 3H₃PO₄.
• Forms salts like NaH₂PO₂ (used in electroless plating).

Reactions:

• With halogens:
  H₃PO₂ + Cl₂ → H₃PO₄ + 2HCl (oxidation to +5).
• With diazonium salts:
  Reduces to benzene (ArN₂⁺ + H₃PO₂ → ArH + N₂ + H₃PO₃).
• Strong reducing agent due to the two P-H bonds.
  4H₃PO₂ → PH₃ + 3H₃PO₄

Uses: Reducing agent in organic synthesis; in pharmaceuticals.

Exam Tip (JEE/GATE): Emphasize its monobasic nature and strong reducing power due to two P-H bonds.

2. Phosphorous Acid (H₃PO₃)

Structure: Tetrahedral; H-P(=O)(OH)₂ (two acidic H from P-OH, one P-H).

Preparation:

1. From PCl₃:
   PCl₃ + 3H₂O → H₃PO₃ + 3HCl.
2. From P₄O₆:
   P₄O₆ + 6H₂O → 4H₃PO₃.

Properties:

• White crystalline solid; deliquescent; m.p. 74°C.
• Dibasic; forms phosphites like Na₂HPO₃.
• Reducing agent: Reduces I₂ to HI, Ag⁺ to Ag.
• Disproportionates: 4H₃PO₃ → 3H₃PO₄ + PH₃ (on heating).

Reactions:

• With strong oxidants: Oxidized to H₃PO₄.
• Dehydration:
  2H₃PO₃ → H₄P₂O₅ + H₂O (pyrophosphorous acid).
• Strong reducing agent due to the P-H bonds.
  4H₃PO₃ → PH₃ + 3H₃PO₄

Uses: Antioxidant in food; reducing agent.

Exam Tip (CSIR): Tautomerism: Exists mainly as P(OH)₃ in gas phase but HPO(OH)₂ in solid/aqueous.

3. Orthophosphoric Acid (H₃PO₄)

Structure: Tetrahedral; P(=O)(OH)₃ (three acidic H).

Preparation:

1. Laboratory:
   P₄O₁₀ + 6H₂O → 4H₃PO₄.
2. Industrial (Wet process):
   Ca₃(PO₄)₂ + 3H₂SO₄ → 2H₃PO₄ + 3CaSO₄.
3. Thermal process:
   P₄ + 5O₂ → P₄O₁₀, then hydrate.

Properties:

• Colorless, viscous liquid; tribasic (pKa: 2.1, 7.2, 12.7).
• Weak acid; forms phosphates: NaH₂PO₄, Na₂HPO₄, Na₃PO₄.
• Mild oxidizing agent: Oxidizes HBr/HI to Br₂/I₂.

Reactions:

• Dehydration:
  2H₃PO₄ → H₄P₂O₇ + H₂O (at 250°C); further to (HPO₃)_n.
• With metals:
  Forms H₂ with active metals.
• With Ca(OH)₂:
  Forms hydroxyapatite in fertilizers.

Uses: Fertilizers (85%), detergents, food additive (E338), rust remover.

Exam Tip (JEE): Buffer systems: H₂PO₄^-/HPO₄^2- in biological systems.

4. Pyrophosphoric Acid (H₄P₂O₇)

Structure: (HO)₂P(=O)-O-P(=O)(OH)₂ (P-O-P linkage; four acidic H).

Preparation: Dehydration of H₃PO₄:
2H₃PO₄ → H₄P₂O₇ + H₂O.

Properties:

• White crystalline solid; tetrabasic.
• Hydrolyzes to H₃PO₄.
• Forms pyrophosphates like Na₄P₂O₇ (used in detergents).

Reactions: Similar to H₃PO₄ but stronger acid due to P-O-P.

Uses: Water softener, catalyst.

5. Hypophosphoric Acid (H₄P₂O₆)

Structure: (HO)₂P(=O)-P(=O)(OH)₂ (P-P bond; four acidic H).

Preparation:
2P + 4NaClO₂ + 2H₂O → H₄P₂O₆ + 2NaCl

Properties:

• Colorless solid; tetrabasic.
• Reducing agent; stable P-P bond.

Reactions: Disproportionates to H₃PO₃ + H₃PO₄ on heating.

Exam Tip: Distinguish from pyrophosphoric: P-P vs P-O-P bond.

6. Metaphosphoric Acid (HPO₃)

Structure: Cyclic or polymeric (HPO₃)_n (e.g., trimetaphosphoric: ring structure).

Preparation: Strong heating of H₃PO₄ or H₄P₂O₇.

Properties:

• Glassy solid; monobasic per unit.
• Hydrolyzes slowly to H₃PO₄.

Uses: In dental cements; as dehydrating agent.

7. Peroxo Acids

Peroxomonophosphoric Acid (H₃PO₅): Structure: (HO)₂P(=O)-O-OH (peroxy linkage).

• Preparation:
  H₃PO₄ + H₂O₂ ⇌ H₃PO₅ + H₂O
• P₂O₅ + 2H₂O₂ + H₂O → 2H₃PO₅
• Strong oxidant; dibasic.

Peroxodiphosphoric Acid (H₄P₂O₈): (HO)₂P(=O)-O-O-P(=O)(OH)₂.

• Preparation: Electrolysis of KHSO₄ + H₃PO₄.
• Strong oxidant; tetrabasic.

Exam Tip (GATE/CSIR): Peroxo acids have O-O bond; stronger oxidants than regular oxoacids.

Comparisons and Trends

• Thermal Stability: H₃PO₄ > H₃PO₃ > H₃PO₂.
• Reducing Power: H₃PO₂ > H₃PO₃ > H₃PO₄ (inversely proportional to oxidation state).
• Acidic Strength: For same oxidation state, increases with more O atoms; overall: Peroxo > Ortho > Pyro > Meta.
• Disproportionation: Common in +3 and +1 states to +5 and lower (e.g., PH₃).

Property	H3PO2	H3PO3	H3PO4
P-H Bonds	2	1	0
Reducing Nature	Strong	Moderate	Weak
Basicity	1	2	3

Important Reactions for Exams

• Action of heat on oxoacids and their salts (e.g., NH₄H₂PO₄ → (NH₄PO₃)_n).
• Conversion cycles: P → P₄O₁₀ → H₃PO₄ → phosphates.
• Test for phosphates: Yellow ppt with (NH₄)₂MoO₄ in HNO₃.

Preparation Strategy: Draw structures repeatedly; practice reactions with oxidants/reductants. For JAM/CSIR: Focus on mechanisms; for JEE: Numerical on equivalents/acidity.

Oxides of Phosphorous MCQs
NEET, JEE, JAM, GATE, CSIR