Phosphorus Halides 

1.0Introduction

Phosphorus halides are compounds formed when phosphorus reacts with halogens. There are mainly two types of phosphorus halides: PX₃ and PX₅, where X stands for a halogen such as fluorine (F), chlorine (Cl), bromine (Br), or iodine (I). However, iodine is an exception—due to its large ionic radius, phosphorus does not form phosphorus pentaiodide (PI₅).

Among these, phosphorus chlorides are the most common and widely studied. These halides are generally covalent and exhibit a range of chemical and physical properties depending on the halogen involved.

2.0Phosphorus Trichloride (PCl₃)

Phosphorus trichloride (PCl₃) is a colorless to pale yellow fuming liquid with a sharp, pungent, and irritating odor. It is highly toxic and reacts vigorously with water, releasing fumes of hydrogen chloride (HCl).

Structure of Phosphorus Trichloride (PCl₃)

In PCl₃, the phosphorus atom is sp³ hybridized. Out of the four hybrid orbitals:

• Three orbitals form σ-bonds with the p-orbitals of chlorine atoms.
• The fourth orbital contains a lone pair of electrons.

Due to the presence of this lone pair, the molecule adopts a trigonal pyramidal geometry, rather than a perfect tetrahedral shape. The presence of bulky chlorine atoms causes steric repulsion, resulting in a slightly greater Cl–P–Cl bond angle than typical trigonal pyramidal structures.

Physical Properties of Phosphorus Trichloride (PCl₃)

Molecular Geometry: Trigonal pyramidal

• Boiling Point: 349 K
• Melting Point: 161 K
• State: Liquid at room temperature
• Solubility: Reacts with water to form phosphorus acid (H₃PO₃) and hydrogen chloride (HCl)
  PCl₃+3H₂O→H₃PO3+3HCl

3.0Preparation of Phosphorus Trichloride (PCl₃)

• From White Phosphorus and Dry Chlorine Gas

 Phosphorus trichloride is prepared by passing dry chlorine gas over heated white phosphorus:

P₄+6Cl₂→4PCl₃

This reaction must be carried out in a dry and inert atmosphere, as PCl₃ is highly reactive with moisture.

• From Thionyl Chloride (SOCl₂) and White Phosphorus

Phosphorus trichloride can also be synthesized by reacting thionyl chloride (SOCl₂) with white phosphorus:

P₄  + 8SOCl₂   →  4PCl₃  +  4SO₂ +  S₂Cl₂

This method is helpful in laboratory preparation and also yields sulfur dioxide and disulfur dichloride as by-products.

4.0Chemical Properties of Phosphorus Trichloride (PCl₃)

Phosphorus trichloride (PCl₃) is a reactive and versatile compound that undergoes several chemical reactions due to the presence of electrophilic phosphorus and labile chlorine atoms. Its major chemical properties are described below:

These reactions highlight the versatile chemical behaviour of phosphorus trichloride in organic synthesis, redox reactions, and industrial applications.

1. Reaction with Chlorine

Phosphorus trichloride undergoes further chlorination to form phosphorus pentachloride (PCl₅):

• PCl₃+Cl₂→PCl₅

2. Reaction with Water (Hydrolysis)

PCl₃ reacts vigorously with water, forming phosphorus acid (H₃PO₃) and hydrogen chloride (HCl):

• PCl₃+3H₂O→H₃PO₃+3HCl

3. Reaction with Concentrated Sulfuric Acid

With concentrated H₂SO₄, PCl₃ forms chlorosulphonic acid (ClSO₃H):

• PCl₃+H₂SO₄→ClSO₃H + other products

4. Reducing Nature

PCl₃ acts as a reducing agent in various reactions:

• It reduces sulfur trioxide (SO₃) to sulfur dioxide (SO₂).
• It also reduces sulfuryl chloride (SO₂Cl₂) to SO₂.
• SO₃+PCl₃→SO₂+other products
• SO₂Cl₂+PCl₃→SO₂+ other products

5. Reaction with Grignard Reagents

PCl₃ reacts with Grignard reagents to form substituted phosphines. For example, with phenyl magnesium chloride (C₆H₅MgCl), it forms triphenylphosphine:

• PCl₃+3C₆H₅MgCl → P(C₆H₅)₃+3MgCl₂

6. Reaction with Reactive Metals

Phosphorus trichloride reacts with finely divided reactive metals (like sodium or magnesium) to produce metal chlorides:

• PCl₃+Metal → Metal chloride + phosphoruscompounds 

7. Reaction with Organic Hydroxy Compounds

PCl₃ reacts with organic compounds containing hydroxyl groups (-OH), replacing -OH with chlorine atoms and forming phosphorus acid as a byproduct:

• 3ROH + PCl₃ → 3RCl + H₃PO₃

8. Reaction with Oxygen

When exposed to oxygen, phosphorus trichloride gets oxidized to phosphorus oxytrichloride (POCl₃):

• PCl₃+O₂→POCl₃

5.0Uses of Phosphorus Trichloride (PCl₃)

• Used to produce phosphorus oxychloride (POCl₃) by oxidation.
• Serves as an intermediate in making phosphate ester-based insecticides.
• Acts as a chlorinating agent to convert alcohols to alkyl chlorides and acids to acyl chlorides.
• Used in the synthesis of compounds like PCl₅, POCl₃, PSCl₃, pseudohalogens, and phosphonic acids.
• Also functions as a catalyst in various chemical reactions.

6.0Phosphorus Pentachloride (PCl₅)

Phosphorus pentachloride is a yellowish-white crystalline solid that is highly sensitive to moisture. It readily dissolves in organic solvents such as carbon tetrachloride (CCl₄), benzene, carbon disulfide (CS₂), and diethyl ether.

Structure:

In the gaseous and liquid states, phosphorus pentachloride (PCl₅) exists as a covalent molecule with a trigonal bipyramidal geometry.

The phosphorus atom in PCl₅ is sp³d hybridised, utilising five hybrid orbitals to form five sigma (σ) bonds with the chlorine atoms.

The structure has:

• Three equatorial P–Cl bonds in one plane at 120° to each other.
• Two axial P–Cl bonds are perpendicular to the equatorial plane.

Due to greater electron repulsion at axial positions, the axial bonds (242 pm) are longer and weaker than the equatorial bonds (202 pm).

Solid State:

• In the solid state, PCl₅ exists as an ionic compound:
  [PCl₄]⁺   [PCl₆]^–

Here, the [PCl₄]⁺ cation has a tetrahedral geometry, while the [PCl₆]^– anion is octahedral.

Physical Properties

• Appearance: Yellowish-white crystalline compound.
• Solubility:
• • Soluble in non-polar organic solvents such as carbon tetrachloride (CCl₄), carbon disulfide (CS₂), benzene, and diethyl ether.
  • Partially dissociates in polar solvents like nitrobenzene.
• Nature in Solid State: Salt-like structure due to its ionic form.
• Reactivity: Sensitive to moisture; hydrolyzes easily in the presence of water.

7.0Preparation of Phosphorus Pentachloride (PCl₅)

The following methods can prepare phosphorus pentachloride:

1. From White Phosphorus and Dry Chlorine

Phosphorus reacts with an excess of dry chlorine gas to form phosphorus pentachloride:

• P₄+10C_l2→4PCl₅

2. From White Phosphorus and Sulphuryl Chloride (SO₂Cl₂)

PCl₅ can also be prepared by reacting white phosphorus with sulphuryl chloride:

• P₄+10SO₂Cl₂→4PCl₅+10SO₂​

These reactions should be carried out in a dry and controlled environment, as PCl₅ is highly reactive and moisture-sensitive.

8.0Chemical Properties of Phosphorus Pentachloride (PCl₅)

Phosphorus pentachloride exhibits a range of chemical behaviors due to its high reactivity, especially in the presence of moisture and organic functional groups.

1. Hydrolysis in Moist Air

PCl₅ readily reacts with water in moist air. The hydrolysis occurs in two steps:

• First, it forms phosphoryl chloride (POCl₃) and hydrogen chloride:

 PCl₅+H₂O→POCl₃+2HCl

• On further hydrolysis, POCl₃ is converted to phosphoric acid (H₃PO₄):

 POCl₃+3H₂O→H₃PO₄+3HCl

2. Thermal Decomposition

Upon heating, PCl₅ sublimes (converts directly from solid to gas). Under strong heating, it decomposes into phosphorus trichloride (PCl₃) and chlorine gas:

PCl₅  → PCl₃ + Cl₂​

3. Reaction with Metals

When heated with finely divided reactive metals (e.g., silver), PCl₅ reacts to form metal chlorides and phosphorus trichloride:

2Ag + PCl₅ → 2AgCl  + PCl₃

4. Reaction with Organic Hydroxy Compounds

PCl₅ reacts with organic compounds containing hydroxyl groups (-OH), replacing the –OH with a chlorine atom and forming chloro derivatives:

Example with ethanol:

C₂H₅OH + PCl₅ → C₂H₅Cl + POCl₃+HCl

This reaction is widely used in organic synthesis for the chlorination of alcohols and acids.

9.0Uses of Phosphorus Pentachloride (PCl₅)

• Chlorinating Agent: PCl₅ is widely used in organic chemistry for converting –OH and –COOH groups into chloro derivatives, especially in the preparation of acid chlorides.
• Catalyst in Organic Synthesis: It acts as a catalyst in various cyclisation and condensation reactions, aiding in the formation of complex organic compounds.
• Pharmaceutical Industry: PCl₅ plays a crucial role in the manufacture of antibiotics such as penicillin and cephalosporin.
• Plastic and Film Industry: It is used as a catalyst in the production of acetyl cellulose, a material used for making plastic films, including those used in motion picture reels.
• Intermediate in Dye and Chemical Production: PCl₅ serves as an important reagent and catalyst in the preparation of organic intermediates, dyes, and other fine chemicals.