POCl₃ (Phosphorus Oxychloride)

What is POCl₃?

“POCl₃ is an inorganic compound which is commonly referred to as phosphorous oxychloride or phosphoryl chloride”.

At room temperature & pressure, this compound is a colourless liquid that forms fumes in moist air. These fumes are released because POCl₃ undergoes hydrolysis in the presence of moisture to yield phosphoric acid and hydrogen chloride (which evolve as fumes).

The synthesis of POCl₃ on an industrial scale is done by reacting phosphorus trichloride (PCl₃) with oxygen or phosphorus pentoxide (P₂O₅). It is important to note that POCl₃ is not soluble in water since it reacts with it to form hydrogen chloride and phosphoric acid.

Table of Contents

• Preparation of POCl₃
• Structure of POCl₃
• Properties of POCl₃
• Uses of POCl₃

Preparation of POCl₃

The oxidation of PCl₃ (phosphorus trichloride) with oxygen yields phosphorus oxychloride as the product. The chemical equation for this reaction is provided below.

2PCl₃ + O₂ → 2POCl₃

Alternately, this compound can also be prepared from phosphorus trichloride by treating it with potassium chlorate. The chemical equation is:

3PCl₃ + KClO₃ → KCl + 3POCl₃

POCl₃ can also be prepared by reacting phosphorus pentachloride with phosphorus pentoxide. The balanced chemical equation for this reaction is given by:

P₄O₁₀ + 6PCl₅ → 10POCl₃

Structure of POCl₃

POCl₃ molecules feature 3 phosphorus-chlorine single bonds and one phosphorus-oxygen double bond. This molecule assumes a tetrahedral shape. The structure of a POCl₃ molecule is illustrated below.

It can be noted that the P=O bond is much stronger than the P-Cl bond. The bond length of the P=O bond is 158 pm whereas that of the P-Cl bond is 202 pm. It can also be noted that the bond dissociation energy of the P=O bond is approximately 533.5 kJ.mol^-1.

Properties of POCl₃

Physical Properties

• The molar mass of POCl₃ is 153.33 grams per mole.
• This colourless liquid has a musty, pungent odour.
• The density of POCl₃ in its liquid state is 1.645 grams per cubic centimetre.
• The melting and boiling point of this compound corresponds to 274.4 K and 378.9 K respectively. Therefore, the liquid range of phosphorus oxychloride is quite similar to that of water.

Chemical Properties

• POCl₃ is highly soluble in chloroform, benzene, carbon tetrachloride, and carbon disulfide.
• When treated with an excess of some phenols or alcohols, this compound yields organophosphates (or phosphate esters).
• This compound can behave like a Lewis base by forming adducts with several Lewis acids for example, titanium tetrachloride.
• In the presence of Lewis acid catalysts, POCl₃ reacts with HBr to obtain POBr₃.

Uses of POCl₃

• POCl₃ is widely used in the industrial manufacture of phosphate esters. Some important examples include the synthesis of triphenyl phosphate and tricresyl phosphate.
• It is also used in the semiconductor industry as a safe source of liquid phosphorus in diffusion processes.
• POCl₃ is a commonly used dehydrating agent in chemical laboratories. It is used as a dehydrating agent in the preparation of nitriles from primary amides.
• It is also used as a dehydrating agent in the Bischler-Napieralski reaction.
• Phosphorus oxychloride is also used in the preparation of Vilsmeier’s reagent, which is an integral part of the Vilsmeier-Haack reaction.

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