POCl3 (Phosphorus Oxychloride)

What is POCl3?

POCl3 is an inorganic compound which is commonly referred to as phosphorous oxychloride or as 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 evolves as fumes).

The synthesis of POCl3 on an industrial scale is done by reacting phosphorus trichloride (PCl3) with oxygen or phosphorus pentoxide (P2O5). The primary application of this compound is in the preparation of certain phosphate esters (including tricresyl phosphate). It is important to note that POCl3 is not soluble in water since it reacts with it to form hydrogen chloride and phosphoric acid.

Preparation of POCl3

The oxidation of PCl3 (phosphorus trichloride) with dioxygen 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.

POCl3 Structure

Structure of POCl3

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 274.4K and 378.9K 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 (titanium tetrachloride, for example).
  • In the presence of Lewis acid catalysts, POCl3 can be reacted 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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