A Short Note On Phosphorus Trichloride (PCl3)

Phosphorus trichloride is a colorless or slightly yellow fuming liquid that has a pungent and disagreeable odor similar to hydrochloric acid. Skin, eyes, and mucous membranes are severely burned. Ingestion, dermal absorption and Inhalation are all extremely dangerous. When it reacts with water, it produces hydrochloric acid, a corrosive and unpleasant gas that appears as white vapors. Used in the production of insecticides, surfactants, gasoline additives, plasticizers, dyestuffs, textile finishing agents, germicides, medical goods, and other chemicals, as well as in the electrodeposition of metal on rubber.

In the case of alcohols replaced by strong electronegative groups, such as 2-nitro alcohols, 2-amino alcohols, 2,2,2-trichloro-, or 2,2,2-trifluoroethanol, the action of phosphorus trichloride on 3 mol of an alcohol in the absence of a base to produce dialkyl H-phosphonates can fail.

Preparation

White phosphorus is heated in a stream of dry chlorine to produce phosphorus trichloride (PCl₃).

P₄ + 6Cl₂ → 4PCl₄

The retort is filled with dry white phosphorus that is slowly heated in a water bath. Over the phosphorus, a stream of pure, dry chlorine is led. The phosphorus trichloride that is generated is volatile and must be collected in a water-cooled receiver. The phosphorus trichloride dissolves the phosphorus. In the laboratory, it may be more convenient to employ the less dangerous red phosphorus.

As an impurity, PCl₅ is present in the phosphorus trichloride produced as described above. PCl₃ is distilled over white phosphorus to eliminate this. It’s a white or slightly yellow fuming liquid with a harsh, disagreeable odor similar to hydrochloric acid. The process takes place in phosphorus trichloride, which has been supplemented with elemental phosphorus. To react with dissolved phosphorus, chlorine is sprayed into the phosphorus trichloride.

Structure 

A PCl₃ molecule has a trigonal pyramidal form. One lone pair of electrons and three bond pairs of electrons make up the core P atom. Tetrahedral electron pair geometry and trigonal pyramidal molecular geometry emerge from sp³ hybridization.

Physical Characteristics

• Low-boiling colorless liquid
• It emits a pungent odor when exposed to moist air.

Chemical Properties 

Water hydrolyzes it violently, releasing phosphorus acid and hydrochloric acid gas. PCl₃ + 3H₂O → 3HCl + H₃PO₃ 

It reacts in a similar way with organic compounds that have a hydroxyl (OH) group, such as acids and alcohols. 

• PCl₃ + 3CH₃ COOH (Acetic Acid) → 3CH₃COCl + H₃PO₃ (Acetyl Chloride) 
• PCl₃ + 3C₂H₅OH (Ethyl alcohol) → 3 C₂H₅Cl + H₃PO₃ (Ethyl Chloride)

It forms phosphorus pentachloride when it interacts with chlorine or sulphuryl chloride. 

• PCl₃ + Cl₂ → PCl₅
• PCl₃ + SO₂Cl₂ → PCl₅ + SO₂

Phosphorus oxychloride is formed when it reacts with oxygen. 

• 2POCl₃ = 2PCl₃ + O₂

Phosphorus oxychloride and SO2 are formed when it reacts with SO₃.

• SO₃ + PCl₃ → POCl₃ + SO₂

Laboratory Preparation Should Be Done with Caution

• Skin, eyes, and mucous membranes are severely burned.
• The dangers of inhalation, ingestion, and cutaneous absorption are all quite real.
• When hydrogen chloride reacts with water, it produces hydrochloric acid.

Uses

 PCl₃ is essential as a precursor of PCl₅, POCl₃, and PSCl₃, which are utilized in herbicides, insecticides, plasticizers, oil additives, and flame retardants, among other things.

For example, oxidizing PCl₃ yields POCl₃, which is used to make triphenyl phosphate and tricresyl phosphate, both of which are employed as flame retardants and PVC plasticisers. They’re also employed in the manufacture of insecticides like diazinon. The herbicide glyphosate is a phosphonate.

PCl₃ is a precursor to triphenylphosphine, which is utilized in the Wittig reaction, as well as phosphite esters, which can be used as industrial intermediates or in the Horner-Wadsworth-Emmons reaction, all of which are essential techniques for producing alkenes. Although TOPO is normally created via the corresponding reaction, it can be used to make trioctylphosphine oxide (TOPO), which is utilized as an extraction agent.

Conclusion 

 The only possible instrumental method for on-line monitoring of ongoing interactions between phosphorus and chlorine in the production of phosphorus trichloride is Raman spectroscopy.

In this non-ideal, corrosive, and hazardous environment, no other optical spectroscopy technique can provide information on the components of interest – P₄, PCl₃, and PCl₅ – while ensuring ease of sampling.

The technique responds in a reasonable amount of time, providing useful feedback for process control and the ability to track the reaction’s progress. With a sensitivity of 1 percent by weight, it also reports the concentration of each of the components.