Phosphorus Halides - Structure, Preparation, Properties, Uses, Practice Problems and FAQ

Phosphorus halides - You've probably come across this term while reading about the p-block elements in your chemistry textbook.

So, what do you think they're worth? Allow me to make things easier for you.

The gorgeously clean and blue swimming pools you've probably seen are actually cleaned with one of the best chlorinating agents - phosphorus halides!

Apart from that, it has a variety of applications. As a result, it is critical to explore the exciting possibilities that these phosphorus halides offer!

TABLE OF CONTENTS

• What are Phosphorus Halides? 
• Structure of Phosphorus Trichloride (PCl_3-)
• Preparation of Phosphorus Trichloride (PCl_3-)
• Physical Properties of Phosphorus Trichloride (PCl_3-)
• Chemical Properties and Reactions of Phosphorus Trichloride (PCl_3-)
• Uses of Phosphorus Trichloride (PCl_3-)
• Structure of Phosphorus Pentachloride (PCl_5-)
• Preparation of Phosphorus Pentachloride (PCl_5-)
• Physical, Chemical Properties and Reactions of Phosphorus Pentachloride (PCl_5-)
• Uses of Phosphorus Pentachloride (PCl_5-)
• Practice Problems
• Frequently Asked Questions - FAQ

What are Phosphorus Halides?

Phosphorus reacts with halogens to form phosphorus halides which are trivalent and pentavalent 

(PX₃ and PX₅) except for iodine. Owing to the large ionic radii of I^- phosphorus does not form Pl₅.

The most pronouncedly utilised and produced phosphorus halides are phosphorus trichloride, PCl₃ and phosphorus pentachloride, PCl₅.

Structure of Phosphorus Trichloride (PCl₃)

Phosphorus trichloride is an oily, colourless liquid that is highly toxic in nature. P in PCl₃ is SP³ hybridised. Three of the SP³ orbitals overlap with the unhybridised p-orbitals of the chlorine atoms forming sigma-bonds, while the fourth orbital contains a lone pair of electrons. Hence, it attains a pyramidal shape. 

The Cl - P - Cl  bond angle is greater than usual owing to the steric crowding of the adjacent Cl atoms.

Preparation of Phosphorus Trichloride (PCl₃)

• On passing dry chlorine gas over heated white phosphorus, phosphorus trichloride is produced.

• Thionyl chloride reacts with white phosphorus to produce phosphorus trichloride.

Physical properties of Phosphorus Trichloride (PCl₃)

The physical properties of phosphorus trichloride are:

• Phosphorus trichloride is a colourless liquid or sometimes a yellow fuming liquid having a very pungent and irritating smell. 
• The melting point of PCl₃ is 349 K.
• The melting point of PCl₃ is 161 K.
• Its molecular geometry is trigonal pyramidal.
• It is soluble in water.

Chemical Properties and Reactions of Phosphorus Trichloride (PCl₃)

The chemical properties of PCl₃ are as follows:

• Phosphorus trichloride on further chlorination produces phosphorus pentachloride.

• Phosphorus trichloride readily reacts with water and forms phosphorus acid along with hydrochloric acid.

• On reacting with concentrated H₂SO₄, chlorosulphonic acid is produced.

• PCl₃ is a reducing agent. Sulphur trioxide is reduced to sulphur dioxide and sulphuryl chloride to sulphur dioxide.

• It reacts with Grignard reagent to produce substituted phosphines. For example, it forms triphenylphosphine with phenyl magnesium chloride.

• Phosphorus trihalides on reacting with finely divided reactive metals give metal chlorides.

PCl₃ on reacting with organic compounds having (- OH) replaces the hydroxyl groups with chlorine atoms and phosphorus acid is formed as a byproduct.

• It reacts with oxygen to give phosphorus oxytrichloride.

Uses of Phosphorus Trichloride (PCl₃)

• Phosphorus trichloride is extensively used for making phosphorus oxychloride by oxidising it with oxygen.
• It is a significant intermediate used in the production of phosphate ester-based insecticides.
• It is used as a chlorinating agent for converting alkyl alcohols to alkyl chlorides and organic acids to organic acid chlorides and a catalyst.
• It is useful in producing important compounds like phosphorus pentachloride, phosphoryl chloride, thiophosphoryl chloride, pseudohalogens and phosphonic acids.

Structure of Phosphorus Pentachloride (PCl₅)

• Phosphorus pentachloride is a yellowish-white compound with a salt-like structure in the crystalline state and partly dissociated in solution, especially in polar solvents such as nitrobenzene. 
• It is soluble in carbon tetrachloride, carbon disulfide, benzene, and diethyl ether. 
• It has a trigonal bipyramidal structure in gaseous and liquid phases. 
• In the solid-state, it exists as an ionic solid, [PCl4]⁺[PC₆l]^-  in which the cation, [PCl4]⁺ is tetrahedral and the anion, [PCl₆]^- is octahedral.
• In PCl₅, phosphorus is SP³d hybridised. The 5 hybridised orbitals of phosphorus overlap with p orbitals of chlorine atoms forming 5 sigma bonds. Its molecular geometry is trigonal bipyramidal. 
• Due to greater repulsion at axial positions as compared to that of the equatorial positions, the two axial bonds (242 pm) are longer and hence weaker than the equatorial bonds (202 pm).

Preparation of Phosphorus pentachloride (PCl₅)

Phosphorus pentachloride can be prepared in the following ways:

• Phosphorus pentachloride is prepared by reacting sulphuryl chlorides with phosphorus or phosphorus trichloride.

• In the laboratory, it can be prepared by passing excess dry chlorine on phosphorus trichloride.

Physical, Chemical Properties and Reactions of Phosphorus pentachloride (PCl₅)

The Physical properties of phosphorus pentachloride can be enlisted as follows:

The chemical properties can be visualised in terms of the various chemical reactions that it can undergo. Phosphorus exists in +5 oxidation state in PCl₅.

• PCl₅ sublimes on heating, but on strong heating it undergoes dissociation.

• Phosphorus pentachloride reacts violently with water. Phosphorus oxychloride is formed when there is insufficient water, whereas phosphoric acid is formed when there is an excess of water.

• On heating PCl₅ with finely divided metals, the corresponding metal chlorides are produced.

• It reacts with concentrated sulphuric acid to form chlorosulphonic acid.

• Phosphorus pentachloride reacts with sulphur dioxide to give thionyl chloride.

• Phosphorus pentachloride reacts with organic compounds with - OH groups and replaces these groups with chlorine atoms.

• It undergoes reactions with P₄O₁₀,  P₄S₁₀ and sulphur dioxide in the following manner:

• On reacting with KF, phosphorus pentachloride forms potassium phosphorus hexafluoride.

• With some Lewis acids or chloride ion accepting species like boron trichloride and niobium tetrachloride, it produces additional compounds containing tetrachloride species [PCl₄]⁺.

Uses of Phosphorus pentachloride

• PCl₅ acts as a chlorinating agent and as a catalyst in preparing organic chemicals, intermediates, dye-based stuff, etc.
• It acts as a catalyst in the manufacture of acetyl cellulose, which is basically the plastic film on which motion pictures are printed.
• PCl₅ is a chlorinating agent in organic chemistry.
• It is widely used in the manufacture of penicillin and cephalosporin in the pharmaceutical industry.
• It is used in the production of acid chlorides.
• It acts as a catalyst for cyclisation and condensation reactions.

Practice Problems 

Q. 1. Is PCl₃ acidic or basic?

Answer: PCl₃ has three SP³ hybridised orbitals bonded with the three chlorine atoms to form a sigma bond while one SP³ hybridised orbital contains a lone pair of electrons. Hence, it can easily donate the lone pair of electrons and therefore it acts as a Lewis base.

Q. 2. What is observed when alcohol comes in contact with PCl₅?

Answer: Organic alcohols readily react with solid phosphorus pentachloride forming clouds of hydrogen chloride gas.

Q. 3. Why does solid phosphorus halide behave as an ionic compound?

Answer: In the solid-state, phosphorus pentahalide exists as an ionic compound, [PCl4]⁺[PC₆l]^-  in which the cation, [PCl4]⁺ is tetrahedral and the anion, [PC₆l]^- is octahedral.

Hence, there is a strong force of attraction between these two ions, which also makes it a non-conductor of heat and electricity in solid-state.

Q. 4. Why are the axial bonds in PCl₅ longer than the equatorial bonds?

Answer:  PCl₅ is SP³d hybridised, with 5 hybridised orbitals of phosphorus overlapping with p orbitals of chlorine atoms forming 5 sigma bonds. Its molecular geometry is trigonal bipyramidal. The two axial p - Cl bonds are longer than the three equatorial bonds due to the higher repulsion at hub positions compared to central positions.

Q. 5. Which is more stable, PCl₅or PCl₃?

Answer: PCl₅ readily dissociates on strong heating and produces PCl₃. Hence, PCl₃ is more stable.

Frequently Asked Questions - FAQ

Why does phosphorus pentaiodide not exist?

Answer: Due to the large ionic radii of iodide ions, the formation of phosphorus pentaiodide would lead to so much steric crowding and hence, is highly unstable. This is why it does not exist.

Why do pentahalides have greater strength of covalent bonding than trihalides?

Answer: Due to the greater polarisation of the sigma bonds in the pentahalide state as compared to the trihalide state, pentahalides are more covalent than trihalides.

Are phosphorus halides, compounds of phosphorus?

Answer: Yes, phosphorus halides are basically PX₃(trihalide) or PX₅(pentahalide) which are formed by the reaction of halogens with phosphorus.

Related Topics to Phosphorous Halides in Chemistry

• Oxygen
• Phosphorus
• Ozone
• Allotropes of Phosphorous

NCERT Class 12 Chemistry Chapters