Halides of Phosphorus: Structure, Properties, Reactions

Halides of Phosphorus: Chlorine is commonly used in swimming pools, as you are probably aware. Halides of Phosphorus, on the other hand, can be utilised as a cleaning and bleaching agent. Have you ever tried to understand the meaning of phosphorus halides?

A phosphorus halide can be described as a compound formed when phosphorus reacts with halogen. In this article, we will provide detailed information about the halides of phosphorus. Continue reading to learn more!

Define Phosphorus Halide

A phosphorus halide is a compound formed when phosphorus reacts with halogen. Phosphorus forms two types of halides, \(\rm{PX}_3\) and \(\rm{PX}_5\). Here, \(\rm{X}\) is nothing but halides such as fluorine, chlorine, bromine, and iodine. However, the main types of phosphorus halide are phosphorus trichloride and phosphorus pentachloride.

Phosphorus Trichloride (PCl₃)

Phosphorus trichloride is an inorganic compound with the chemical formula \(\rm{PCl}_3\).

Preparation of Phosphorus Trichloride

In the laboratory, phosphorus trichloride can be prepared by passing dry chlorine gas overheated red phosphorus.

\({{\text{P}}_{\text{4}}}{\text{ + 6C}}{{\text{l}}_{\text{2}}} \to {\text{4PC}}{{\text{l}}_{\text{3}}}\)

I. It is also prepared by treating phosphorus with thionyl chloride.

\({{\text{P}}_{\text{4}}}{\text{ + 8SOC}}{{\text{l}}_{\text{2}}} \to {\text{4PC}}{{\text{l}}_{\text{3}}}{\text{ + 4S}}{{\text{O}}_{\text{2}}}{\text{ + 2}}{{\text{S}}_{\text{2}}}{\text{C}}{{\text{l}}_{\text{2}}}\)

Structure of Phosphorus Trichloride

\(\rm{P}\) in \(\rm{PCl}_3\) is \(\rm{sp}^3\)-hybridized. Three of its \(\rm{sp}^3\)- orbitals overlap with the p-orbitals of three chlorine atoms to form three \(\rm{P-Cl}\), sigma-bonds, while the fourth orbital contains a lone pair of electrons. Therefore, \(\rm{PCl}_3\) is pyramidal. The \(\rm{Cl-P-Cl}\) bond angle in \(\rm{PCl}_3\) is, however, greater \((100.4^\circ)\) due to the steric crowding of the two \(\rm{Cl}\) atoms.

Properties of Phosphorus Trichloride

Let’s discuss the physical and chemical properties of Phosphorus Trichloride in this section

Physical Properties of Phosphorus Trichloride

The physical properties of phosphorus trichloride are as follows:

1. Phosphorus trichloride is a colourless liquid with a pungent smell. The boiling point of phosphorus trichloride is \(349\;\rm{K}\).
2. It has a pungent odour.
3. The melting point of phosphorus trichloride is (161\;\rm{K}\)

Chemical Properties of Phosphorus Trichloride

The chemical properties of phosphorus trichloride are as follows:

1. It reacts readily with water and forms phosphorus acid and hydrochloric acid.
   \({\text{PC}}{{\text{l}}_3} + 3{{\text{H}}_2}{\text{O}} \to {{\text{H}}_3}{\text{P}}{{\text{O}}_3} + 3{\text{HCl}}\)

2. When it reacts with concentrated sulphuric acid, chlorosulphonic acid is formed.
\(\mathop {2{\text{HO}} – {\text{SO}}_2 – {\text{OH}}}\limits_{{\text{Sulphuric}}{\mkern 1mu} {\text{acid}}}  + {\text{PCl}}_3 \to \mathop {{\text{HO}} – {\text{S}}{{\text{O}}_2} – {\text{Cl}}}\limits_{{\text{Chlorosulphonic}}{\mkern 1mu} {\text{acid}}}  + {\text{HP}}{{\text{O}}_3} + {\text{S}}{{\text{O}}_2} + 2{\text{HCl}}\)

3. It acts as a reducing agent. For example, it reduces Sulphur trioxide to Sulphur dioxide and sulphuryl chloride to Sulphur dioxide.
\({\text{PC}}{{\text{l}}_{\text{3}}}{\text{ + S}}{{\text{O}}_{\text{3}}} \to {\text{POC}}{{\text{l}}_{\text{3}}}{\text{ + S}}{{\text{O}}_{\text{2}}}\)
\({\text{PC}}{{\text{l}}_{\text{3}}}{\text{ + }}\mathop {{\text{S}}{{\text{O}}_2}{\text{C}}{{\text{l}}_2}}\limits_{{\text{Sulphuryl}}\,{\text{chloride}}} \to {\text{PC}}{{\text{l}}_5}{\text{ + S}}{{\text{O}}_{\text{2}}}\)

4. Phosphorus trichloride reacts with Grignard reagents to form substituted phosphine. For example,
\({\text{PC}}{{\text{l}}_{\text{3}}}{\text{  +  }}\mathop {{\text{3}}{{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{MgCl}}}\limits_{{\text{Phenylmag}}{\mkern 1mu} {\text{chloride}}}  \to \mathop {{\text{P}}{{\left( {{{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}} \right)}_{\text{3}}}}\limits_{{\text{Triphenyl}}{\mkern 1mu} {\text{phosphine}}} {\text{ +  3MgC}}{{\text{l}}_{\text{2}}}\)

5. Phosphorus trichloride on heating with finely divides metals forms metal chlorides. For example,
\(12{\text{Ag}} + 4{\text{PC}}{{\text{l}}_3}\xrightarrow{\Delta }12{\text{AgCl}} + {{\text{P}}_4};\,6{\text{Na}} + {\text{PC}}{{\text{l}}_3}\xrightarrow{\Delta }3{\text{NaCl}} + {\text{N}}{{\text{a}}_3}{\text{P}}\)

6. Phosphorus trichloride reacts with organic compounds containing hydroxyl groups and replaces the hydroxyl groups with chlorine atoms. For example,

\(\mathop {3{{\text{C}}_2}{{\text{H}}_5}{\text{OH}}}\limits_{{\text{Ethyl}}{\mkern 1mu} {\text{alcohole}}}  + {\text{PC}}{{\text{l}}_3} \to \mathop {3{{\text{C}}_2}{{\text{H}}_5}{\text{Cl}}}\limits_{{\text{Ethyl}}{\mkern 1mu} {\text{chloride}}}  + \mathop {{{\text{H}}_3}{\text{P}}{{\text{O}}_3}}\limits_{{\text{Phosphorus}}{\mkern 1mu} {\text{acid}}} \)
\(\mathop {3{\text{C}}{{\text{H}}_3}{\text{COOH}}}\limits_{{\text{Acetic}}{\mkern 1mu} {\text{acid}}}  + {\text{PC}}{{\text{l}}_3} \to \mathop {3{\text{C}}{{\text{H}}_3}{\text{COCl}}}\limits_{{\text{Acetyl}}{\mkern 1mu} {\text{chloride}}}  + \mathop {{{\text{H}}_3}{\text{P}}{{\text{O}}_3}}\limits_{{\text{Phosphorus}}{\mkern 1mu} {\text{acid}}} \)

7. It reacts with chlorine and forms phosphorus pentachloride.
\({\text{PC}}{{\text{l}}_{\text{3}}}{\text{ + C}}{{\text{l}}_{\text{2}}} \to {\text{PC}}{{\text{l}}_{\text{5}}}\)

8. It reacts with oxygen to give phosphorus oxytrichloride.
\({\text{2PC}}{{\text{l}}_{\text{3}}}{\text{ + }}{{\text{O}}_{\text{2}}} \to {\text{2POC}}{{\text{l}}_{\text{3}}}\)

Uses of Phosphorus Trichloride

Phosphorus trichloride is widely used as an important reagent in organic chemistry for replacing hydroxyl groups with chlorine atoms in organic reactions.

\(\mathop {3{{\text{C}}_2}{{\text{H}}_5}{\text{OH}}}\limits_{{\text{Ethyl}}{\mkern 1mu} {\text{alcohole}}}  + {\text{PC}}{{\text{l}}_3} \to \mathop {3{{\text{C}}_2}{{\text{H}}_5}{\text{Cl}}}\limits_{{\text{Ethyl}}{\mkern 1mu} {\text{chloride}}}  + \mathop {{{\text{H}}_3}{\text{P}}{{\text{O}}_3}}\limits_{{\text{Phosphorus}}{\mkern 1mu} {\text{acid}}} \)

\(\mathop {3{\text{C}}{{\text{H}}_3}{\text{COOH}}}\limits_{{\text{Acetic}}{\mkern 1mu} {\text{acid}}}  + {\text{PC}}{{\text{l}}_3} \to \mathop {3{\text{C}}{{\text{H}}_3}{\text{COCl}}}\limits_{{\text{Acetyl}}{\mkern 1mu} {\text{chloride}}}  + \mathop {{{\text{H}}_3}{\text{P}}{{\text{O}}_3}}\limits_{{\text{Phosphorus}}{\mkern 1mu} {\text{acid}}} \)

Phosphorus Pentachloride

Phosphorus pentachloride is a chemical compound with the formula \(\rm{PCl}_5\). It is a very important phosphorus chloride.

Preparation of Phosphorus Pentachloride

The preparation of Phosphorus Pentachloride are as follows:

I. Phosphorus pentachloride may be prepared by the action of sulphuryl chlorides on phosphorus or phosphorus trichloride.
\({{\text{P}}_4} + \mathop {10{\text{S}}{{\text{O}}_2}{\text{C}}{{\text{l}}_2}}\limits_{{\text{Suphuryl}}{\mkern 1mu} {\text{chloride}}}  \to 4{\text{PC}}{{\text{l}}_5} + 10{\text{S}}{{\text{O}}_2}\)
\({\text{PC}}{{\text{l}}_3} + \mathop {{\text{S}}{{\text{O}}_2}{\text{C}}{{\text{l}}_2}}\limits_{{\text{Suphuryl}}{\mkern 1mu} {\text{chloride}}}  \to {\text{PC}}{{\text{l}}_5} + {\text{S}}{{\text{O}}_2}\)

ii. Laboratory Preparation: In the laboratory, it is prepared by the action of an excess of dry chlorine on phosphorus trichloride.
\(\mathop {{\text{PC}}{{\text{l}}_3}}\limits_{{\text{Phosphorus}}{\mkern 1mu} {\text{trichloride}}} {\text{ +  C}}{{\text{l}}_2} \to \mathop {{\text{PCl}}_5}\limits_{{\text{Phosphorus}}{\kern 1pt} {\text{pentachloride}}} \)
The apparatus used is depicted in Fig. It is made up of a wide-mouthed flask with a dropping funnel and two tubes. With the help of a dropping funnel, phosphorus trichloride is slowly dropped into the flask.
Through one of the two tubes fitted into the flask’s mouth, a current of pure and dry chlorine is passed into the flask. The other tube is an outlet for the used gas. Azing mixture is used to cool the flask. Phosphorus trichloride reacts with chlorine to form solid phosphorus pentachloride. The chlorine that is not used escapes through the outlet.

Structure of Phosphorus Pentachloride

In \(\rm{PCl}_5\), phosphorus undergoes \(\rm{sp}^3\rm{d}\)-hybridization and has a trigonal bipyramidal geometry in gaseous and liquid states. It has three \(\rm{P-Cl}\) bonds that are equatorial and two \(\rm{P-Cl}\) bonds that are axial. Since two axial \(\rm{P-Cl}\) bonds are repelled by three bond pairs, while three equatorial bonds are repelled by two bond pairs. Therefore, axial bonds are longer \((240\;\rm{pm})\) than equatorial bonds \((202\;\rm{pm})\).

Properties of Phosphorus Pentachloride

The physical and chemical properties of Phosphorus Pentachloride are explained below:

Physical Properties of Phosphorus Pentachloride

Although pure phosphorus pentachloride is nearly colourless, impure samples are typical greenish-yellow in colour. It has a pungent smell.

Chemical Properties of Phosphorus Pentachloride

The chemical properties of Phosphorus Pentachloride is as follows:

I. Dissociation: When phosphorus pentachloride is heated, it sublimes but decomposes on stronger heating into \(\rm{PCl}_3\) and \(\rm{Cl}_2\).
\({\text{PC}}{{\text{l}}_{\text{5}}} \rightleftharpoons {\text{PC}}{{\text{l}}_{\text{3}}}{\text{ + C}}{{\text{l}}_{\text{2}}}\)

II. The action of water: 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.

III. Action on metals: When heated, it reacts with finely divided metals to produce the corresponding chlorides.
For example,
\({\text{PC}}{{\text{l}}_{\text{5}}}{\text{ + Zn}} \to {\text{PC}}{{\text{l}}_{\text{3}}}{\text{ + ZnC}}{{\text{l}}_{\text{2}}}\)
\({\text{PC}}{{\text{l}}_5} + 2{\text{Cu}} \to {\text{PC}}{{\text{l}}_3} + 2{\text{CuCl}}\)
\({\text{PC}}{{\text{l}}_5} + 2{\text{Ag}} \to {\text{PC}}{{\text{l}}_3} + 2{\text{AgCl}}\)
\({\text{PC}}{{\text{l}}_5} + {\text{Cd}} \to {\text{PC}}{{\text{l}}_3} + {\text{CdC}}{{\text{l}}_2}\)

IV. Reaction with sulphuric acid: It reacts with concentrated sulphuric acid to form chlorosulphonic acid.
\({\text{PC}}{{\text{l}}_5} + {{\text{H}}_2}{\text{S}}{{\text{O}}_4} \to \mathop {{\text{Cl}}.{\text{S}}{{\text{O}}_3}{\text{H}}}\limits_{{\text{Chloro}}\,{\text{sulphonic}}\,{\text{acid}}} + {\text{POC}}{{\text{l}}_3} + {\text{HCl}}\)

V. Reaction with Sulphur dioxide: It reacts with sulphur dioxide to form thionyl chloride.
\({\text{PC}}{{\text{l}}_5} + {\text{S}}{{\text{O}}_2} \to \mathop {{\text{SOC}}{{\text{l}}_2}}\limits_{{\text{Thionyl}}\,{\text{chloride}}} + {\text{POC}}{{\text{l}}_3}\)

VI. Reaction with hydroxyl compounds: It reacts with compounds, particularly organic compounds containing hydroxy \((-\rm{OH})\) groups, and replaces the \(-\rm{OH}\) groups with chlorine atoms. For example,
\(\mathop {{\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{OH}}}\limits_{{\text{Ethyl}}\,{\text{alcohol}}} + {\text{PC}}{{\text{l}}_5} \to \mathop {{\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{Cl}}}\limits_{{\text{Ethyl}}\,{\text{chloride}}} + {\text{POC}}{{\text{l}}_3} + {\text{HCl}}\)
\(\mathop {{\text{C}}{{\text{H}}_3}{\text{COOH}}}\limits_{{\text{Acetic}}\,{\text{acid}}} + {\text{PC}}{{\text{l}}_5} \to \mathop {{\text{C}}{{\text{H}}_3}{\text{COCl}}}\limits_{{\text{Acetyl}}\,{\text{chloride}}} + {\text{POC}}{{\text{l}}_3} + {\text{HCl}}\)
\(\mathop {{\text{HO}} – {\text{S}}{{\text{O}}_2} – {\text{OH}}}\limits_{{\text{Sulphuric}}\,{\text{acid}}} + 2{\text{PC}}{{\text{l}}_5} \to \mathop {{\text{Cl}} \cdot {\text{S}}{{\text{O}}_2}{\text{Cl}}}\limits_{{\text{Sulphuryl}}\,{\text{chloride}}} + 2{\text{POC}}{{\text{l}}_3} + 2{\text{HCl}}\)
\(\mathop {{\text{N}}{{\text{O}}_2} \cdot {\text{OH}}}\limits_{{\rm{Nitric}}\,{\rm{acid}}} + {\text{PC}}{{\text{l}}_5} \to \mathop {{\text{N}}{{\text{O}}_2}{\text{Cl}}}\limits_{{\text{Nitrosyl}}\,{\text{chloride}}} + {\text{POC}}{{\text{l}}_3} + {\text{HCl}}\)

VII. Reaction with diphosphorus decaoxide, diphosphorus decasulphide and sulphur dioxide: The reactions occurs as follows:
\(6{\text{PC}}{{\text{l}}_5} + {{\text{P}}_4}{{\text{O}}_{10}} \to \mathop {10{\text{POC}}{{\text{l}}_3}}\limits_{{\text{Phosphorus}}\,{\text{oxychloride}}} ;\,6{\text{PC}}{{\text{l}}_5} + \mathop {{{\text{P}}_4}{{\text{S}}_{10}}}\limits_{{\text{Phosphorus}}\,{\text{pentasulphide}}} \to \mathop {10{\text{PSC}}{{\text{l}}_3}}\limits_{{\text{Phosphorus}}\,{\text{triochloride}}} \)
\({\text{PC}}{{\text{l}}_5} + {\text{S}}{{\text{O}}_2} \to {\text{POC}}{{\text{l}}_3} + \mathop {{\text{SOC}}{{\text{l}}_2}}\limits_{{\text{Thionyl}}\,{\text{chloride}}} \)

VIII. Reaction with chloride ion acceptors: With chloride ion acceptors like boron trichloride and niobium tetrachloride, it forms additional compounds containing tetrachloride \([\rm{PCl}_4]^+\) species.
\({\text{PC}}{{\text{l}}_5} + {\text{BC}}{{\text{l}}_3} \to {\left[ {{\text{PC}}{{\text{l}}_4}} \right]^ + }{\left[ {{\text{BC}}{{\text{l}}_4}} \right]^ – };{\text{PC}}{{\text{l}}_5} + {\text{NbC}}{{\text{l}}_4} \to {\left[ {{\text{PC}}{{\text{l}}_4}} \right]^ + }{\left[ {{\text{NbC}}{{\text{l}}_5}} \right]^ – }\)

IX. Reaction with potassium fluoride: With \({\rm{KF}},\,{\rm{PCl}}_5\) forms potassium phosphorus hexafluoride, \({\rm{K}}^+[\rm{PF}_6]^-\).
\({\text{PC}}{{\text{l}}_5} + 6{\text{KF}} \to {{\text{K}}^ + }{\left[ {{\text{P}}{{\text{F}}_6}} \right]^ – } + 5{\text{KCl}}\)

Uses of Phosphorus pentachloride

In Organic chemistry, phosphorus pentachloride is widely used as a chlorinating agent, replacing \(-\rm{OH}\) groups with chlorine atoms.

Summary

Phosphorus halide is a compound formed when phosphorus reacts with halogen. Phosphorus forms two types of halides, \(\rm{PX}_3\) and \(\rm{PX}_5\). Phosphorus trichloride is an inorganic compound with the chemical formula \(\rm{PCl}_3\). Phosphorus pentachloride is a chemical compound with the formula \(\rm{PCl}_5\). It is a very important phosphorus chloride. Phosphorus trichloride is widely used as an important reagent in organic chemistry for replacing hydroxyl groups with chlorine atoms in organic reactions. In organic chemistry, phosphorus pentachloride is widely used as a chlorinating agent, replacing \(-\rm{OH}\) groups with chlorine atoms.

FAQs on Halides of Phosphorus

Q.1. What is the formula for phosphorus halide?
Ans: The formula for Phosphorus halide is \(\rm{PX}_3\) and \(\rm{PX}_5\). Where \(\rm{X = F},\,\rm{Br},\,\rm{Cl}\) and \(\rm{I}\).

Q.2. When alcohol reacts with phosphorus halides, what product is formed?
Ans: When phosphorus halides react with alcohol, an alkyl halide is formed. For example,
\(\mathop {3{{\text{C}}_2}{{\text{H}}_5}{\text{OH}}}\limits_{{\text{Ethyl}}\,{\text{alcohol}}} + {\text{PC}}{{\text{l}}_3} \to \mathop {3{{\text{C}}_2}{{\text{H}}_5}{\text{Cl}}}\limits_{{\text{Ethyl}}\,{\text{chloride}}} + \mathop {{{\text{H}}_3}{\text{P}}{{\text{O}}_3}}\limits_{{\text{Phosphorus}}\,{\text{acid}}} \)

Q.3. Is phosphorus halide a compound of phosphorus?
Ans: Yes, phosphorus halide is a compound of phosphorus.

Q.4. What is the function of phosphorus in the human body?
Ans: The primary function of phosphorus is the formation of bones and teeth.

Q.5. What type of halides does phosphorus form?
Ans: Phosphorus forms two types of halides, \(\rm{PX}_3\) and \(\rm{PX}_5\).

Q.6. Which halide of phosphorus does not exist?
Ans: Phosphorus pentaiodide \((\rm{PI}_5)\) does not exist.

Study About Allotropes of Phosphorus Here

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