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Phosphorus Halides: Phosphorus Trichloride and Phosphorus Pentachloride

An Introduction to Halides of Phosphorus

The halides (or halogenides) of phosphorus are the elements phosphorus, phosphorus monoxide, phosphorus dioxide, phosphorus trioxide, and phosphorus pentoxide. Some of the elements have multiple forms (e.g., phosphorus and phosphorus) and are represented as such in this database.


The compounds with phosphorus are sometimes called organophosphates, with triphosphates, tetra phosphines, etc., being used. Organic compounds are often called organophosphine or organophosphine oxides (e.g., olefins) if they contain phosphorus; inorganic compounds are organophosphorus (e.g., inorganic phosphoric acid), organophosphates, or organophosphates.


All the compounds of phosphorus that were discovered by Lavoisier and Antoine Lavoisier are given in the table below (along with some compounds discovered since then). Phosphorus forms multiple binary compounds and has more organic phosphorus compounds than any other element. Phosphorus can be characterized by different parameters including the P to H ratio, covalent radius, bond valence, ionic radius, atomic radius, polarizability, and so on.


A phosphorus compound can also be called a phosphide. Phosphides are compounds with the general formula MPn. Phosphides are a group of compounds with strong chemical and physical properties. Phosphides are used in a range of applications including flame retardants, lubricants, alloys, and pigments for glass, paint, and inks.


The oxidation states of phosphorus are +5, +3, 0, and -3. Phosphorus is a chalcophile, and it prefers to coordinate with the groups of the elements with higher electronegativity than phosphorus.


Most halides of phosphorus have a monoclinic structure, while those of the higher oxidation state have a rhombohedral or orthorhombic structure.

Synthesis

Synthesis of phosphorus from its elemental form, P4 (tetraphosphorus), is an extremely difficult process. The simplest way of producing phosphorus is from phosphorus trichloride PCl3. The compound is volatile, making it a difficult substance to handle. The compound can also be produced by treating trimethyl phosphine with chlorine. The compound can also be produced by fusing phosphorus in solid argon at high temperatures. This method is used commercially.


A simple method for making phosphorus metal was discovered in the 19th century. Here, phosphorus metal is heated in a sealed bottle with a stream of hydrogen gas to create elemental phosphorus and is used to ignite phosphorus bombs. This method is used for commercial synthesis.

Compounds

All three phosphorus oxides are strong bases.


The compounds of phosphorus can be classified based on the number of phosphorus atoms they contain, and their oxidation states. Group, I compounds are a group of inorganic phosphorus compounds with two phosphorus atoms with a formal oxidation state of 0. Group II compounds are a group of inorganic phosphorus compounds with two phosphorus atoms with a formal oxidation state of 2. Group III compounds are a group of inorganic phosphorus compounds with four phosphorus atoms with a formal oxidation state of 3. Phosphorus in group III compounds can exist in three structural forms: tetrahedral, trigonal planar, or octahedral. The tetrahedral form has higher energy and is more stable than the other forms. P4 and higher polyphosphates are often denoted by the letters Pn where n is the atomic number, e.g., P4. Polyhalogenated phosphorus compounds are denoted by the prefixes PF, where F is the symbol for fluorine, SF, where S is the symbol for sulfur, and so on. For example, octal chlorophosphate (Cl2P8) is a polychloride or polychloride anion.


Most poly phosphorus compounds are cations (positive ions) of one type of polycation with anions. Poly Phosphorus compounds often are cationic. The highest valency at phosphorus is eight, which explains the prefixes PF, SF, PS, and P4. Many poly phosphorus compounds have high coordination numbers because of phosphorus's ability to form many covalent bonds to other atoms, allowing phosphorus to be easily chelated and single atom phosphorus of higher coordination number. The general formula for coordination polyphosphates (Pn)2(Xn)2(2+), where n is 2, 3, or 4 and Xn are metal ions, can also be denoted as MPn X 2n+2.

Properties

Many poly phosphorus compounds do not crystallize easily, but in some cases, crystallization is easy enough to obtain good crystals. As a result of difficulties in obtaining good crystals, and the structural chemistry of phosphorus, much of the available experimental and theoretical data for phosphorus compounds are obtained in solution. This is because many of the compounds are not stable at ambient temperatures. Solutions of many phosphorus compounds are acidic, as in the case of phosphoric acid (H3PO4).


In the case of poly phosphorus compounds, there are usually eight molecules of phosphorus-containing cations present. Most poly phosphorus compounds contain some phosphorus-oxygen bonds. The bond can be one or two single P-O bonds, one P-O-P single bond, or one P-O-P-O tetrahedral chain. Many poly phosphorus compounds do not have isolated atoms of phosphorus, but rather the oxygen atoms are bonded to multiple phosphorus atoms, especially when the polycation has a charge. Poly Phosphorus compounds have been studied extensively, but there are very few compounds that are purely non-metal polyanions, as most of the phosphorus-containing compounds have either no negative charge or carry multiple negative charges. The only other metal polyanion that has been extensively studied is the polyborate ion.


Poly Phosphorus compounds include organo phosphonium ions, organophosphates, organophosphates, and a few other compounds. Of these, the organic phosphonium ions are the most common, and the organophosphates are some of the most well-known compounds of this class of compound. Phosphine oxide is another example of a phosphorus-oxygen bond.

Phosphorus Halide

When phosphorus reacts with halide, it forms phosphorus halides. Halides of phosphorus are of two types: PX3 and PX5. X here is denoted for halogen and it can be fluorine, chlorine, bromine, and iodine.

 

The most common type of phosphorus halide is phosphorus trichloride (PCl3) and phosphorus pentachloride (formula of phosphorus pentachloride - PCl5).

Phosphorus Trichloride PCl3

  • Phosphorus trichloride is highly poisonous and is colourless in nature. 

  • The phosphorus trichloride formula (chemical) is PCl3.

  • This compound exhibits hybridization and is triangular pyramidal in shape.

Phosphorus Trichloride Structure

  • It consists of three bonded chlorine atoms to phosphorus and one lone pair.

  • Phosphorus in the structure of PCl3 shows sp3 hybridization. These three sp3 hybridized orbits of phosphorus overlap with the p orbital of each chlorine atom thus forming a three-sigma P-Cl bond.

  • The fourth sp3 hybridized orbital of phosphorus contains a lone pair.

 

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Preparation of Phosphorus Trichloride

  1. Phosphorus trichloride is formed when dry chlorine is passed over warmed white phosphorus.

 

P4   +   6Cl2   →   4PCl3

 

  1. Phosphorus trichloride is produced when thionyl chloride reacts with white phosphorus.

 

P    +     8SOCl2     →    4PCl3  +    4SO2   +    2S2Cl2

 

Chemical Properties

  1. Hydrolysis of phosphorus trichloride in the presence of moisture.

 

PCl3 +   3H2O   →   H3PO3  +   3HCl

 

  1. To produce its chloro-derivatives, it reacts with organic compounds containing the OH group.

 

3C2H5OH   +   PCl3   →   3C2H5Cl   +   H3PO3

 

Phosphorus Pentachloride(PCl5)

  • It is a yellowish-white powder in colour with a pungent smell.

  • It is very sensitive to water.

  • It is soluble in carbon tetrachloride, benzene, diethyl ether, and carbon disulfide.

  • The phosphorus pentachloride formula is PCl5

Structure of Phosphorus Pentachloride

  • Phosphorus pentachloride shape is trigonal bipyramidal (in liquid and gaseous form).

  • There are two axial bonds and three equatorial bonds in the structure of phosphorus pentachloride PCl5. There is more repulsion in axial bonds as compared to equatorial bonds. So, axial bonds are longer than the equatorial bonds in the structure.

 

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Note: Phosphorus pentachloride exists in ionic form in solid-state. Its ionic form consists of [PCl4]+[PCl6]-. The cation [PCl4]+ is tetrahedral while [PCl6]- is octahedral in shape.

 

Preparation:

  1. Phosphorus pentachloride is formed when white phosphorus reacts with an excess of dry chlorine.

 

P4    +    10Cl2    →    4PCl5

 

  1. By the reaction of SO2Cl2 and phosphorus, we can also produce it.

 

P4 + 10SO2Cl2  →      4PCl5 + 10SO2

 

Chemical Properties:

  1. Phosphorus pentachloride hydrolyzes into POCl3 in the presence of moist air. Over a period of time, this compound converts to phosphoric acid.

 

PCl5   +   H2O   →   POCl3   +   2HCl

 

 POCl3  +    3H2O   →   H3PO4    +     3HCl

 

  1. It sublimates and further disintegrates into phosphorus trichloride as we heat it.

 

PCl5    →   PCl3  +    Cl2

 

  1. Under the influence of heat, it reacts with finely partitioned metals to produce metal chlorides.

 

2Ag    +    PCl5    →    2AgCl    +    PCl3

 

  1. It reacts and produces its 'chloro' subordinates with natural compounds containing-OH group.

 

C2H5OH    +    PCl5   →    C2H5Cl   +    POCl3   +    HCl

 

Uses of Phosphorus Halides:

  • It is used as an agent for chlorinating.

  • It is used as an intermediate in the manufacturing of phosphorus acid, chloro-anhydrides, and phosphoric acid derivatives.

  • It is used for the manufacturing of organophosphate pesticides.

  • It is used in water treatments and paint additives.

FAQs on Phosphorus Halides: Phosphorus Trichloride and Phosphorus Pentachloride

1. Is phosphorus trichloride an acid or base?

According to Lewis’s acid-base theory, compounds that accept lone pairs of electrons are lewis acids, and the compound that donates lone pairs of electrons is Lewis base. Phosphorus trichloride structure contains three sp3 hybridized orbitals bonded with three chlorine atoms to form a sigma bond while one sp3 hybridized orbital contains a lone pair. So it can readily donate the lone pair of electrons and hence acts as a Lewis base.

2. How is PCl3 formed?

Through the reaction of chlorine with a reflux solution of white phosphorus in phosphorus trichloride, with continuous removal of PCl3 as it is formed, Phosphorus trichloride is industrially prepared. Using less toxic red phosphorus could be more convenient in the laboratory.

3. Is PCl a liquid? If yes, then explain.

Yes, it is a liquid since between the molecules there are only van der Waals dispersion forces and dipole-dipole attractions.


Note: There is a permanent dipole in the phosphorus(III) chloride molecule, so dipole-dipole attractions are feasible.

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