Halides and Oxides of Phosphorus

Halides and Oxides of Phosphorus – The p-Block Elements – Class 12

Halides of Phosphorus

 

Phosphorus forms two types of halides i.e., phosphorus trihalides (covalency of P =3) PX₃ (X = F, CI, Br, I) and phosphorus pentahalides PX₅ (X = F, Cl, Br) (covalency of P=5). 

 

With chlorine it forms:

(1) Phosphorus trichloride

(2) Phosphorus pentachloride.

 

Phosphorus Trichloride, PCl₃

Preparation of Phosphorus Trichloride

 

(1) It is prepared in the laboratory by passing dry chlorine gas over heated white phosphorus. The vapours of PCl3 distil over and are collected in a receiver cooled by water.

 

P₄ + 6Cl₂ ——> 4PCl₃

 

The receiver has a calcium chloride tube attached to it which protects it from the reactions of outside moisture. It can be purified by distilling over white phosphorus to remove the excess of chlorine.

 

(2) It can also be obtained by the reaction of thionyl chloride with white phosphorus.

 

P₄ + 8SOCl₂ —–> 4PCl₃ + 4SO₂ + 2S₂Cl₂

    Thionyl chloride

 

Structure of Phosphorus Trichloride

(1) Phosphorus in PCl₃ undergoes sp³ hybridisation. Three of the sp³ hybrid orbitals overlap with p-orbitals of chlorine to form three P-Cl σ bonds while the fourth sp³ hybrid orbital contains a lone pair of electrons. 

 

(2) PCl₃ has pyramidal structure. 

 

(3) The Cl P Cl bond angle in PCl₃ is 100.4° which is greater than HPH bond angle in PH₃ (93.6°). This is due to stearic crowding of two large Cl atoms in comparison to H atoms. 

 

(4) The P-Cl bond is much larger (240 pm) than P-H bond (142 pm) because of larger size of CI atom.

 

Properties of Phosphorus Trichloride

 

Physical Properties

 

(i) It is a colourless oily liquid fuming constantly is the moist air.

(ii) Its specific gravity is 1.6.

(iii) Its boiling point and freezing point values are 347 K and 161 K respectively.

(iv) It has a highly pungent smell.

 

Chemical Properties of Phosphorus Trichloride

 

(1) Action with water

It fumes in moist air and reacts with water violently to form phosphorus acid.

 

PCl₃ + 3H₂O ——> H3PO₃ + 3HCl

                    Phosphorous acid

(2) Action with atmospheric air or oxygen

 

It slowly combines with dry oxygen to form phosphorus oxychloride.

 

2PCl₃ + O₂ ——> 2POCl₃

                      Phosphorous oxychloride

 

 

(3) Action with sulphur trioxide

 

It reacts with sulphur trioxide to form phosphorus oxychloride.

 

PCl₃ + SO₃ —–> POCl₃ + SO₂

               Phosphorous oxychloride

 

(4) Action with chlorine or sulphur monochloride

 

It combines with chlorine or sulphur monochloride to form phosphorus pentachloride.

 

PCl₃ + Cl₂ —–>PCl₅ + SO₂

 

PCl₃ + S₂Cl₂ —-> PCl₅ + 2PSCl₃

 

(5) Action with organic compounds

 

It reacts with organic compounds containing -OH group such as acetic acid, ethyl alcohol, etc.

 

(i) With organic acid, it forms acid chlorides.

 

3CH₃COOH + PCl₃ ——> 3CH₃COCl + H₃PO₃

              Acetic acid                  Acetyl chloride Phosphorus acid

 

(ii) With alcohols, it forms alkyl chlorides.

 

3C₂H₅OH + PCl₃ ——> 3C₂H₅ +  H₃PO₃

 

(6) It is readily oxidized to the important phosphorus (V) derivatives PCl5, POCl₃ and PSCl₃. It is oxidized by As₂O₃ to P₂O₅.

 

(7) PCl₃ undergoes many substitution reactions and is main source of organophosphorus compounds.

 

Uses

It is mainly employed in the organic chemistry as an important reagent to replace the hydroxyl group (-OH) by chlorine atom in organic reactions.

 

Phosphors Pentachloride, PCl₅

 

 

Preparation of Phosphors Pentachloride

 

(1) Phosphorus pentachloride is prepared by the reaction of white phosphorus with excess of dry chlorine.

 

P₄ + 10Cl₂ —–> 4 PCl₅

(2) It can also be obtained by the reaction of dry chlorine on phosphorus trichloride.

 

PCl₃ + Cl₂ ——> PCl₅

 

(3) Phosphorus pentachloride can also be prepared by the action of SO₂Cl₂ on phosphorus.

 

P4 + 10 SO₂Cl_2. ——–> 4PCl₅ + 10SO₂

 

Structure Phosphors Pentachloride

(1) In PCl5,  phosphorus undergoes sp³d hybridisation and has trigonal bipyramidal structure. 

 

(2) It has three equatorial P-Cl bonds and two axial P-Cl bonds which are different.

Therefore, all the five P-Cl bonds are not equal. The axial bonds are larger than equatorial

bonds as :

P-Cl (axial) = 219 pm

P-Cl (equatorial) = 204 pm

 

(3) In gaseous and liquid phases, PCl₅ has trigonal bipyramidal structure. However, in the solid state, it exists as an ionic solid [PCl₄]⁺ [PCl₆]¯. In this, the cation [PCl₄]⁺ is tetrahedral and the anion [PCl₆]¯ is octahedral. 

 

(4) In solution, it exists either molecular or ionically dissociated depending on the nature of the solvent.

 

Physical Properties of Phosphorus Pentachloride, PCl₅

 

(i) It is a yellowish white powder having pungent smell.

 

(ii) It fumes in moist air due to its strong affinity for water.

 

(iii) On heating, it sublimes at 433 K and can be melted (m.p. 318 K) only by heating it under pressure.

 

Chemical properties of Phosphorus Pentachloride, PCl₅

 

(1) Stability

PCl₅ is thermally less stable than PCl₃. On heating it sublimes but decomposes on stronger heating into trichloride and chlorine.

 

PCl₅ ⇔ PCl₃

(2) Action with water

In moist air, it hydrolyses to POCl₃ and finally gets converted to phosphoric acid.

PCl₅ + H₂O —-> POCl₃ + 2HCl

               Phosphorus oxychloride

POCl₃ + 3 H₂O —> H₃PO₄ + 3HCl

 

(3) Action with compounds containing OH group: It reacts with compounds containing OH group (-OH) to give the corresponding chloro compounds in which each-OH group is replaced by a chlorine atom. 

 

For example

(i) With organic acid 

CH₃COOH +PCl₅ ——> CH₃COCl + POCl₃ + HCl

 

(ii) with ethyl alcohol 

C₂H₅OH + PCl₅ —–> C₂H₅Cl + POCl₃ + HCl

 

(iii) With sulphuric acid

OH-SO₂-OH + 2PCl₅ —-> ClSO₂Cl + 2 POCl₃ + 2HCl

 

(4) Reaction with phosphorus pentoxide and sulphur dioxide

 

PCl₅ reacts with P₄O₁₀ forming phosphorus oxychloride and with SO₂ forming phosphorus oxychloride and thionyl chloride.

 

6PCl₅ + P₄O10 ——> 10 POCl₃

                  Phosphorus oxychloride

 

PCl₅ + SO₂ ——> POCl₃ + SOCl₂

 

(5) Reaction with phosphorus pentasulphide

PCl₅ reacts with phosphorus pentasulphide to form phosphorus thiochloride.

6PCl₅ + P₄S₁₀ ——> 10 PSCl₃

 

(6) Reaction with metals

PCl5 reacts with finely divided metals on heating to give corresponding chlorides.

 

2Ag + PCl₅ ——> 2 AgCl + PCl₃

Sn + 2PCl₅ —–> SnCl₅ + 2PCl₃

 

(7) Reduction

Phosphorus pentachloride is reduced with hydrogen to form PCl₃ 

PCl₅ + 2H ——-> PCl₃ + 2HCl

 

 

(8) Reaction with potassium fluoride

With KF, PCl5 forms potassium phosphorus hexafluoride, K+[PF₆]¯ + 5 KCl

 

Uses of Phosphorus Pentachloride

It is extremely useful in organic reactions to replace a hydroxyl group (-OH) by chlorine atom such as for the synthesis of C₂H₅Cl, CH₃COCl etc.

 

Oxides of Phosphorus

Phosphorus forms two common oxides namely
(i) phosphorus trioxide (P₄O₆) and  (ii) Phosphorus Pentaoxide (P₄0₁₀)

 

(1) Phosphorus (III) oxide (P₄O₆)

 

Preparation of Phosphorus Trioxide

Phosphorus trioxide is formed when phosphorus is burnt in a limited supply of air.

P₄ + 3O₂ (limited) ———-> P₄O₆

 

Properties of Phosphorus Trioxide

(i) Phosphorus (III) oxide is a crystalline solid with garlic odour.

 

(ii)  It is soluble in carbon disulphide, ether and chloroform.

 

(iii) Heating in air: On heating in air, it forms phosphorous pentaoxide.

P₄O₆ + 2 O₂ —–> P₄O₁₀

 

(iv) Action of water: It dissolves in cold water to give phosphorous acid.

P₄O₆ + 6H₂O (hot) —–>4H₃PO₄

 

It is, therefore, considered as anhydride of phosphorous acid. With hot water, it gives phosphoric acid and inflammable phosphine.

 

P₄O₆+ 6H₂O (hot) ——> 3H₃PO₄ + PH₃

 

(v) Action with Chlorine

It reacts vigorously with chlorine to form a mixture ‘of phosphoryl chloride and metaphosphoryl chloride.

 

P4O₆ + 4Cl₂ —->2POCl₃ + 2PO₂Cl

            Phosphoryl chloride                   Metaphosphoryl chloride

                        

 

 

Structure of Phosphorus Trioxide

(1) Each atom of phosphorus in P₄O₆ is present at the corner of a tetrahedron. 

(2) Each phosphorus atom is covalently bonded to three oxygen atoms and each oxygen atom is bonded to two phosphorus atoms. 

(3) The six oxygen atoms lie along the edges of the tetrahedron of P atoms.

 

Phosphorus (V) Oxide (P₄O₁₀)

 

 

Preparation of Phosphorus Oxide

It is prepared by heating white phosphorus in excess of air.

P₄ + 5O₂ (excess) → P₄O₁₀

 

Properties of Phosphorus Oxide

(i) It is snowy white solid.

(ii) Action with water

 

It readily dissolves in cold water forming metaphosphoric acid

P₄O_{10 +} 2H₂O (cold) —–> 4HPO₃

                                 Metaphosphoric acid

 

With hot water it gives phosphoric acid.

P₄O_{10 +} 6H₂O (hot) —–>  4H₃PO₄

                                    Phosphoric acid

 

(iii) Dehydrating Nature

Phosphorus pentaoxide has strong affinity for water and therefore, acts as a powerful dehydrating agent. It extracts water from many organic and inorganic compounds including sulphuric acid and nitric acid.

 

H₂SO₄ ——-> SO₃

2HNO₃ ——-> N₂O₅

2HClO₄ —-> Cl₂O₇

CH₃CONH₂ ——-> CH₃CN

 

Structure of Phosphorus Oxide

Its structure is similar to that of P4O10 . In addition, each phosphorus atom forms a double bond with oxygen atom.