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Group 15: Phosphorus - Halides

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Group 15: Phosphorus - Halides - Lesson Summary

Phosphorous forms 2 types of halides, namely, PX 3 and PX 5. Here, X represents a halogen, which may be fluorine, chlorine, bromine or iodine. Of these, chlorides are more common. These chlorides are covalent in nature.

Preparation of PCl3:
In the laboratory, phosphorus trichloride is prepared by heating white phosphorus in a current of dry chlorine.

     P 4               Δ→            6Cl 2        +                 4PCl 3
   White                           Chlorine                     Phosphorus
        Phosphorus                                                tricloride

Phosphorus trichloride can also be prepared by the action of white phosphorus on thionyl chloride.

     P 4                  +      8SOCl 2        Δ→            4PCl 3        +        4SO 2    +     2S 2Cl 2  
    White                       Tionyl                      Phosphorus          Sulphur      Disulphur
     Phosphorus              Chloride                   tricloride              dioxide         dichloride

Structure of PCl3:
The central phosphorus atom in PCl 3 undergoes sp3 hybridisation with 3 bond pairs and 1 lone pair of electrons. On account of this, it has a pyramidal shape. It acts as a Lewis base due to its ability to donate its lone pair of electrons to other electron-deficient ions or molecules.

Properties of PCl3:
It is a colourless liquid with a pungent smell. It has a very low boiling point of 74 0c. It reacts violently with water and gets hydrolysed to form phosphorus acid and fumes of hydrochloric acid.

      PCl 3              +                3H 2O          →      H 3PO 3       +          3HCL
Phosphorus                          Moisture                   Phosphorous               Hydrochloric
      trichloride                                                                     acid                          acid

It reacts with organic compounds that contain hydroxyl groups such as acetic acid and ethyl alcohol to form phosphorus acid as the main product.

PCl 3            +    3CH 3COOH       → H 3PO 3                + 3CH 3COCl
Phosphorus         Acetic acid            Phosphorous          Acetyl chloride
 trichloride                                                     acid

PCl 3            +    3C 2H 5OH       → H 3PO 3                + 3C 2H 5Cl
Phosphorus         Ethyl            Phosphorous          Ethyl chloride
 trichloride              Alcohol                    acid

Preparation of PCl5:
In the laboratory, PCl 5 is generally prepared by passing excess chlorine into a solution of phosphorus trichloride in carbon tetrachloride.
                                             CCl 4
PCl 3                +      Cl 2            →            PCl 5
Phosphorus            Chlorine                      Phosphorus
    trichloride                                              pentachloride

Properties of PCl5:
It is a yellowish-white crystalline powder. On strong heating it dissociates into phosphorus trichloride and chlorine.

PCl 5                 →        PCl 3             +      Cl 2
Phosphorus                 Phosphorus          Chlorine
 pentachloride                  trichloride

Phosphorus pentachloride reacts violently with excess of water to form phosphoric acid.

                                      Complete Hydrolysis
     PCl 5        +       4H 2O            →             H 3PO 4       +         5HCl
Phosphorus               Excess                     Phosphorus            Hydrochloric
     pentachloride          Water                                    acid              acid

PCl 5 reacts with organic compounds that contain hydroxyl groups. It replaces the hydroxyl groups in the compounds with chlorine atoms. EX: The hydroxyl groups in ethyl alcohol and acetic acid are replaced by chlorine atoms to form chloro derivatives, namely, ethyl chloride and acetyl chloride.

PCl 5               +       C 2H 5OH           →       C 2H 5Cl     +        POCl 3       +    HCl
Phosphorus                 Ethyl                         Ethyl                 Phosphoryl      Hydrochloric
 pentachloride                 alcohol                   chloride              chloride                   acid

PCl 5               +       CH 3COOH           →       CH 3COCl     +        POCl 3       +    HCl
Phosphorus                 Acetic                        Acetyl                 Phosphoryl      Hydrochloric
 pentachloride                 acid                              chloride              chloride                   acid

When heated with finely divided metals such as copper, silver and tin, phosphorus pentachloride forms the corresponding chlorides.

PCl 5                 +       2Cu       →       PCl 3      +      2CuCl
Phosphorus                Copper           Phosphorus        Cuprous
  pentachloride                                      trichloride           chloride

PCl 5                 +       2Ag       →       PCl 3      +      2AgCl
Phosphorus                Silver           Phosphorus        Argentous
  pentachloride                                      trichloride           chloride

PCl 5                 +       2Sn       →       PCl 3          +      SnCl 4
Phosphorus                  Tin             Phosphorus            Stannic
  pentachloride                                        trichloride           chloride

Structure of PCl5:
The central phosphorus atom in phosphorus pentachloride undergoes sp3d hybridisation. All the five electron pairs in these hybrid orbitals are bond pairs. The geometry of the molecule is trigonal   bipyramidal.

Trigonal   bipyramidal geometry is observed for phosphorus pentachloride only in its liquid and gaseous state. It exists as a salt in the solid state.


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