write a short note on any 2 polyhalogen compounds

Chloroform:

It is an important trihalogen derivative of methane. It was discovered by Liebig in 1831 and its name chloroform was proposed by Dumas as it gave formic acid on hydrolysis. In the past, it was extensively used as anaesthetic for surgery but now it is rarely used as it causes liver damage.

 

(1) Preparation

(i) Chloroform is prepared both in the laboratory and on large scale by distilling ethyl alcohol or acetone with bleaching powder and water. The yield is about 40%. The available chlorine of bleaching powder serves both as oxidising as well as chlorinating agent.

 CaOCI₂ + H₂O ——? Ca (OH)₂ + CI₂          

Bleaching powder

 

(a) From alcohol

 [ Cl₂ + H₂O ——? 2HCl + O ]

CH₃ CH₂ OH + O ——? CH₃ CHO + H₂O            

Ethylacohol                         Acetaldehyde           

 

CH₃ CHO + 3CI₂ ——? CCI₃ CHO + 3HCI           

Acetaldehyde                            Chloral

 

 

 

Freons

The chloro fluoro derivatives of methane and ethane are called freons. Some of the derivatives are: CHF₂Cl  (monochlorodifluoromethane), CF₂Cl₂ (dichlorodifluoro-methane), HCF₂CHCl₂ (1,1-dichloro-2,2-difluoroethane). These derivatives are non-inflammable, colourless, non-toxic, low boiling liquids. These are stable upto 550°C. The most important and useful derivative is CF₂Cl₂ which is commonly known asfreon and freon-

 

12. Freon or freon-12 (CF₂Cl₂) is prepared by treating carbon tetrachloride with antimony trifluoride in the presence of antimony pentachloride as a catalyst.

 

 

                                     SbCl₃

         3CCl₄ + 2SbF₃ —————? 3CCl₂F₂ + 2SbCl₃

                                  Catalyst

 

Or it can be obtained by reacting carbon tetrachloride with hydrofluoric acid in presence of antimony pentafluoride.

 

                                  SbF₅

         CCl₄ + 2HF —————? CCl₂F₂ + 2HCl

 

Under ordinary conditions freon is a gas. Its boiling point is –29.8°C. It can easily be liquified. It is chemically inert. It is used in air-conditioning and in domestic refrigerators for cooling purposes (As refrigerant). It causes depletion of ozone layer.