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$10/53$ HALOALKANES 10.10. POLYHALOGEN COMPOUNDS Carbon compounds containing more than one halogen atom are called poly halogen colppounds. Many polyhalogen compounds are useful in industry and agriculture. Some of these are described below : 10.10.1. Dichloromethane (Methylene Chloride), ${\mathrm{CH}}_2{\mathrm{Cl}}_2$ Uses. (i) It is widely used as a solvent for paint remover, as a propellant in aerosols and as a process solvent in the manufacture of drugs. (ii) It is also used as metal cleaning and finishing solvent. Physiological effects. Methylene chloride acts on central nervous system (CNS). (i) Exposure to small levels of methylene chloride in air causes slightly impaired hearing and vision. Higher levels of methylene chloride in air causes dizziness, nasea, tingling and numbness in the fingers and toes. (ii) Direct contact of methylene chloride with skin causes intense burming and mild redness of skin. Direct contact with the eyes can bum cornea. The comea of animals was damaged when exposed to vapours of methylene chloride. 10.10.2. Trichloromethane (Chloroform), ${\mathrm{CHCl}}_3$ * Preparation. 1. By distilling ethanol (ethyl alcohol) or propanone (acetone) with a suspension of bleaching powder in water (Labonatory method) : The reaction in case of ethyl alcohol is supposed to take place as under : (i) Bleaching powder supplies both chlorine and calcium hydroxide. $\underset{\text{ Bleaching powder }}{{\mathrm{CaOCl}}_2}+{\mathrm{H}}_2\mathrm{O}⟶{\mathrm{Cl}}_2+\mathrm{Ca}(\mathrm{OH}{)}_2$ (ii) Chlorine acts both as an oxidising as well as a chlorinating agent. Oxidation : ${\mathrm{CH}}_3\mathrm{CHO}+3{\mathrm{Cl}}_2\frac{\mathrm{Ca}({\mathrm{OH}}_2}{(\text{ Ehlorination })}\underset{\text{ Acetaldehyde }}{{\mathrm{CH}}_2}{\mathrm{CCl}}_3\mathrm{CHO}+3\mathrm{HCl}$ Chlorination : or Chloral (iii) Chloral is then hydrolysed by calcium hydroxide to give chloroform. In case of acetone, the reactions are : (i) ${\mathrm{CaOCl}}_2+{\mathrm{H}}_2\mathrm{O}⟶{\mathrm{Cl}}_2+\mathrm{Ca}(\mathrm{OH}{)}_2$ (ii) ${\mathrm{CH}}_3{\mathrm{COCH}}_3+3{\mathrm{Cl}}_2\underset{(\mathrm{Ca}(\mathrm{OH}{)}_2}{(\text{ Chlorination })}{\mathrm{CCl}}_3{\mathrm{COCH}}_3+3\mathrm{HCl}$ Propanone (Aceone (iii) Acetone is preferred to ethyl alcohol, as the yield is better.

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Updated on: Apr 21, 2023