How is trichloromethane prepared in the laboratory? Why is it discouraged to use chloroform as an anesthesia? [5]

Preparation of Trichloromethane (Chloroform) in the Laboratory

Trichloromethane (chloroform, $\mathrm{CHCl}{}_3$) is prepared in the laboratory by the haloform reaction of ethanol with bleaching powder (calcium hypochlorite, $\mathrm{Ca}(\mathrm{OCl}){}_2$). The reaction proceeds as follows:

chloroformchloroform

1. Reaction Mechanism: Ethanol ($\mathrm{C}{}_2\mathrm{H}{}_5\mathrm{OH}$) is first oxidized to acetaldehyde ($\mathrm{CH}{}_3\mathrm{CHO}$) by the hypochlorite ion ($\mathrm{OCl}{}^{-}$). The acetaldehyde then undergoes halogenation in the presence of excess chlorine (from bleaching powder), replacing all three hydrogen atoms of the methyl group ($\mathrm{CH}{}_3$) with chlorine atoms, forming chloroform ($\mathrm{CHCl}{}_3$). The balanced chemical equation is: $$\mathrm{C}{}_2\mathrm{H}{}_5\mathrm{OH}+4\mathrm{Ca}(\mathrm{OCl}){}_2\to \mathrm{CHCl}{}_3+4\mathrm{Ca}(\mathrm{OH}){}_2+\mathrm{CaCO}{}_3$$ (Note: The exact stoichiometry may vary slightly depending on conditions.)

ethanolethanol+4Ca(OCl)₂heattrichloromethanetrichloromethane+4Ca(OH)₂+CaCO₃

2. Procedure:
   • Mix ethanol with bleaching powder (a suspension of $\mathrm{Ca}(\mathrm{OCl}){}_2$ in water).
   • Heat the mixture gently in a flask.
   • Chloroform is formed as a dense, colorless liquid, which can be separated by distillation (bp = 61°C).
   • The crude chloroform is then dried with anhydrous calcium chloride and redistilled to obtain pure chloroform.

---

Why Chloroform is Discouraged as an Anesthetic

Chloroform was once widely used as an anesthetic but is now discouraged due to severe health risks:

1. Toxicity and Metabolism:

   • Chloroform is metabolized in the liver to phosgene ($\mathrm{COCl}{}_2$), a highly toxic gas that causes pulmonary edema (fluid accumulation in the lungs) and hepatic necrosis (liver damage).
   • It also forms carbon monoxide ($\mathrm{CO}$), leading to hypoxia (oxygen deficiency in tissues).

2. Cardiovascular and Respiratory Depression:

   • Chloroform depresses the central nervous system, slowing heart rate and respiration, which can be fatal if not carefully monitored.
   • It causes arrhythmias (irregular heartbeat) and hypotension (low blood pressure).

3. Carcinogenicity:

   • Long-term exposure to chloroform is linked to cancer due to its metabolic byproducts (e.g., phosgene and free radicals).

4. Flammability and Explosive Risks:

   • Chloroform vapors can form explosive peroxides when exposed to air and light, posing a fire hazard.

5. Modern Alternatives:

   • Safer anesthetics like halothane, enflurane, and propofol have replaced chloroform due to their controlled metabolism, non-toxic byproducts, and lower systemic risks.

---

acetaldehydeacetaldehyde+3Cl₂lighttrichloromethanetrichloromethane+3HCl