Chloroform: Unit 8 Class 12 Chemistry (NEB)

Chapter 8: Chloroform

For Class 12 NEB Chemistry, Unit 8 covers the preparation of chloroform (8.11), its physical properties (8.12), and its chemical properties (8.13).

Introduction to Chloroform

Chloroform, also known as trichloromethane (CHCl₃), is a colorless, sweet-smelling liquid widely used as a solvent and in the past as an anesthetic. It is prepared through various methods, including the reaction of chlorine with methane or acetone.

Preparation of Chloroform

Chloroform is prepared in the laboratory by heating ethanol or acetone with an aqueous paste of bleaching powder (CaOCl₂). In this process, bleaching powder acts as an oxidizing agent, chlorinating agent, and hydrolyzing agent.

CaOCl₂+ H₂O –^Δ⟶ Ca(OH)₂+ Cl₂
Bleaching powder calcium hydroxide

1. From Ethanol (Ethyl Alcohol)

1. Oxidation of Ethanol: Chlorine oxidizes ethanol into ethanal. $$\text{CH}_3\text{CH}_2\text{OH} + \text{Cl}_2 \xrightarrow{\Delta} \text{CH}_3\text{CHO} + 2\text{HCl} $$
   • (ethanol) ⟶ (ethanal)
2. Chlorination of Ethanal: Excess chlorine reacts with ethanal to give chloral.$$\text{CH}_3\text{CHO} + 3\text{Cl}_2 \xrightarrow{\Delta} \text{CCl}_3\text{CHO} + 3\text{HCl}$$
   • (ethanal) ⟶ (chloral, 2,2,2-trichloroethanal)
3. Hydrolysis of Chloral: Chloral is hydrolyzed with Ca(OH)₂ to form chloroform. $$ 2\text{CCl}_3\text{CHO} + \text{Ca(OH)}_2 \xrightarrow{\Delta} 2\text{CHCl}_3 + (\text{HCOO})_2\text{Ca} $$
   • (chloral) ⟶ (chloroform) (calcium formate)

2. From Propanone (Acetone)

1. Chlorination of Acetone: Acetone is chlorinated by passing excess chlorine to form trichloroacetone. $$\text{CH}_3\text{COCH}_3 + 3\text{Cl}_2 \xrightarrow{\Delta} \text{CCl}_3\text{COCH}_3 + 3\text{HCl}$$
   • (acetone) (trichloroacetone)
2. Hydrolysis of Trichloroacetone: Trichloroacetone is hydrolyzed with calcium hydroxide to form chloroform. $$ 2\text{CCl}_3\text{COCH}_3 + \text{Ca(OH)}_2 \xrightarrow{\Delta} 2\text{CHCl}_3 + (\text{CH}_3\text{COO})_2\text{Ca}$$
   • tri-chloroacetone ⟶ chloroform (trichloromethane)

Physical Properties of Chloroform

1. Appearance:
   • Chloroform is a colorless, mobile, oily liquid.
2. Boiling and Melting Points:
   • Boiling point: 61°C
   • Melting point: -60°C
3. Density:
   • Chloroform is heavier than water, with a specific gravity of 1.48.
4. Smell and Taste:
   • It has a characteristic sweet smell and taste.
5. Solubility:
   • Chloroform is insoluble in water but soluble in almost all organic solvents such as benzene and ether.
6. Solvent Properties:
   • It is an excellent solvent for dissolving fats, oils, resins, waxes, and similar substances.
7. Anesthetic Properties:
   • The vapor of chloroform causes temporary unconsciousness when inhaled in small amounts. This happens because chloroform depresses the central nervous system, leading to a loss of sensation and consciousness. It was historically used as an anesthetic drug for this reason.

Chemical Properties of Chloroform

1. Action with Air (Oxidation):
   • When chloroform is exposed to air in the presence of sunlight, it forms highly poisonous phosgene gas (carbonyl chloride).
   $$ 2\text{CHCl}_3 + \text{O}_2 \xrightarrow{\text{sunlight}} 2\text{COCl}_2 + 2\text{HCl}$$
   • (chloroform) ⟶ (phosgene gas)
   • Precautions for Storage:
     • Chloroform is stored in dark brown bottles to protect it from sunlight.
     • Bottles are completely filled to exclude air.
     • A small amount (1%) of ethyl alcohol is added to react with any formed phosgene gas, converting it to non-toxic diethyl carbonate.

2. Reaction with Silver Powder:

• Heating chloroform with silver powder produces acetylene. $$2\text{CHCl}_3 + 6\text{Ag} \xrightarrow{\Delta} \text{H-C≡C-H} + 6\text{AgCl}$$
  • (chloroform) ⟶ (acetylene)

3. Reaction with Concentrated HNO₃ (Nitration):
   • Chloroform treated with concentrated nitric acid forms chloropicrin, used as an insecticide and tear gas.
   • CHCl₃​+Conc. HNO₃​→CCl₃​NO₂​+H₂​O
     • (chloroform) ⟶ (chloropicrin, trichloronitromethane)

4. Reaction with Acetone:
   • Heating chloroform in the presence of aqueous NaOH or KOH with acetone forms chloretone, a sleep-inducing hypnotic drug.

5. Reduction:

• Acidic Medium (Zn/HCl):
  • Reduction of chloroform in an acidic medium forms methylene chloride.
  $$\text{CHCl}_3 + \text{Zn/HCl} \xrightarrow{\Delta} \text{CH}_2\text{Cl}_2 + \text{HCl} $$
  • chloroform ⟶ methylene chloride (dichloromethane)
• Basic Medium (Zn/H₂O):
  • Reduction in a neutral medium forms methane. $$\text{CHCl}_3 + \text{Zn/H}_2\text{O} \xrightarrow{\Delta} \text{CH}_4 + 3\text{HCl}$$
    • chloroform ⟶ methane

Purity Test of Chloroform:

6. Reaction with Silver Nitrate (AgNO₃):
   • Pure Chloroform:
     • Pure chloroform does not give a white precipitate with AgNO₃ because the C-Cl bond is a strong covalent bond and does not release Cl⁻ ions.
     CHCl3+AgNO3→No white ppt.
     • (pure chloroform)
   • Impure Chloroform:
     • Impure chloroform gives a white precipitate of AgCl with AgNO₃ due to the presence of Cl⁻ ions after the oxidation of chloroform in the air.
     $$ \text{CHCl}_3 + \frac{1}{2}\text{O}_2 \rightarrow \text{COCl}_2 + \text{HCl} $$
     • ​(impure chloroform) ⟶ (white precipitate)

Uses of Chloroform

Here are some uses of chloroform:

1. Anesthetic (historically)
2. Solvent
3. Production of Freon
4. Chemical intermediate

Question – Reaction Summary

Chloroform Reaction Summary Answer

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Read More:

• Haloalkanes – Class 12 Chemistry Notes
• Heavy Metals Notes – Class 12
• Class 12 Chemistry – All Chapter Notes