CHCl₃ — Chloroform

Chloroform (CHCl₃), also known as trichloromethane, is a colorless, dense, and volatile liquid with a sweet odor. It belongs to the family of halogenated hydrocarbons and is widely used in the laboratory and industry as a solvent for fats, oils, resins, and alkaloids. Chloroform gained historical importance in medicine as one of the earliest anesthetics, though it is no longer used for that purpose due to safety concerns.

Chloroform is non-flammable under ordinary conditions but can decompose upon exposure to light or air, producing phosgene gas (COCl₂), which is toxic. For this reason, chloroform is stored in dark-colored bottles containing a small amount of ethanol as a stabilizer. In modern applications, chloroform remains valuable in organic synthesis, particularly in the production of chlorodifluoromethane (HCFC-22) and other refrigerants.

Chloroform’s molecular structure consists of a central carbon atom bonded to one hydrogen and three chlorine atoms. Its Lewis structure can be represented as \(CHCl_3\). The carbon atom is sp³ hybridized, forming four single covalent sigma (σ) bonds — one with hydrogen and three with chlorine atoms.

Because chlorine atoms are highly electronegative, the C–Cl bonds are polarized, creating a net dipole moment. The molecular geometry of chloroform is tetrahedral with bond angles close to 109.5°. The molecule’s polarity explains its ability to dissolve both nonpolar and slightly polar substances, making it a versatile solvent.

Under ultraviolet light or in the presence of oxygen, chloroform can oxidize to form phosgene (COCl₂) as shown below:

This reaction highlights the importance of proper storage to avoid hazardous decomposition.

Chloroform can be prepared both in laboratories and on an industrial scale using several methods:

1. Laboratory Preparation from Ethanol or Acetone

Chloroform can be prepared by the action of bleaching powder (calcium hypochlorite) on ethanol or acetone. This process is known as the haloform reaction.

In this reaction, ethanol is oxidized and chlorinated in the presence of chlorine and calcium hydroxide, forming chloroform and formic acid as a byproduct.

2. From Methane or Chloromethane

Chloroform can also be produced by stepwise chlorination of methane or chloromethane in the presence of ultraviolet light:

This sequential process yields chloroform as an intermediate before forming carbon tetrachloride (CCl₄) if over-chlorinated.

3. Industrial Production

Industrially, chloroform is produced as a byproduct during the manufacture of chloromethanes from methane and chlorine at elevated temperatures (400–500°C). The product mixture is then separated by fractional distillation.

Physical Properties:

• Chloroform is a colorless, heavy liquid with a sweet, pleasant odor.
• It has a boiling point of 61.2°C and a melting point of -63.5°C.
• It is only slightly soluble in water but miscible with alcohol, ether, and benzene.
• Chloroform is denser than water and forms a separate layer when mixed with it.
• When exposed to air and sunlight, it slowly decomposes to produce toxic phosgene gas.

Chemical Properties:

• 1. Reaction with Air and Light: Chloroform oxidizes to phosgene gas when exposed to light or air:

\(2CHCl_3 + O_2 \xrightarrow{light} 2COCl_2 + 2HCl\)

• 2. Reaction with Sodium Hydroxide: On heating with sodium hydroxide, chloroform yields sodium formate and sodium chloride:

\(CHCl_3 + 4NaOH \rightarrow HCOONa + 3NaCl + 2H_2O\)

• 3. Reaction with Primary Amines (Carbylamine Reaction): When chloroform reacts with a primary amine and alcoholic KOH, it produces an isocyanide (carbylamine), which has a very offensive odor:

\(CHCl_3 + RNH_2 + 3KOH \rightarrow RNC + 3KCl + 3H_2O\)

• 4. Combustion: Chloroform burns with a green-edged flame, forming carbon dioxide and hydrogen chloride:

\(2CHCl_3 + O_2 \rightarrow 2CO_2 + 2HCl + 2Cl_2\)

• 1. Solvent: Chloroform is extensively used as a solvent for fats, waxes, resins, and alkaloids in laboratory and industrial applications.
• 2. Chemical Intermediate: It is used in the production of chlorodifluoromethane (HCFC-22), which is a precursor to Teflon (PTFE) and refrigerants.
• 3. Extraction: Employed in pharmaceutical and biochemical research for extracting active compounds from natural materials.
• 4. Laboratory Reagent: Used in organic synthesis as a reagent and solvent in reactions such as the Reimer–Tiemann reaction for producing salicylaldehyde from phenol.
• 5. Historical Anesthetic: Once used as an inhalation anesthetic during surgeries, though discontinued due to its toxic and carcinogenic effects on the liver and heart.

Chloroform is toxic when inhaled, ingested, or absorbed through the skin. Short-term exposure can cause dizziness, nausea, and central nervous system depression, while prolonged exposure can damage the liver, kidneys, and heart. Chronic exposure is linked to carcinogenic effects.

Because chloroform can oxidize to phosgene (COCl₂), a highly toxic gas, it must be stored in dark, tightly sealed containers with ethanol as a stabilizer. Environmentally, chloroform contributes to air pollution and poses a hazard to aquatic life if released in large amounts.