CCl₄ — Carbon Tetrachloride

Carbon Tetrachloride (CCl₄), also known as tetrachloromethane, is a halogenated hydrocarbon consisting of one carbon atom bonded to four chlorine atoms. It appears as a colorless, heavy liquid with a sweet odor similar to chloroform. Although once used extensively as a cleaning agent, dry-cleaning solvent, and refrigerant precursor, its use has declined due to health and environmental concerns.

Carbon tetrachloride is non-flammable and chemically stable, making it an excellent solvent for nonpolar substances like fats, oils, and resins. However, it is toxic to the liver and kidneys and contributes to ozone layer depletion, leading to its restriction under international environmental agreements like the Montreal Protocol. Today, it is primarily used in controlled industrial processes and as a laboratory reagent.

Carbon Tetrachloride has a molecular formula \(CCl_4\). The molecule consists of a central carbon atom bonded to four chlorine atoms through single covalent bonds. Each carbon-chlorine bond involves the overlap of an sp³ hybrid orbital of carbon with a p-orbital of chlorine, forming four sigma (σ) bonds.

The molecule has a tetrahedral geometry with bond angles of approximately 109.5°, typical of sp³ hybridization. Although each C–Cl bond is polar due to chlorine’s high electronegativity, the overall molecule is nonpolar because of its symmetrical structure, resulting in the dipole moments canceling out.

Its nonpolar nature explains why carbon tetrachloride is an excellent solvent for nonpolar compounds and does not mix with water.

Carbon Tetrachloride can be prepared using several laboratory and industrial methods:

1. Chlorination of Methane

The most common industrial method involves stepwise chlorination of methane in the presence of ultraviolet light or heat. The reaction proceeds through a free radical mechanism:

This series of substitution reactions leads from methane to chloromethane, dichloromethane, chloroform, and finally to carbon tetrachloride.

2. From Carbon Disulfide

Carbon tetrachloride can also be produced by chlorinating carbon disulfide (CS₂) at elevated temperatures using iron or copper chloride as catalysts:

This process yields carbon tetrachloride along with sulfur chlorides as byproducts.

3. Laboratory Preparation from Chloroform

Chloroform (CHCl₃) can be further chlorinated to form carbon tetrachloride in the presence of sunlight or heat:

Physical Properties:

• Carbon tetrachloride is a colorless, dense liquid with a sweet odor.
• It is insoluble in water but dissolves many organic compounds like fats, waxes, and oils.
• It has a boiling point of 76.7°C and a melting point of -23°C.
• It is non-flammable but decomposes when heated strongly, releasing toxic gases.

Chemical Properties:

• 1. Stability: Carbon tetrachloride is chemically stable and does not react with acids, bases, or most oxidizing agents at room temperature.
• 2. Decomposition: When heated or exposed to ultraviolet radiation, CCl₄ decomposes to form phosgene (COCl₂), a highly toxic gas:

\(CCl_4 \xrightarrow{heat} CCl_2O + Cl_2\)

• 3. Reaction with Steam: At very high temperatures, CCl₄ reacts with steam to form carbon dioxide, hydrogen chloride, and chlorine gas:

\(CCl_4 + 2H_2O \xrightarrow{700°C} CO_2 + 4HCl + Cl_2\)

• 4. Reaction with Metals: On heating with sodium or potassium, carbon tetrachloride forms metal chlorides and carbon:

\(2Na + CCl_4 \rightarrow 2NaCl + C + Cl_2\)

• 5. Non-combustibility: CCl₄ is non-flammable and was once used in fire extinguishers, though its toxic fumes make it unsafe for modern use.

• 1. Solvent: CCl₄ is used as a solvent for oils, greases, fats, and resins due to its nonpolar nature and stability.
• 2. Fire Extinguishers (Historical): Once used in hand-held fire extinguishers and bombs, although now banned due to the production of phosgene gas when decomposed by heat.
• 3. Refrigerant Production: Used in the synthesis of chlorofluorocarbons (CFCs), particularly Freon-12 (CCl₂F₂).
• 4. Laboratory Reagent: Utilized as a solvent for spectroscopic and organic reactions.
• 5. Industrial Applications: Employed in the production of propellants, pesticides, and as a feedstock for various chlorinated compounds.

Carbon tetrachloride is a toxic compound that can cause severe damage to the liver, kidneys, and central nervous system upon prolonged exposure. Inhalation of high concentrations may lead to dizziness, nausea, or unconsciousness. Chronic exposure has been linked to hepatotoxicity and nephrotoxicity.

Environmentally, CCl₄ is an ozone-depleting substance (ODS) and contributes to the breakdown of the stratospheric ozone layer. As a result, its use is regulated under the Montreal Protocol (1987). Modern industries are shifting to safer alternatives and controlled processes to limit emissions.