Carbon tetrachloride has the molecular formula \(\text{CCl}_{4}\). It is classified as an organic molecule, specifically a halomethane, where chlorine atoms fully substitute the hydrogen atoms of methane. For many decades, this colorless liquid was a widely used industrial agent. Its applications have been drastically curtailed due to concerns about human health and environmental damage.
Explaining the Molecular Structure
The formula \(\text{CCl}_{4}\) indicates a structure where one carbon atom is bonded to four chlorine atoms through single covalent bonds. The carbon atom sits at the center, acting as the foundation for the molecule. The four chlorine atoms are positioned symmetrically in a three-dimensional shape known as a tetrahedron.
While the individual bonds between carbon and chlorine are polar due to electronegativity differences, the molecule as a whole is nonpolar. The precise symmetry of the tetrahedral shape causes the slight electrical charges from each bond to cancel each other out. This overall lack of electrical charge distribution influences the molecule’s behavior when interacting with other substances.
Defining Key Characteristics and Behavior
Carbon tetrachloride is a volatile substance that exists as a clear, colorless liquid under standard conditions. It emits a noticeable, sweet odor often compared to chloroform. It has a high density, measuring approximately 1.59 grams per cubic centimeter, making it significantly heavier than water. The compound has a low boiling point, transitioning into a gas at about 76.7 degrees Celsius.
Its nonpolar nature makes it an effective solvent, capable of dissolving nonpolar materials like fats, oils, and greases. The molecule is also non-flammable, a characteristic valued in industrial processes and fire suppression efforts. However, when heated to high temperatures, such as in a fire, carbon tetrachloride can decompose into highly reactive and toxic compounds, including phosgene gas.
Past Uses and Regulatory Status
The non-flammability and strong solvent power of carbon tetrachloride led to its widespread adoption across numerous industries. Historically, it was a common solvent in dry cleaning and general cleaning agents, as well as a fumigant for grain storage. Its ability to extinguish fires safely, especially electrical ones, also made it a popular component in fire extinguishers.
A major industrial use was as a precursor chemical in the manufacture of chlorofluorocarbons (CFCs), which were used extensively as refrigerants and propellants. However, its widespread use was curtailed by two significant problems: its severe toxicity to humans and its environmental impact. Exposure to carbon tetrachloride, even at low levels, can cause damage to the central nervous system, liver, and kidneys.
The compound is metabolized in the liver to highly reactive free radicals, making it a potent hepatotoxin, and it is classified as a probable human carcinogen. Its chemical stability allows it to persist in the atmosphere, where it acts as a powerful ozone-depleting substance.
The international community addressed this environmental threat by including carbon tetrachloride in the Montreal Protocol on Substances that Deplete the Ozone Layer. This international agreement, signed in 1987, led to the phase-out of the compound in developed countries by 1996 and in developing countries by 2010. Small quantities are still permitted for specific laboratory research and industrial feedstock processes that do not release it into the atmosphere.