Uses, Environmental Impact, and Health Risks for Students
Polyhalogen Compounds
Carbon is a very important element of the periodic table because of its valency and its ability to form long compounds. The entire branch of organic chemistry is the study of carbon-hydrogen compounds. Polyhalogen compounds are carbon compounds which have more than one halogen atom attached to it. Halogens are the group 17 elements and are the most electronegative elements of the periodic table. Some of the more important poly-halogen compounds are methyl chloride, chloroform, carbon tetrachloride, iodoform, DDT, benzene hexachloride, CFCs a.k.a Freon’s, etc. They have wide-spread applications in the fields ranging from agriculture to medicine. In this article, we are primarily focusing on poly-halogens Freons, DDT, and carbon tetrachloride.
Freons
Chlorofluorocarbons also known as Freon’s are considered to be important for poly-halogens. It is obtained when we replace the hydrogen atoms of methane or CH4 with fluorine and chlorine atoms. The properties of CFCs can vary with the presence of different numbers of chlorine and fluorine atoms. The naming of chlorofluorocarbons follows a unique system called the rule of 90. The concerned CFC is named CFC-n, where n is obtained as follows. The number of carbon atoms, hydrogen atoms, and fluorine atoms is written in that order and 90 is subtracted from it to obtain n.
So for example, if the formula of the concerned CFC is CCl3F, then it is named as CFC-11.
Here the number 11 gives information about the constituents of the compound as we can find the number of chlorine, fluorine, and hydrogen atoms from this number. The chlorofluorocarbons were initially manufactured as a refrigerant as the refrigerants used till then were toxic to humans and accidents were common. As these are non-toxic they are now used in sprays, perfumes, aerosols, etc. Their production and use as refrigerants have reduced considerably these days due to the fact that these are not very much environmentally friendly.
DDT
DDT is the short form of dichloro-diphenyl-trichloroethane, an organochlorine. It is a colorless crystalline solid and has no taste or smell. It is primarily used as an insecticide and a pesticide. The manufacture of DDT uses a Friedel Crafts reaction using chloral and chlorobenzene in the presence of an acidic catalyst.
It comprises a benzene ring with a chlorine atom in its para position and diphenyl trichloroethane in its ortho position. Because of its structure, this component exhibits isomerism ortho para and para-para isomerism. These isomers are however considered as impurities and do not exhibit any kind of insecticidal properties.
DDT is not soluble in water due to the presence of the rings but is soluble in organic solvents, fats, and oils. Though it is not used in the agricultural sector very prevalently still it is extensively used in the manufacture of various aerosols and vaporizers, lotions, emulsifiable concentrates, etc. However, for this purpose, a mixture of different isomers of DDT is used.
Carbon Tetrachloride
The chemical formula of carbon tetrachloride is CCl4. We obtain carbon tetrachloride when we replace all the hydrogen atoms of CH4 with chlorine atoms. This is a clear, volatile liquid with a sweet smell. This is denser than water with density = 1.581 g/cubic centimeter and is not soluble in water.
Earlier this was used as refrigerants, coolants, in cleaning products, in fire extinguishers, etc. However, it is toxic upon inhalation and can cause irritation to the skin. Hence, these uses were banned. This is now used primarily as a fumigant and for the manufacture of other chemicals. It is also used in various reactions as a chlorine source like the ‘Appel Reaction’.
It is also used as a source of proton in NMR spectroscopy. Historically this was manufactured by the reaction of chlorine and chloroform, however now the chlorination of various carbon compounds primarily hydrocarbons are used.
This has a tetrahedral structure. Since this is the same structure of methane, carbon tetrachloride is also called halo methane. Hence this is a non-polar, covalent compound. Upon crystallization, this exhibits monoclinic crystal lattice.
Other than being toxic, it is also bad for the environment as it acts as a greenhouse gas and leads to the depleting of the ozone layer.
FAQs on Freons, DDT, and Carbon Tetrachloride Explained
1. What are freons and what are some common examples?
Freons are the trade name for a class of synthetic organic compounds known as chlorofluorocarbons (CFCs). They are generally unreactive, non-flammable, and non-toxic gases or liquids that can be easily vaporised and condensed. These properties made them ideal for use as refrigerants, aerosol propellants, and solvents. A widely known example is Freon-12, which has the chemical formula CCl₂F₂ and is chemically named Dichlorodifluoromethane.
2. What were the primary uses of Carbon Tetrachloride (CCl₄), Freons, and DDT?
These polyhalogen compounds each had significant, though now largely discontinued, applications:
- Carbon Tetrachloride (CCl₄): It was used as a cleaning agent for degreasing, a solvent in chemical manufacturing, a grain fumigant to kill pests, and in fire extinguishers under the trade name 'Pyrene'.
- Freons (CFCs): Their main application was as refrigerants in air conditioning systems and refrigerators. They were also used as propellants in aerosol spray cans and as blowing agents in the production of plastic foams.
- DDT (Dichlorodiphenyltrichloroethane): It was a highly effective and widely used insecticide for agriculture and for controlling insect-borne diseases such as malaria and typhus.
3. How are freons typically prepared?
Freons are most commonly prepared using the Swarts reaction. This chemical reaction involves the exchange of chlorine atoms for fluorine atoms in a chloroalkane by reacting it with a metallic fluoride. For instance, Freon-12 (CCl₂F₂) can be synthesized by reacting carbon tetrachloride (CCl₄) with antimony trifluoride (SbF₃) in the presence of an antimony pentachloride (SbCl₅) catalyst. The balanced chemical equation is:
3CCl₄ + 2SbF₃ → 3CCl₂F₂ (Freon-12) + 2SbCl₃
4. How do freons cause the depletion of the ozone layer? Explain the mechanism.
Freons are stable in the lower atmosphere but are broken down by powerful ultraviolet (UV) radiation in the stratosphere, releasing a chlorine atom as a free radical (Cl•). This radical acts as a catalyst to destroy ozone (O₃) molecules through a chain reaction:
- Step 1 (Initiation): The UV radiation breaks a C-Cl bond in the freon molecule to generate a chlorine radical.
CF₂Cl₂(g) + UV light → Cl•(g) + •CF₂Cl(g) - Step 2 (Propagation): The chlorine radical attacks an ozone molecule, forming chlorine monoxide and an oxygen molecule.
Cl•(g) + O₃(g) → ClO•(g) + O₂(g) - Step 3 (Regeneration): The chlorine monoxide radical reacts with an oxygen atom, regenerating the chlorine radical which can repeat the process.
ClO•(g) + O(g) → Cl•(g) + O₂(g)
Because the chlorine radical is regenerated, a single freon molecule can destroy thousands of ozone molecules, leading to significant ozone layer depletion.
5. Why was DDT considered a highly effective insecticide, and what environmental problems led to its ban?
DDT was highly effective due to its powerful neurotoxic effect on insects, its persistence (meaning it didn't need frequent reapplication), and its low cost. However, these same properties caused severe environmental issues, leading to its ban in many countries. The main problems are:
- Environmental Persistence: DDT is not easily biodegradable and can remain in soil and water for decades.
- Bioaccumulation: Being fat-soluble, it accumulates in the fatty tissues of organisms.
- Biomagnification: Its concentration increases at higher trophic levels of the food chain. This caused significant harm to top predators, especially birds, by interfering with their calcium metabolism, which resulted in thin, fragile eggshells and a decline in their populations.
6. Why is carbon tetrachloride now banned for use in fire extinguishers and as a cleaning agent?
Carbon tetrachloride (CCl₄) was banned primarily due to its severe human toxicity and environmental impact. Exposure to CCl₄ can cause acute damage to the liver, kidneys, and central nervous system. In the body, it is metabolized into the highly reactive trichloromethyl free radical (•CCl₃), which can trigger cancer. Furthermore, when released into the atmosphere, CCl₄ contributes to the depletion of the ozone layer, and its vapour is a potent greenhouse gas. Safer alternatives have since been developed for both fire suppression and cleaning.
7. Compare the environmental impact of Freons and DDT.
While both are harmful, Freons and DDT impact the environment in different ways:
- Primary Impact Zone: The main damage from Freons (CFCs) occurs in the upper atmosphere (stratosphere), where they destroy the protective ozone layer. The primary damage from DDT occurs in terrestrial and aquatic ecosystems (biosphere) through soil and water contamination.
- Mechanism of Harm: Freons cause harm through a catalytic chemical reaction that depletes ozone, increasing UV radiation on Earth. DDT causes harm through biomagnification up the food chain, leading to toxic effects in wildlife, particularly top predators.
- Target of Harm: Freons harm the global atmospheric shield, affecting all life indirectly. DDT directly poisons organisms within the food web.
