Chlorofluorocarbons (CFCs) are a type of hydrocarbons containing chlorine, fluorine, and carbon atoms. They are easy to compress, inert, low toxic, and are not flammable as well. Due to these specific physical properties, they have been vastly used in the last decade. These properties also made them applicable as coolant gas in refrigerators and air conditioners, etc.
Chlorofluorocarbons were also used as propellant and washing gas in dry cleaners. By 1974, CFCs were produced on a very large scale. For example, only USA had half a billion pounds of its annual production. Two scientists Rowland and Molina (Nobel prize in chemistry in 1995 for this effort) published an article, that indicates that CFCs are reaching the stratosphere and damaging the ozone layer. From that point onwards, all environmental hazard-related concerns started and eventually, it all came to put a ban on the production and use of CFCs.
Chemistry of Chlorofluorocarbons (CFCs)
Chlorofluorocarbons (CFCs) diffuse up to the stratosphere as they are relatively inert. Here they interact with electromagnetic radiations, especially ultraviolet radiation to produce free radical chlorine (Cl•). Free radical chlorine reacts with ozone to form oxygen and another free radical in the form of oxychloride radical. These free radicals generate more free radicals and the cycle continues. one single chloride, free radical can reduce up to 3 million ozone molecules.
The figure below shows the impact on ozone in 2004 over Antarctica.
Importance of Ozone (O3)
The ozone layer is a protective layer with a thickness of around 0.12 inches. It is present in the stratosphere and blocks out harmful UV rays coming from the sun that can cause skin cancer and other health problems, if not filtered right there. CFCs break down ozone molecules which leads to holes in the ozone layer. These holes allow UV rays to reach Earth’s surface, eventually causing health problems.
The normal concentration of ozone in the stratosphere is 300 dobson units.
On the left side, it shows the normal ozone layer which filters out the harmful UV rays. On the right side, there’s less ozone, so UV rays enter the atmosphere.
Synthesis of CFCs
Chlorofluorocarbons were first introduced by Thomas Midgley Jr. of General Motors, as safer chemicals for refrigeration purposes in 1928. CFCs can be prepared by the free radical substitution of alkanes.
CH4 + Cl2 → CH3Cl
CH3Cl + Cl2 → CH2Cl2
CH2Cl2 + F2 → CHFCl2
CHFCl2 + F2 → CF2Cl2
Ozone-depleting substances (ODS)
The chemical substances that destroy or damage ozone are termed ozone-depleting substances (ODS). They are relatively inert in the troposphere and get activated upon reaching the stratosphere. Here the UV light breaks them to form chlorine (Cl) and bromine (Br) free radicals. These free radicals damage the ozone O3.
Ozone-depleting potential (ODP)
ODS are classified as per their potential to damage the ozone. For, example, Bromochlorodifluoromethane (CF2ClBr) extensively used in fire extinguishers (Halon 1311) is now considered the most dangerous in this category with an ODP value of 10.
Chlorine source gases in the stratosphere
There is a variety of gases that go into the stratosphere and release chlorine-free radicals (Cl•). Among them, CFCs are the most abundant ones. The sources of chlorine-free radicals have a major role in ozone depletion. This is so because each Cl• can destroy 100,000 molecules of ozone (O3).
The sources of chlorine gases are given below:
Montreal pact 1987
In 1987, 27 nations signed a pact to ban the use of ozone-depleting substances (ODS). They have a consensus to periodically eliminate the production and consumption of primary ODS. This pact was led by the United States and since that agreement, the ozone hole started filling up. This means that the pact is a success. Below are some of the effects of banning ODS.
ALternatives to ODS (CFCs)
After the harmful nature of CFCs was known, scientists started efforts to produce their alternatives. A low boiling gas with a boiling point near -30 °C was required. However, such a gas having properties similar to CFCs is hard to find, therefore, some less harmful yet less effective chemicals are used as alternatives to ODS.
|Use of CFCs||Alternatives|
|Aerosols||Carbon dioxide, hydrocarbons, HCFs|
|Coolants||HCFCs, hydrofluorocarbons, Propane, ammonia|
|Propellents (aircraft etc)||1,3 dichloropropane, steam (gas)|
1,1,1,2,3,3,3,-hepta fluoropropane (C3HF7)
|Propellent in oral Inhallers||1,1,1,2 tetrafluoroethane|
Carbon dioxide is a non-polar gas due to zero dipole moment. It is easy to compress and is a relatively nontoxic gas. Chlorofluorocarbons are almost completely replaced by CO2 in electronic devices production plants.
HFCs have replaced chlorofluorocarbons in many applications being similar in physical properties but different in chemical reactivity. C-Cl is weaker than the C-F bond. This is due to the greater electronegativity difference (E.N) of the C-F bond. Therefore, in the stratosphere, HFCs are less harmful when compared with CFCs.
Propane is used frequently in DC inverter ACs as a coolant gas. The reason is that its compressibility is as easy as in CFCs.
effects of Chlorofluorocarbons on the environment
Chlorofluorocarbons not only cause ozone depletion in the stratosphere but also contribute to the increasing greenhouse effect. The increase in overall Earth’s temperature is known as the greenhouse effect. It is one of the most important environmental issues these days. The presence of greenhouse gases in the upper atmosphere insulates the outgoing heat and results in this effect. This blanket effect is shown in the figure below:
What are the applications of CFC?
In the near past, chlorofluorrocarbons were widely used as aerosols, propellants, and coolant gas. When scientists discovered the harmful impact on the ozone layer their use was minimized. Nowadays, they are only used for special purposes such as they are still utilized in fire extinguishers as propellants on aircraft and battleships, etc.
Why were CFCs banned?
CFCs are banned because they were found to play a major role in ozone depletion in the stratosphere, such as they are the sources of Cl free radicals. Free radicals spontaneously react with ozone (O3) to convert it into oxygen (O2).
What are the advantages of chlorofluorocarbons?
Some of the advantages of CFCs are:
- Low boiling
- Unreactive (Inert)
- Perfect for refrigeration
- Compressible, etc
How are CFCs harmful to the environment and human beings?
CFCs are harmful to the environment as well. Being inert, they stay in the troposphere for years and cause a greenhouse effect. Floods and untimely rains are the major impacts of the greenhouse effect on our environment. When they react stratosphere, they decompose ozone and affect humans indirectly by letting uv rays from the sun, in.
Is chlorofluorocarbon a stable or an unstable compound?
A chlorofluorocarbon is a very stable compound. C-Cl and C-F bonds are not easy to break. They only react in the presence of UV light.
Why do ACs release chlorofluorocarbons?
Air conditioners use (not anymore) CFCs as a coolant gas. Coolant gas works on the principle to absorb heat energy and then release that heat outside. Due to some leakage and recharging, it may also be released into the atmosphere.
How long does CFC stay in the atmosphere?
CFCs are unreactive gases. They can stay in the atmosphere for more than 100 years.
How do CFCs get transported to Antarctica from the Northern Hemisphere?
When the circular movement of the earth and air depressions create air currents. CFCs, though heavier than air, get transported towards Antarctica.
- Oxford IB-Diploma Programme Chemistry (second edition) by Sergey Bylikin, Gary Horner, Brian Murphy, David Tarcy
- Cambridge International AS and A Level By Ryan and Norris (Cambridge International AS and A Level Chemistry Coursebook)
- Environmental Pollution Bachelor Level Course by Dr. Zahidullah (AIOU Pakistan)
- Ozone watch (NASA)
- Chlorofluorocarbons (CFCs) and hydrofluorocarbons (HFCs) (Minnesota Pollution Control Agency-MPCA)
- Chlorofluorocarbons and Ozone Depletion (American Chemical Society- ACS)