The chlorofluorocarbons (or CFCs for short) have recently received a great deal of attention, and notoriety, as a result of being implicated in the destruction of the ozone layer. CFCs (also known as Freons) are a family of chemicals based upon hydrocarbon skeletons (most often methane), where some or all of the hydrogens have been replaced with chlorine and/or fluorine atoms. These compounds are non-flammable, tasteless and odourless, and chemically stable. Their other important property is their volatility, having boiling points close to zero degrees Centigrade.
These physical properties make them ideal for use as refrigerant gases in air conditioners, freezers and refrigerators. Their low boiling points also make them ideal for blowing agents for foam plastics, allowing the foam to expand as the liquid CFC boils. Chlorofluorocarbons are regularly blamed for causing damage to our environment. Although they absorb much more infrared radiation per molecule than carbon dioxide, their contribution to the “greenhouse effect” is very low due to their very low abundance in the atmosphere.
More importantly CFC’s are responsible for a thinning of the protective ozone layer in the stratosphere. CFC’s are still used in air conditioners and were formerly used as refrigerants and aerosol propellants. It is ironic that one of the properties that make CFCs so suitable for use in household items such as refrigerators, their chemical stability, should be the cause of their undoing. Most chemicals, when released into the atmosphere get rapidly broken down into smaller, harmless components by reactions in the lower atmosphere.
The CFCs however, are so stable and unreactive that they survive to reach the highest levels of the atmosphere, and become globally distributed in the stratosphere. At these high altitudes, the intensity of ultra-violet radiation is so great that even the stable CFCs are split apart to release a chlorine atom. It is the atomic chlorine that does the damage, since it can react with ozone (O3) to form oxygen The developments of compounds such as CFCs and BCF illustrate aspects of the work of chemists which benefit society and the environment. Unwanted fires cause considerable economic and environmental damage.
Often chemists respond tot he needs of society by developing new, safer products. This happened in 1928 when Thomas Midgely was asked to find a safer alternative to the early refrigerants sulphur dioxide and ammonia. He suggested the use of CF Cl and demonstrated its lack of toxicity of inhaling the gas and blowing out a candle. In recent years we have learnt that the introduction of CFCs like CF Cl were not without environmental consequences. These consequences have been identified by chemists and other scientists. An understanding of the processes involved has also helped in the search for safer replacements.
Nowadays, chemists are designing new “ozone friendly” chemicals to replace the destructive CFCs. The compound 1,1,1,2-tetrafluroethene, CF CH F, is now being manufactured as an appropriate alternative. The presence of the hydrogen atoms increases the reactivity of this compound relative to CFCS, so that it is broken down in the lower atmosphere much more rapidly. If it does reach the stratosphere, it does not produce the damaging chlorine free radicals. The use of such alternatives should allow the ozone layer to recover, although this process may be slow.
The Cl atom is regenerated in this reaction, and so the break up of only one CFC molecule can initiate the subsequent removal of thousands of ozone molecules. This has recently become a major cause for concern, since the ozone forms a vital protective layer in the upper atmosphere, shielding the Earth and all its plants and animals from the harmful effect of the Sun’s UV radiation. Damage to the ozone layer would let more UV through to reach the planet’s surface, leading to an increase in skin cancers in animals and humans, and damage to vegetation.
The discovery of the ‘hole’ in the ozone layer above the Antarctic in the 1980s, led to a United Nations protocol to reduce the use of CFCs and other chlorocarbons as solvents, refrigerants and aerosols by 50% by 1999, and a total ban soon after. CFCs are now banned from aerosols, and replacements for their use in refrigeration (based on ammonia or the less dangerous hydrofluorocarbons, HFCs) are now being introduced. However CFCs are so tenacious, that it will take many years before they are all removed from the atmosphere. Some estimates say that the ozone layer may not re-establish itself properly for a century or more.