chlorofluorocarbon

Chlorofluorocarbons (or CFCs are as the name suggests materials that are composed of chlorine and fluorine (halogens)combined with carbon. They are readily condensed gases at room temperature and pressure. They are useful in several applications because of their inert nature and the relative ease with which they can be evaporated and condensed.

• Halon is used used as a fire extinguisher gas in computer systems because it is non-conductive (unlike water), does not mess up systems (unlike powder), and does not scramble magnetic information (unlike carbon dioxide).
• Freon was used for a very long time as the primary refrigerant in most systems.
• CFCs were also used as the primary propellant in most aerosol cans as they did not react with the active ingredients.

Unfortunately it has been discovered that they are one of the primary gases that attack the ozone layer (protects the Earth from excessive ultraviolet irradiation). Due to their relative inactivity they also persist in the environment for a considerable period of time. Nowadays there is a major push to replace them with other more environmentally friendly materials.

Every wonder what CFC-12 and CFC-134A and all those numbers mean? There's a simple rule for finding out the chemical structure. Take for example CFC-134, the stuff that they're using now in cars. Start with the number 134 and add 90:

          1 3 4
        +   9 0   the magic conversion number
          _____
          2 2 4

Here's the rule, the first 2 is the number of carbons, the second 2 the number of hydrogens, and the 4 the number of fluorines. Any leftover valences are filled with chlorines. Thus, CFC-134 looks like:

          F F
          | |
        H-C-C-H  
          | |
          F F

or some isomer thereof. An A after the name signifies a asymmetric stereoisomer. Since all the valences are filled with hydrogens and fluorines, there is no room left for chlorine. That's why this compound is used now instead of CFC-12, the refrigerant that's now been banned:

          1 2
        + 9 0
        _____
        1 0 2

So - one carbon, no hydrogens, 2 fluorines and the rest chlorines. This looks like:

           F
           |
        Cl-C-F
           |
           Cl

This is the stuff that's so bad for the ozone layer.

A little piece of 'industrial espionage' (TWAJS) I got from a friend at Dupont.

In 1972, the chemical industry was producing about 700,000 metric tons of chlorofluorocarbons (CFCs) per year. A chlorofluorocarbon is a compound composed of just carbon, chlorine, and fluorine. Most of the CFCs produced in the early 70’s were either CFC-11 (CFCl₃,) or CFC-12 (CF₂Cl₂.)

When these substances reach the stratosphere, they come into contact with radiant energy (ie. ultraviolet light) strong enough to break them down. For instance, radiant energy of wavelength less than 215 nm will break the covalent bond between one of the chlorine atoms and the carbon atom in CF₂Cl₂.

        CF2Cl2(g) -> CF2Cl(g) + Cl(g)

The chlorine atoms released in this type of reaction can destroy ozone molecules, i.e:

	Cl(g) + O3(g) -> ClO(g) + O2(g)
        ClO(g) + O(g) -> Cl(g) + O2(g)

Each chlorine atom is thought to destroy an average of 1000 ozone molecules before it is converted into an inactive form. Two important inactive chlorine compounds are HCl and ClONO₂. They are formed in reactions such as:

        CH4(g) + Cl(g) -> CH3(g) + HCl(g)
        ClO(g) + NO2(g) + M -> ClONO2(g) + M 
       (M = molecule to carry off excess energy)

In recent times, much research has been conducted into finding substitutes for CFCs; for example, since 1995 carbon dioxide has been used to blow polystyrene foam as opposed to CFCs. By reducing the output of CFCs, the depletion of the ozone layer will stop increasing in speed, and hopefully the original equilibrium between the formation of ozone and the natural destruction of ozone will be restored.

The chloro and fluoro carbons, when going into the air, tend to migrate towards the poles, where they will remain frozen during the course of arctic and antarctic winter, once the sun hits the poles, large clouds of frozen chemical undergo the reaction in the write-up above producing chlorine in a gaseous form. (which is actually Cl2(g). When hit with light, chlorine molecules undergo a chain reaction activated by light, which involes chlorine radicals, (an atom with an uncoupled electron in it's outer shell - ususally denoted by the element name with a suprescripted dot), radicals are VERY reactive, and are attracted to the conjugated bonds of the ozone, breaking it up.

The danger is: these compounds get released in large enough amounts at once because they are able to accumilate in its frozen form over half a year of winter

Also, ozone takes a LOT of energy to regenerate and it does not happen nearly at same rate as it si destroyed. Curiously enough, when you read that the hole has gotten smaller over the last decade, don't be mislead. Ozone, out atmosphere in general, is made up of gases, and naturaly over the period of time, the will diffuse from higher concentration to lower concentration, thus, the ozone hole, so as the hole has gotten smaller, the overall atmosphere has gotten thiner over this century.