Chlorofluorocarbons and Ozone

Substituted Hydrocarbons

Chloromethane, CH3Cl, is produced by marine organisms and released into the atmosphere. Because it has C-H bonds, it reacts with HO and O2 like hydrocarbons do.

Chlorofluorocarbons are unreactive towards oxidation

Because of their oxidation reactions, hydrocarbons have a limited lifetime in the troposphere and don't migrate to the stratosphere. Carbon-containing molecules without C-H bonds can't react with hydroxyl radical. These molecules have a much longer lifetime in the troposphere and can migrate to the stratosphere.

X3C-H + OH X3C + H2O H is negative; low activation barrier
X3C-F + OH X3C + HOF H is positive; high activation barrier
X3C-Cl + OH X3C + HOCl H is positive; high activation barrier

Applications of CFCs

Because chlorofluorocarbons don't react in oxidation reactions, they are not flammable. CFCs were developed during the 1920s as replacements for more toxic refrigerants such as NH3, CH3Cl, and SO2. Dichlorodifluoromethane (CFC-12) is a non-flammable and unreactive gas that makes a good refrigerant. It is also called Freon-12.

Other applications for CFCs include:
  • fire retardant
  • blowing agent for plastics (polystyrene foam)
  • cleaning solvent for electronics
  • propellant in spray cans

CFC-11 or CCl3F is an even better refrigerant than CFC-12 because it has a higher boiling point. However, it also has the greatest percentage of chlorine and has the greatest ozone depletion potential. One estimate is that every chlorine atom in the stratosphere destroys 100,000 ozone atoms before the chlorine is converted to an unreactive form. Chlorofluorocarbons trap more heat in the atmosphere per molecule than other substances. The global warming potential of CFC-11 is 4600 times that of CO2!

Compound Name BP Environmental Hazard
CFCl3 CFC-11 23.8 deg C Ozone depletion= 1
Global warming= 4600
CF2Cl2 CFC-12 -29.8 Ozone depletion= 1
Global warming= 8100
CF3Cl CFC-13 -81.5 Ozone depletion= 1
Global warming= 14000
CF2ClCFCl2 CFC-113 47.7 Ozone depletion= 1
Global warming= 5000

Bromofluorocarbons are called Halons. These are particularly useful in fire extinguishers. The lifetime in the troposphere and the effect on the ozone layer are similar for chlorofluorocarbons and bromofluorocarbons.

Reactions with Ozone

When chlorofluorocarbons migrate to the stratosphere they are photolyzed. Higher energy UV radiation in the stratosphere breaks C-Cl bonds and chlorine atoms are formed. Chlorine atoms, and other radical species, react catalytically with ozone to convert it to molecular oxygen. One atom of chlorine can destroy many ozone molecules.

In the next section, we'll cover the reactions that result in decreases of stratospheric ozone and the annual formation of an ozone hole above the arctic. Both energy and rate are important in these reactions.

Professor Patricia Shapley, University of Illinois, 2012