CFC can cause ozone to break down, as you read in Making and Destroying Ozone. Remember that all the mayhem begins when a CFC molecule breaks down to release chlorine atoms.

Keep this reaction in mind while we talk about a consequence of the work done by Mario Molina. Since CFCs were wreaking havoc on the ozone layer, they are being phased out in the United States and other industrialized countries. That sounds easy, but remember that since CFCs won't be used in refrigerators or air conditioners anymore, someone now has to find new compounds to use as refrigerants. How might you make an ozone-safe refrigerant? Remember that the properties of any material are determined by the molecular structure of its molecules. So a good place to start would be by changing the molecular structure of the CFC. How would you change the molecular structure of the CFC molecule shown in the reaction above to make it safe for ozone?

Here's an idea: Get rid of that chlorine atom and replace it with some other kind of atom. Let's suppose we made a compound similar to the CFC you saw above, but without the chlorine atom. Swap the chlorine (Cl) atom for a hydrogen (H) atom, and the result is what you see below.

chlorotrifluoromethane (a CFC)	trifluoromethane (an HFC)

Since the new compound, called a hydrofluorocarbon (HFC), has no chlorine atoms, it should be safer for the ozone layer. This is wonderful, but do HFCs make good refrigerants? To be a good refrigerant, a compound must have a low boiling point, because it has to vaporize from a liquid to a gas without much difficulty in the refrigerator. Also, the compound has to be nontoxic. If not, a broken refrigerator could poison everyone in the house!

Let's take a look at the properties of trifluoromethane to see if it could be a good refrigerant. Specifically, we want to look at its boiling point and its toxicity. Trifluoromethane boils at -84°C, so it passes the first test. As for toxicity, the compound is practically nontoxic. So it is safe to use in home refrigerators and air conditioners. Since it meets these requirements, trifluoromethane has become a common refrigerant, and is sold by DuPont under the name Freon^¨-23. Two other common HFC refrigerants are shown below.

HFC-134a (boiling point = –26.5°C)	HFC-125 (boiling point = –48.5°C)

Sign found on the air-conditioning unit of a Philadelphia commuter train, equipped with new HFC refrigerant.

These compounds seem to be safer alternatives to CFCs, though they aren't perfect. To avoid repeating the mistakes of the past, HFCs and other potential refrigerants are studied with great scrutiny to spot any potential danger before a new refrigerant is put into use. One scientist who works to catch the trouble before it starts is Joseph Francisco.

Next: Meet Joseph Francisco