Molecule of the Day

So, as we mentioned yesterday, good old Thomas Midgley had made quite a mess of things with his organolead snafu. By way of apology, he gave us CFCs. Why would we want them?

To make a refrigerator, you need a gas that can go pretty easily between liquid and gas - so, something with a near-room temperature boiling point at atmospheric pressure. If you’re a chemist, you’re thinking: ether, anhydrous ammonia, sulfur dioxide, and a bunch of other nasties. That was, in fact, the stuff we used first (Well, not ether. But loads of SO2 and NH3). Trouble was, it was so toxic it kept killing people. We needed something that balanced volatility with low toxicity.

Well, we had these, too. The simple low-number alkanes (butane, propane) aren’t actually that toxic, but they are flammable. So, we needed something lacking flammability as well. The CFCs fit this fine. Carbon-halogen bonds are weak; weaker as you go down the series (Fluorine, Chlorine, Bromine, Iodine). This made them good fire suppressants for reasons we won’t discuss here (See halon 1201 below, note the C-Br bond), as well as ozone depleting chemicals.

The weaker C-Cl and C-Br bonds break, leaving a trihalocarbon neutral radical, which reacts with ozone to form plain old dioxygen. Ozone is an okay UV absorber; only because there is loads of it can we filter out most of the “hard” (high energy/carcinogenic/mutagenic) UV photons coming from the sun. Since halons have C-Br bonds, they are TERRIBLE for the ozone and are outright banned; very very few places are allowed to use halon. Airplanes are the only exception I can think of; even museums are forced to use stuff like CO₂. Interestingly, concentrations of halon on the order of 5% are adequate for fire suppression. This means that a halon fire extinguisher can work in a room with air you can breathe in (for awhile, at least - a minute or so - long enough to get out). CO2 extinguishers will leave you gasping literally instantly. You don’t have oxygen sensors for your blood, you have CO2 sensors.
R-12, above, was an ubiquitous refrigerant which has been supplanted by R-134a. Note that there are no C-Cl bonds, and only strong C-F bonds remain. This decreases its tendency to form a neutral radical in the ozone markedly. I’ve been told that there has been some experimental work on IODOcarbon refrigerants, which would seem like they’d be the absolute worst. Apparently, though, they break down before they even reach the upper atmosphere, so no harm done. I don’t believe any of these ever made it to market though.

R-12 still finds its way into some uses, and I believe you’re still allowed to use it pretty freely as long as it’s recycled. I am told that virgin R-12 makes its way into the country via Mexico and other sources, being “laundered” into the “recycled” R-12 pathway. Yep, black market CFCs!