R12 has the ozone depleting chemical chlorine in it and has been replaced with a refrigerant known as R134a. A boiling fluid absorbs heat and changes its state to a vapor. The expanding vapor/gas has a lower pressure than the liquid and therefore lower temperature. For heated air to be absorbed it must transfer to a cooler surface. The trick is to find a liquid that boils at such a low temperature that the vapor is cold enough to absorb the amount of heat generated inside an automobile. Freon-12 boils at -22º F and its expanding vapor, in the evaporator core, absorbs the heated air circulating around it. Freon-134a boils at about the same temperature as R12 and therefore is a good replacement.