Chlorofluorocarbon - Structure, Properties, Production

Structure, Properties, Production

As in simpler alkanes, carbon in the CFCs and the HCFCs is tetrahedral. Since the fluorine and chlorine atoms differ greatly in size from hydrogen and from each other, the methane derived CFCs deviate from perfect tetrahedral symmetry.

The physical properties of the CFCs and HCFCs are tunable by changes in the number and identity of the halogen atoms. In general they are volatile, but less so than parent alkane. The decreased volatility is attributed to the molecular polarity induced by the halides and the polarizability of halides, which induces intermolecular interactions. Thus, methane boils at −161 °C whereas the fluoromethanes boil between −51.7 (CF₂H₂) and −128 °C (CF₄). The CFCs have still higher boiling points because the chloride is even more polarizable than fluoride. Because of their polarity, the CFCs are useful solvents. The CFCs are far less flammable than methane, in part because they contain fewer C-H bonds and in part because, in the case of the chlorides and bromides, the released halides quench the free radicals that sustain flames.

The densities of CFCs are invariably higher than the corresponding alkanes. In general the density of these compounds correlates with the number of chlorides.

CFCs and HCFCs are usually produced by halogen exchange starting from chlorinated methanes and ethanes. Illustrative is the synthesis of chlorodifluoromethane from chloroform:

HCCl₃ + 2 HF → HCF₂Cl + 2 HCl

The brominated derivatives are generated by free-radical reactions of the chlorofluorocarbons, replacing C-H bonds with C-Br bonds. The production of the anesthetic 2-bromo-2-chloro-1,1,1-trifluoroethane ("halothane") is illustrative:

CF₃CH₂Cl + Br₂ → CF₃CHBrCl + HBr