Refrigerants for the 21st Century
13. Synthesis Challenge
|THE SYNTHESIS CHALLENGE FOR HFC-134a AND HFC-125 GOES BEYOND THE CORE CONCEPTS OF CARBON-BASED MATERIALS AND MAIN GROUP ELEMENTS|
In developing processes for the early CFC's and HCFC's, chemists had learned how to attach F to carbon-containing molecules to form C-F bonds. This area of chemistry, called "Organofluorine Chemistry", is quite different from classical organic chemistry. New rules had to be learned. One that became apparent early on is that exchange of F for multiple Cl's attached to carbon becomes increasingly difficult as more C=F bonds are formed. Thus, in the preparation of CFC-11/-12 (CFCl3/CF2Cl2) from CCl4 using AHF and an Sb(V) halide catalyst, going beyond two C-F bonds is very difficult. Special techniques are required to prepare CF3Cl (CFC-13) and CF4 (FC-14).
One technique that has become commercially important is high temperature (300-400oC.) F exchange using AHF, a vaporized chlorinated raw material and solid metal oxide or metal fluoride catalysts. These catalysts can be iron (III) oxide (Fe2O3), chromium (III) oxide (Cr2O3) or fluoride (CrF3), or aluminum fluoride (AlF3). This vapor-phase process is especially useful for the less reactive C-Cl bonds and has been used to prepare CFC-13 and FC-14, among others.
Another characteristic of the F for Cl exchange process is that exchange is easier when all the substituents attached to C are halogens or another carbon, rather than hydrogen. In the extreme, when there is only one carbon-halogen bond in the starting material, exchange is quite difficult. Thus, preparation of methyl fluoride (CH3F) from the corresponding chloride (CH3Cl) is almost impossible by F exchange with AHF.
One type of reaction which is common to both organofluorine chemistry and classical organic chemistry is addition to alkenes. AHF can add to most alkenes - even those highly substituted with other halogens, such as trichloroethylene (= triclene; CCl2=CHCl) or tetrachloroethylene (= perclene; CCl2=CCl2).
An awareness of the special features of organofluorine chemistry, as well as classical organic chemistry, led to processes for HFC-134a and HFC-125.
Concept Map for this ChemCase
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Principal Investigator Laurence Peterson; Project Director Matthew Hermes;
Author of this module William Gumprecht.