Q5(a). Tetrafluoroethylene from methane; uses of PTFE

Introduction. The building block (monomer) of PTFE, poly(tetrafluoroethylene) or Teflon, is tetrafluoroethylene, CF₂=CF₂. Industrially it is made from methane through chloroform and chlorodifluoromethane (HCFC-22).

Scheme: Methane → chloroform → chlorodifluoromethane → tetrafluoroethylene → PTFE.

1. Chlorination. CH₄ + 3Cl₂ →CHCl₃ + 3HCl, a radical chain substitution (excess Cl₂; the mixture of chloromethanes is separated by distillation).
2. Fluorination (Swarts reaction). CHCl₃ + 2HF →CHClF₂ + 2HCl. Antimony(V) halide exchanges Cl for F; the reaction stops cleanly at CHClF₂.
3. Pyrolysis. 2CHClF₂ → CF₂=CF₂ + 2HCl. At high temperature HCl is eliminated (α-elimination) to give difluorocarbene,:CF₂, which dimerises to tetrafluoroethylene.
4. Polymerisation. CF₂=CF₂ is polymerised by a radical chain (ammonium persulfate initiator, water, pressure) to -(CF₂CF₂)_(n)⁻.
5. Why PTFE is special. The C–F bond is the strongest single bond to carbon, and the fluorine atoms form a tight helical sheath round the carbon chain. PTFE is therefore chemically inert, non-polar, non-wetting, has a very low coefficient of friction and is stable to high temperature.

Applications of PTFE

• Non-stick coatings on cookware and moulds.
• Chemical-resistant gaskets, valve and pipe linings, seals, laboratory ware, stirring bars and thread-sealing tape.
• Electrical insulation of high-frequency and high-temperature cables (low dielectric loss).
• Low-friction bearings, bushes and slide plates; porous expanded PTFE in waterproof breathable fabrics and filters; vascular grafts in medicine.

Conclusion. CH₄ → CHCl₃ (Cl₂/hν) → CHClF₂ (HF/SbCl₅) → CF₂=CF₂ (pyrolysis, via:CF₂), which is polymerised by radicals to PTFE: an inert, non-stick, low-friction, insulating polymer.

Application. PTFE is chosen wherever hot concentrated acids, alkalis or oxidants must be handled (chemical plant linings) and in aerospace wiring; its inertness also makes it the standard material for laboratory sample containers.