Thermal Decomposition of Carbon Tetrachloride

Article abstract of DOI:10.1021/j100111a032

The first rate measurements of the thermal dissociation of CCl4 are reported.Three detection techniques were used in monitoring the reaction rate for various dilutions over a wide temperature range: (i) ARAS of product Cl atoms in reflected shock waves using 3.2-6.4 ppm of CCl4 in Ar over 1084-1705 K and 150-908 Torr, (ii) decay of CCl4 by molecular absorption of O-atom resonance radiation in reflected shock waves using 48 - 173 ppm of CCl4 in Ar over 1192 - 1733 K and 219 - 855 Torr, and (iii) laser schlieren density gradients in incident shock waves using 0.5 and 2 percent CCl4 in Kr over 1470 - 2186 K and 90 - 660 Torr.The second-order rates from ARAS and molecular absorption measurements for the bond fission reaction CCl4 -> CCl3 + Cl are in complete agreement with the laser schlieren results where they overlap.The temperature and pressure dependence of these rates is well characterized by Gorin model RRKM calculations using a current Δ H⁰₀ = 67.71 kcal/mol for E₀, derived from Δ_fH⁰₂₉₈ = 17.0 kcal/mol for CCl3.The low-pressure rate constant (k₀) derived from this RRKM fit is log k₀ (cm³/mol s)) = 54.980 - 10.624 log T - 74.796 (kcal/mol)/2.303RT.These low-pressure rates require unusually large β_c corresponding to a <ΔE>_down = 1200 cm^-1.This may be a general feature of chlorocarbon dissociations.The ARAS data indicate that two Cl atoms are ultimately produced for each CCl4 that dissociates, with second Cl atom forming slower than the first.Here all measurements are consistent with a further dissociation of CCl3, CCl3 -> CCl2 + Cl, as the dominant source of secondary Cl-atom at a rate about 0.1 that of the primary fission.