Studies in gas‐phase thermal hydrogenolysis. Part IV: Chlorobenzene and o‐dichlorobenzene

The gas‐phase thermolysis of chlorobenzene 1 and o‐dichlorobenzene 6 in H2 at atmospheric pressure between 773‐1280 K, τ = 3‐9 s, has been carried out in tubular and tank flow reactors. On a per‐chlorine basis, 1 and 6 react at almost identical rates; the major end products are HCl and benzene. Small amounts of biphenyl (3) and, at incomplete dechlorination levels, chlorobiphenyls are also formed. Dechlorination levels of 99.95% have been achieved. In an excess of H2, the pseudo‐first‐order rate constant for HCl production follows approximately log(k/s−1) ⋍ 10.9 ‐ 241.kJ/mol/(2.3. R. T) between 928 and 1155 K. The mechanism involves hydrogen atoms, which dechlorinate 1 and 6 via both ipso substitution and chlorine abstraction. At temperatures ≥ 900 K, atomic hydrogen is almost in equilibrium with H2, while at lower temperatures, production of H• can be chemically induced, leading to chain reactions and a steady‐state concentration of H• greater than that for equilibrium.