Relative gas-phase desubstitution rates of chlorobenzene derivatives by hydrogen atoms near 1 000 K

Abstract

Thermolysis of arene/H₂ mixtures at atmospheric pressure near 1 000 K leads to desubstitution via attack of hydrogen atoms: (Ar–X + H˙→ ArH). Rates of dechlorination (and, where appropriate, dehydroxylation and defluorination) have been examined for a variety of Chlorobenzene derivatives, including o,m,p-dichlorobenzenes, 1,2,4-trichlorobenzene, o-,m-chlorofluorobenzenes, o-,m-,p-chlorophenols, and 2,4-dichlorophenol. Employing competitive studies and computer analysis of product distributions, the rates of desubstitution have been determined relative to (1), (C₆H₅Cl + H˙→ C₆H₆). Observed per-site rate constants for dechlorination of the substrates were 0.66–1.44 times that of C₆H₅Cl. Electron-withdrawing substituents retard the reaction rate, and electron-donating groups lead to rate enhancement. Substituent effects on dehydroxylation were found to be similar; rates of defluorination were less affected by ring substituents. The results are discussed in terms of the reaction mechanisms.