Gas‐phase oxygenation of benzene derivatives around 300 K with O(3P) atoms produced by microwave discharge of N2O. Part 2. Kinetic H/D isotope effects

The possible pathways for the formation of (chloro)phenol, following the addition of O(3P) to (chloro)benzene, have been examined using deuterated substrates: C6D6 (also in admixture with C6H6) and p-deuterochlorobenzene. Whereas with O-C6H6 adduct biradicals, loss of H to give phenoxy radicals predominates, only one-third of the O-C5D6 intermediates undergo the corresponding reaction. Phenoxy radicals lead to phenol by transfer of an H(D) atom from cyclohexadienyl-type radicals, formed from H (D) and substrate. Analogously, in reactions of p-deuterochlorobenzene, loss of H is a major reaction after addition of an oxygen atom to a meta position, whereas loss of D (to give p-chlorophenol) occurs only with 35% of the corresponding O(3P) adduct biradicals. The isotopic composition of phenol formed from p-DC6H4Cl (via p-DC6H4O; generated by ipso substitution) revealed that H transfer to phenoxy radicals primarily gives the keto tautomers as major products. Isomerization of (chloro)-benzene-O(3P) adduct biradicals to the corresponding phenols also appears to involve mainly, keto tautomers. The reaction of O(3P) with p-deuterochlorobenzene showed a slight change in the o/m/p distribution; this can be explained by the absence of a net secondary H/D kinetic isotope effect for O(3P) addition to the para site and a normal secondary isotope effect for meta addition.