We have shown previously that hydroxyl (or alkoxyl) and carbon-centered quinone ketoxy radicals can be produced during the metal-independent decomposition of H2O2 (or organic hydroperoxides) by tetrachloro-1,4-benzoquinone (TCBQ) and other halogenated quinones,and a novel nucleophilic substitution coupled with homolytical decomposition mechanism was proposed.Hydroxamic acids were found to markedly inhibit the formation of these radicals via an unexpected two-step Lossen rearrangement mechanism.Due to the instability of the proposed quinone-hydroxamic acid intermediate,we can not isolate and identify it.Recently we found that the interaction between benzohydroxamic acid (BHA) and 2,5-dichloro-1,4-benzoquinone (DCBQ) could form a relative stable DCBQ-BHA intermediate,which could also decompose slowly,under alkaline conditions or with higher temperature,to produce phenyl isocyanate,via Lossen rearrangement.Here we show that the N-methyl derivative of BHA (N-MeBHA) could also react with DCBQ to form a similar stable product DCBQ-N-MeBHA.However,the slow decomposition of this product was found,unexpectedly,to be through a radical pathway,not through the Lossen rearrangement.The major reaction product between N-MeBHA and DCBQ was identified as an unusual C-N bonding compound 2-OH-3-[ArC(O)-N(CH3)]-5-chloro-1,4-benzoquinone [CBQ(OH)-N(CH3)-CO-Ar],which is the rearranged isomer of initial transient product,the C-O bonding compound 2-[ArC(O)-N(CH3)-O]-5-chloro-1,4-benzoquinone.Interestingly,a new 18-line nitrogen-centered radical and the carbon-centered quinone ketoxy radical were observed by the ESR spin-trapping study with BMPO (an analog of DMPO) as the spin-trapping agent.Analogous results were also observed for other halogenated quinones.Based on these data,a unique mechanism for the reaction between DCBQ and N-MeBHA was proposed: A nucleophilic reaction may take place between DCBQ and N-MeBHA,forming a relatively stable CBQ-O-N-MeBHA initial product,which can decompose slowly and homolytically to produce the nitrogen-centered radical ?N(CH3)-COAr and the oxygen-centered quinone enoxy radical CBQ-O?.CBQ-O? could isomerize to form the carbon-centered quinone ketoxy radical ?CBQ=O,which then coupled with ?N(CH3)-COAr to produce the major reaction product CBQ(OH)-N(CH3)-COAr via keto-enol tautomerization.These findings may have broad biological and environmental implications for future research on the widely-used hydroxamate pharmaceuticals and the ubiquitous polyhalogenated quinoid carcinogens.