Generation of Phenol and Molecular Hydrogen through Catalyst-Free C-H Activation of Benzene by Water Radical Cations.

Here, we report on the abundant formation of phenol and molecular hydrogen when benzene vapor was exposed to gas plasma generated by +5.5 kV corona discharge of water vapor in argon in the absence of oxygen. Systematic analysis using a series of isotopic standards ( d 6 -benzene, D 2 O, and H 2 18 O) and benzene derivatives (mono-, di-, trichlorobenzene, and N , N -dimethylaniline) indicated that the formation of phenol occurred through the reaction between neutral benzene and the radical cation of water dimer, (H 2 O) 2 +• . A two-step reaction mechanism was proposed based on the results of experiments and DFT calculations: (1) the formation of (C 6 H 6 ...H 2 O) +• intermediate through electrophilic addition; (2) the formation of C 6 H 5 OH +• through the release of H 2 from the (C 6 H 6 ...H 2 O) +• intermediate. Our findings offer a novel catalyst-free method to prepare phenol from benzene with phenol selectivity >90%.