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Benzene is not formed in flames by high-pressure-limit addition reactions, as had been implied previously, but by chemically activated addition and isomerization reactions. Possible mechanisms were tested by comparing predicted benzene formation rates (from measured reactant concentrations and predicted rate constants) against measured net formation rates of benzene. First, mole-fraction and rate data for molecules and free radicals were measured in a lightly sooting, laminar, premixed flame of C2H2/02/Ar at 1000-1700 K and 2.67 kPa (20 Torr) by using molecular-beam mass spectrometry. Second, mechanisms were screened in this flame and a similar 1,3-butadiene flame by using high-pressure-limit rate constants. Third, pressure-dependent rate constants for all channels of successful mechanisms were analyzed by bimolecular quantum-RRK calculations. Finally, data tests with these more accurate rate constants showed that only additions of vinylic 1-C4H5 and 1-C4H3 radicals to C2H2 were fast enough to account for the highest observed rates of benzene formation, forming benzene and phenyl "directly" by chemically activated channels. These reactants have been suggested before, but the pathways are crucially different from high-pressure-limit routes. A similar addition of C2H3 to C4H6 also contributed in the butadiene flame at lower temperatures.

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