The Reactions of Hydroxyl Radicals with 1,4-and 1,3-Cyclohexadiene in Aqueous Solution

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Abstract
The reactions of radiolytically generated hydroxyl radicals and H atoms with 1,4- and 1,3-cyclohexadiene were studied by pulse radiolysis and product analysis. Hydrogen abstraction from these substrates by the OH radical yields the cyclohexadienyl radical (.epsilon. (310 nm) = 4400 dm3 mol-1 cm-1 from the reaction of the H atom with benzene) with an efficiency of 50% (0.29 .mu.mol J-1) in the case of 1,4-cyclohexadiene and 25% (0.15 .mu.mol J-1) in the case of 1,3-cyclohexadiene as determined by pulse radiolysis. The remaining OH radicals add to the olefin. In 1,4-cyclohexadiene the yield of the resulting adduct radicals has been determined in a steady-state 60Co-.gamma.-irradiation experiment by reducing it with added 1,4-dithiothreitol (DTT) to 4-hydroxycyclohexene. There are two sites of OH radical attack in the case of 1,3-cyclohexadiene, and only the alkyl radical is reduced quantitatively by DDT (G(3-hydroxycyclohexene) = 0.15 .mu.mol J-1). From material balance considerations it is concluded that the allylic radical must be formed with a G value of 0.28 .mu.mol J-1 but largely escapes reduction by DTT (G(4-hydroxycyclohexene) = 0.03 .mu.mol J-1). H atoms add preferentially to the double bonds of 1,4- and 1,3-cyclohexadiene (78% and 93%, respectively), while the O.- radical (the basic form of the OH radical) undergoes mainly H-abstraction (92% and 83%, respectively). The radicals formed in these systems decay bimolecularly (2k = 2.8 .times. 109 dm3 mol-1 s-1). In their combination reactions the cyclohexadienyl radicals form the four possible dimers in proportions such that the dienyl radical moiety shows a 2:1 preference to react from its central (1a) rather than from a terminal carbon atom (1b). Cyclohexadienyl radicals and the OH- and H-adduct radicals also cross-terminate by disproportionation and dimerization. Material balance has been obtained for the 1,4-cyclohexadiene system in N20-saturated solution (10-2 mol dm-3) at a dose rate of 0.14 Gy s-1, the products (G values in .mu.mol J-1 being: benzene (0.085), 4-hydroxycyclohexene (0.25), cyclohexadienyl-dimers (0.144), cyclohexadienyl-OH-adduct-dimers (0.02), OH-adduct-dimers (0.02). Some of the 4-hydroxycyclohexene is formed in an H-abstraction reaction by the OH-adduct radical from 1,4-cyclohexadiene.

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