The UV photolysis (λ = 254 and 185/254 nm) of cyclohexene in oxygen-saturated and oxygen-free cyclopentane solution

Abstract

Cyclohexene was photolysed in oxygenated and oxygen-free cyclopentane solutions using the light of a low pressure mercury arc. At 254 nm cyclohexene does not absorb and no product formation is observed in deoxygenated solutions, but in the presence of oxygen peroxyl radicals are formed via a solute-to-oxygen charge transfer absorption. The quantum yields are concentration dependent. The main products (with the quantum yields at 1 per cent cyclohexene content given in parentheses) are cyclopentanol (0.61), cyclopentanone (0.56), cyclohexen-3-ol (0.46), cyclohexen-3-one (0.31), organic hydrogperoxides (0.3) and cyclohexene oxide (0.22). There is radical transfer in this system. The 185 nm light is absorbed by cyclohexene, and in the absence of oxygen the products are hydrogen (2 × 10 −2), tricyclo [6.4.0.0 2, 7] dodecane (7 × 10 −3), 3-cyclopentylcyclohexene (4.6 × 10 −3), 3.3′-bicyclohexenyl (2.6 × 10 −3), cyclopentylcyclohexane (1.7 × 10 −3), 3-cyclohexylcyclohexene (1.2 × 10 −3) and bicyclohexyl (4 × 10 −4). In the presence of oxygen, direct photolysis of the solute by the 185 nm component of the unfiltered light also leads to peroxyl radical formation, as well as to hydrogen and hydrocarbon products via non-radical pathways. The main products due exclusively to the action of the 185 nm component are hydrogen (2 × 10 −2), organic hydroperoxides (1.5 × 10 −2), hexanedial (6 × 10 −3), tricyclo [6.4.0.0 2, 7] dodecane (3 × 10 −3), 3-cyclopentylcyclohexene (2.3 × 10 −3), cyclohexanol (2.3 × 10 −3) and cyclohexanone (1.3 × 10 −3). The oxygenated products are formed from the peroxyl radicals in the course of radical transfer and termination reactions. An interesting difference regarding the reactivity of cyclopentylperoxyl radicals in the hydrocarbon solvent vs. water is pointed out.