Kinetics of Ketyl Radical Reactions Occurring in the Photoreduction of Benzophenone by Isopropyl Alcohol

Abstract

AbstractBenzophenone – isopropyl alcohol – acetonitrile, benzophenone – benzhydrol – acetonitrile and acetone – isopropyl alcohol – acetonitrile systems were studied by laser flash photolysis transient absorption technique and by steady‐state quantum yield determinations. Kinetic parameters were derived for reactions of the ketyl radicals formed in the primary photoreduction steps. The room temperature overall rate coefficients at 0.001 mol dm−3 benzophenone and 2.6 mol dm−3 isopropyl alcohol in acetonitrile are in 108 dm3 mol−1 s−1 units: 12.8 ± 0.6, 8.0 ± 0.4 and 0.59 ± 0.01 for Mc2ĊOH + Me2ĊOH, Me2ĊOH + Ph2ĊOH and Ph2ĊOH + Ph2ĊOH, respectively. (The error limits represent 2 standard deviations.) Combination was shown to be the only significant reaction in the interaction of two Ph2ĊOH radicals, while disproportionation is the dominating path in the reactions inolving Me2ĊOH radicals. Evidences are presented for the existence of four reaction channels in the cross reaction of the two ketyl radicals. These are combination (4a), disproportionations (4b) and (4c) and formation of a “light absorbing transient” (4d), with branching ratios k4a/k4 = 0.18 ± 0.08, k4b/k4 = 0.61 ± 0.11, k4c/k4 = 0.05 ± 0.02 and k4d/k4 = 0.18 ± 0.03, respectively. – The radical transfer reaction Me2ĊOH + Ph2CO → Me2CO + Ph2ĊOH, which may change considerably the product distribution and quantum yields in the photoreduction of benzophenone by isopropyl alcohol at medium and low light intensities, was investigated by quantum yield determinations and a room temperature rate coefficient of k5 = (7.3 ± 1.2) · 104 dm3 mol−1 s−1 was extracted through computer model simulation. This rate coefficient is considerably lower than the values reported previously.