Effects of Concentration of Amine and of Medium in Photoreduction of Ketones by Amines

Abraham H. Parola, Anita W. Rose, Saul G. Cohen

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Quantum yields have been determined for photoreduction, over wide ranges of amine concentration (0.005-4.5 M) of benzophenone by cyclohexylamine (a) in benzene and (b) in tert-butyl alcohol and of 4-benzoylbenzoate (c) by triethylamine and (d) by 2-butylamine in 1:1 water-pyridine at pH 12. At low concentrations of amine (<0.02 A/), in a and b, linear plots of φ-1 vs. (amine)-1 are obtained, leading to low, useful, values of kd/kir, 0.0012 M, and to low extrapolated values of φim, 0.60 and 0.79, respectively. At high concentrations of amine, values of φ are higher, and the plots are curved, leading to values of 1.09 and 1.49, respectively, and to high apparent values of kd/kir, Inhibition by piperylene of photoreduction by high concentration of amine leads, in system a, to kir = 3.7 × 108 M-1 sec-1, similar to the value determined by quenching of phosphorescence, and kd = 4.5 × 105 sec-1; in system b, it leads to kir = 1.4 × 108 M-1 sec-1 and to kd = 1.7 × 105 sec-1, compared with 1.1 × 105 sec-1 observed by decay of phosphorescence. System c shows a smaller enhancement of quantum yield by high concentration of amine; kir = 8.5 × 108 M-1 sec-1 at both 1 and 0.02 M triethylamine, determined by naphthalene quenching. Increased quantum yield at high amine concentration may arise from excitation of ground-state ketone-amine complex, charge-transfer interaction of this with amine in solution, and catalysis by the initial amine of the proton transfer from the radical cation to the radical anion in the charge-transfer complex. Equilibrium constants for ground-state benzophenone-cyclohexylamine complex are estimated to be 8 ± 2 M-1 in benzene, 3 ± 1 M-1 in tert-butyl alcohol. This effect is less important when the aqueous pyridine may assist the transfer and, in system d, a linear plot of φ-1vs. (amine)-1 is observed over the entire concentration range. Kinetic expressions are developed.

Original languageEnglish
Pages (from-to)6202-6209
Number of pages8
JournalJournal of the American Chemical Society
Issue number21
StatePublished - 1 Oct 1975
Externally publishedYes


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