Quenching, Radical Formation, and Disproportionation in the Photoreduction of 4-Carboxybenzophenone by 4-Carboxybenzhydrol, Hydrazine, and Hydrazinium Ion

Article abstract of DOI:10.1021/ja00414a047

Laser flash and steady irradiation studies have been carried out on the 4-carboxybenzophenone (CB) - 4-carboxybenzhydrol (CBH2) and CB - hydrazine systems over the pH range 5.8-12.In reaction with CBH2, CB triplet is quenched in part by the carboxylate, k_q ca. 4 * 10⁶ M^-1 s^-; total reaction, k₄ = 3.0 * 10⁷ M^-1 s^-1, leads to the ketyl radical CBH^., φ = 1.7.Absorption spectra were recorded for CB triplet, λ_max = 535 nm (ε = 6600 M^-1 cm^-1), for CBH^., λ_max = 570 nm (ε = 5500), and for the radical anion CB^-., λ_max = 660 (ε = 8100).The pK_a of CBH^. was determined, 8.2.Rate constants were determined: for combination of CBH^. to pinacol, 2k₆ = 1.8 * 10⁹ M^-1 s^-1; for reactions of CBH^. with CB^-., k₇ = 1.1 * 10⁹ M^-1 s^-; for disproportionation of 2CB^-., 2k₈ = 9.8 * 10⁶ M^-1 s^-.Factors to convert observed yields of photoreduction by hydrazine to yields of ketyl radical leading to pinacol and hydrol were determined from effects of pH on quantum yields of reduction of CB by CBH2.Reduction of CB triplet by neutral hydrazine occurs with k_ir = 6.8 * 10⁸ M^-1 s^-1, forming ketyl radical with φ = 1.0, measured relative to the CBH2 reaction.The reaction with protonated hydrazine is slower, k'_ir = 4.4 * 10⁶ M^-1 s^-1.Computer simulation, using the measured rate constants, pK_a and primary yields, and literature constants for pure hydrazine systems, leads to calculated reduction yields very similar to the observed values, with φ_ketyl ca. 0.0 for reaction of triplet with protonated hydrazine, and ratios of rate constants for ketyl-hydrazyl disproportionations appropriately close to unity.