Flash photolysis of 5-methyl-1,4-naphthoquinone in aqueous solution: Kinetics and mechanism of photoenolization and of enol trapping

5-Methyl-1,4-naphthoquinone (1) is a remarkable probe to study hydrogen and proton transfer reactions. The photoenol 4-hydroxy-5-methylidene naphthalen-1(5H)-one (2) is formed in the ground state within 2 ps of excitation and with a quantum yield of unity, presumably through a conical intersection of the S-0 and S-1 hypersurfaces. In aqueous acid, enol 2 is hydrated to 5-(hydroxymethyl)naphthalene-1,4-diol 3 (X = OH, Scheme 1). The rate of hydration of 2 increases linearly with acid concentration from ca. 1.5 x 10(4) s(-1) at pH 6 to reach a maximum value of 9 x 10(7) s(-1) when the remaining carbonyl function is protonated, pK(a)(2(+)) = 1.1. Contrary to an earlier suggestion, the rate-determining step in the acid-catalyzed hydration of 2 is addition of water to the conjugate acid 2(+). Pronounced acceleration of the decay rate of 2 by hydrazoic-acid buffers indicates competitive trapping of 2(+) by the azide ion. In neutral-to-weakly-basic solutions, enol 2 reacts by ionization, pK(a)(2) = 6.5, and nearly diffusion-controlled condensation of the carbanionic species 2(-) with quinone 1. Protonation at the methylidene C-atom does not compete measurably with protonation on carbonyl O-atom, despite a substantial thermodynamic bias for carbon protonation of ca. 50 kJ mol(-1) for 2, and 100 kJ mol(-1) for 2(-).