Flash Photolysis of 5‐Methyl‐1,4‐naphthoquinone in Aqueous Solution: Kinetics and mechanism of photoenolization and of enol trapping

Abstract 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 S0 and S1 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 × 104 s−1 at pH 6 to reach a maximum value of 9 ×107 s−1 when the remaining carbonyl function is protonated, pKa(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, pKa(2) = 6.5, and nearly diffusion‐controlled condensation of the carbanionic species 2− with quinone 1. Proto‐nation at the methylidene C‐atom does not compete measurably with protonation on carbonyl O‐atom, despite a Substsial thermodynamic basic for carbon Portoation of ca. 50 kJ mol−1 for 2 and 100 mol−1 for 2−.