ISSN:
0018-019X
Keywords:
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
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-.
Additional Material:
6 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/hlca.19970800408
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