ja020367z_si_001.pdf (382.41 kB)
Comparison of Oxygen and Sulfur Effects on Keto−Enol Chemistry in Benzolactone Systems: Benzo[b]-2,3-dihydrofuran-2-one and -2-thione and Benzo[b]-2,3-dihydrothiophene-2-one and -2-thione
journal contribution
posted on 2002-07-11, 00:00 authored by A. J. Kresge, Q. MengCarbon-acid ionization constants,
(concentration quotient at ionic strength = 0.10 M), were
determined by spectrophotometric titration in aqueous solution for benzo[b]-2,3-dihydrofuran-2-one (3,
p
= 11.87), benzo[b]-2,3-dihydrothiophene-2-one (2, p
= 8.85), and benzo[b]-2,3-dihydrofuran-2-thione (1, p
= 2.81). Rates of approach to keto−enol equilibrium were also measured for the latter two
substrates in perchloric acid, sodium hydroxide, and buffer solutions, and the rate profiles constructed
from these data gave the ionization constants of the enols ionizing as oxygen or sulfur acids p
= 5.23
for 2 and p
= 2.69 for 1. Combination of these acidity constants with the carbon-acid ionization
constants according to the relationship
/
= KE then gave the keto−enol equilibrium constants pKE =
3.62 for 2 and pKE = 0.12 for 1. The fourth, all-sulfur, member of this series, benzo[b]-2,3-dihydrothiophene-2-thione (4), proved to exist solely as the enol in aqueous solution, and only the enol ionization constant
p
= 3.44 could be determined for this substance; the limits pKE < 1.3 and p
< 2.1, however, could
be set. The unusually high acidities and enol contents of these substances are discussed, as are also the
relative values of the ketonization and enolization rate constants measured; in the latter cases, Marcus
rate theory is used to determine intrinsic kinetic reactivities, free of thermodynamic effects.