- Flash photolysis of α-diazonaphthoquinones in aqueous solution: Determination of rates and equilibria for keto-enol tautomerization of 1-indene-3-carboxylic acid
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Flash photolysis of either 1-diazo-2(1H)naphthalenone (1a) or 2-diazo-1(2H)naphthalenone (1b) generates benzofulven-8-one (2). Hydrolysis of ketene 2 forms benzofulvene-8,8-diol (3), the enol tautomer of indene-3-carboxylic acid (4). pH rate profiles for the reactions 2 → 3 and 3 → 4 were determined in aqueous solution. Ketonization of 3 is catalyzed by acid and by base. Catalysis by protons saturates in strongly acidic solutions, thereby defining the first ionization constant of the enol, pKaE = 1.90 ± 0.05, catalysis by hydroxyl ions saturates in dilute base, defining the second ionization constant, pK′aE = 8.3 ± 0.2. The first (OH) and second (CH) ionization constants of 4 were determined by spectrophotometric titration, pKaK = 4.50 ± 0.03 and pK′aK = 15.2 ± 0.2. Two independent estimates of the enolization constants of 4 and 4-, the first based on thermodynamic cycles, the second on the ratio of enolization and ketonization rates, were combined to give pKE = 9.3 ± 0.3, pK′E = 6.6 ± 0.3. Ketene 2 is formed by irradiation of 1-bromo-2-naphthol at 12 K in an argon matrix, but neither it nor its isomer 2-bromo-1-naphthol were suitable for the generation and observation of 2 and 3 by flash photolysis in aqueous solution.
- Almstead, Ji-In Kim,Urwyler, Bernhard,Wirz, Jakob
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p. 954 - 960
(2007/10/02)
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- Kinetics and mechanism of the isomerization of 1H-indene-1-carboxylic acid to 1H-indene-3-carboxylic acid in aqueous solution and determination of their keto-enol equilibrium constants and acid dissociation constants of the keto and enol forms. Implication on the photolysis of diazonaphthoquinones
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Rates of isomerization of 1H-indene-1-carboxylic acid to 1H-indene-3-carboxylic acid were measured in dilute aqueous solutions of HClO4, NaOH, and CH3CO2H and H2PO4-, (CH3)3CPO3H-, and HCO3- buffers. This gave a rate profile which, together with the occurrence of general base catalysis and sizable primary kinetic isotope effects, indicates that the isomerization takes place through an enolization-reketonization reaction sequence. The equilibrium constant of the isomerization reaction is K = [indene-3-carboxylic acid]/[indene-1-carboxylic acid] = 200 in aqueous acid solution and K = 100 in base. The ratio of products formed by ketonization of the indenecarboxylic acid enol intermediate generated in the photolysis of 2-diazo-1 (2H)-naphthalenone is R = [indene-3-carboxylic acid]/[indene-1-carboxylic acid] = 0.47 in aqueous acid solution and R = 20 in base. The failure of previous investigations of the photolysis reaction to detect any indene-1-carboxylic acid as the product is attributed to the facile isomerization of this substance to indene-3-carboxylic acid and the preponderance of the latter at equilibrium. The enol intermediate of this isomerization reaction was also generated by flash photolysis of 2-diazo-1 (2H)-naphthalene and rates of its ketonization were measured in dilute aqueous HClO4 solutions. Analysis of the data gave the enol acidity constant pKaE = 2.09. The results, in combination with those for the isomerization reaction, also provided carbon acid acidity constants (KaK) and keto-enol equilibrium constants (KE) for the two acids: pKaK = 9.35 and pKE = 7.26 for indene-1-carboxylic acid and pKaK = 11.69 and pKE = 9.60 for indene-3-carboxylic acid.
- Andraos,Kresge,Popik
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p. 961 - 967
(2007/10/02)
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