Proton Transfer from Carbon Acids to Carbanions. 2. Reaction of Phenylnitromethane with Carbanions, Enolate, and Nitronate Ions in 90% Me2SO-10% Water. Carbon to Carbon or Carbon to Oxygen Proton Transfer? Test of the Marcus Equation

1994 ◽  
Vol 59 (17) ◽  
pp. 4910-4916 ◽  
Author(s):  
Claude F. Bernasconi ◽  
Jiu Xiang Ni
Keyword(s):  
Proton Transfer ◽  
Transfer Test ◽  
Carbon Acids ◽  
Marcus Equation

Potentiometric study of equilibrium acidities of some carbon acids in acetonitrile

2002 ◽  
Vol 80 (9) ◽  
pp. 1259-1264 ◽  
Author(s):  
M Stanczyk-Dunaj ◽  
W Galezowski ◽  
A Jarczewski
Keyword(s):  
Proton Transfer ◽  
Transfer Reaction ◽  
Proton Transfer Reaction ◽  
Glass Electrode ◽  
Buffer Solution ◽  
Tetrabutylammonium Hydroxide ◽  
Potentiometric Study ◽  
Tetrabutylammonium Salt ◽  
Influence Of Water ◽  
Carbon Acids

The pKa values for 30 C-acids of the type CH·R1·R2·R3 — where the R1, R2, and R3 substituents are H, alkyl, and aryl, and the most common electron withdrawing groups used in studies of proton-transfer reaction — are given herein. The pKa values have been measured potentiometrically in acetonitrile, using a glass electrode. These values were determined in a buffer solution consisting of a C-acid and its tetrabutylammonium salt, at constant ionic strength (0.001 M), at 25°C. The measurements were done under nitrogen, to prevent oxidation of the anion. The influence of water within the sample of the tetrabutylammonium hydroxide base was negligible. The lowest pKa value of 19.1 was found for bis(4-nitrophenyl)cyanomethane, while the largest pKas that could be measured were limited by the basicity of the hydroxide ion, and did not exceed 28. A satisfactory agreement between spectrophotometric and potentiometric acidities was achieved. Key words: C-acids, pKa values, acetonitrile, potentiometry, proton transfer.

Ketonization equilibria of phenol in aqueous solution

1999 ◽  
Vol 77 (5-6) ◽  
pp. 605-613 ◽  
Author(s):  
Marco Capponi ◽  
Ivo G Gut ◽  
Bruno Hellrung ◽  
Gaby Persy ◽  
Jakob Wirz
Keyword(s):  
Aqueous Solution ◽  
Proton Transfer ◽  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Flash Photolysis ◽  
Equilibrium Constants ◽  
Thermodynamic Cycle ◽  
Acidity Constant ◽  
Kinetic Isotope ◽  
Carbon Acids

The two keto tautomers of phenol (1), cyclohexa-2,4-dienone (2) and cyclohexa-2,5-dienone (3), were generated by flash photolysis of appropriate precursors in aqueous solution, and the pH-rate profiles of their enolization reactions, 2 –> 1 and 3 –> 1, were measured. The rates of the reverse reactions, 1 –> 2 and 1 –> 3, were determined from the rates of acid-catalyzed hydron exchange at the ortho- and para-positions of 1; the magnitude of the kinetic isotope effect was assessed by comparing the rates of hydrogenation of phenol-2t and -2d. The ratios of the enolization and ketonization rate constants provide the equilibrium constants of enolization, pKE(2, aq, 25°C) = -12.73 ± 0.12 and pKE(3, aq, 25°C) = -10.98 ± 0.15. Combination with the acidity constant of phenol also defines the acidity constants of 2 and 3 through a thermodynamic cycle. These ketones are remarkably strong carbon acids: pKa(2) = -2.89 ± 0.12 and pKa(3) = -1.14 ± 0.15. They disappear by proton transfer to the solvent with lifetimes, τ(2) = 260 μs and τ(3) = 13 ms, that are insensitive to pH in the range from 3-10.Key words: proton transfer, tautomers, flash photolysis, kinetic isotope effect, pH-rate profiles.

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