Evidence for concerted general acid catalysis in an SN2-like transition state for the reduction of sulfilimines by thiols

1985 ◽  
Vol 107 (25) ◽  
pp. 7578-7581 ◽  
Author(s):  
Paul R. Young ◽  
Arvydas P. Zygas ◽  
I Wen Evelyn Lee
Keyword(s):  
Transition State ◽  
Acid Catalysis ◽  
General Acid

Reactivity of proton and general acid with 1,3-bis-(4-methylphenyl)triazene in aqueous methanol

1990 ◽  
Vol 55 (11) ◽  
pp. 2701-2706 ◽  
Author(s):  
Oldřich Pytela ◽  
Taťjana Nevěčná ◽  
Jaromír Kaválek
Keyword(s):  
Benzoic Acid ◽  
Transition State ◽  
Rate Constant ◽  
Acid Catalysis ◽  
Methanol Concentration ◽  
Binary Solvent ◽  
Aqueous Methanol ◽  
Solvent Water ◽  
General Acid ◽  
Activated Complex

The effect of concentration of benzoic acid and composition of the binary solvent water-methanol on the rate of decomposition of 1,3-bis(4-methylphenyl)triazene has been studied. It has been found that both general acid catalysis by undissociated benzoic acid and catalysis by the proton are significant. The rate constant kHA of general acid catalysis decreases monotonously with decreasing amount of water in the mixture due to preferred solvation of the activated complex as compared with the educts. The rate constant kH of the catalysis by proton in its dependence on methanol concentration exhibits a minimum for 80% (by wt.) of methanol in the mixture. This phenomenon is caused by formation of the conjugated acid from more basic methanol and proton with simultaneous solvation by water and methanol; the particle thus formed is a weaker acid as compared with the complexes existing in water or in methanol. The kH value is higher in methanol than in water due to preferred solvation of the educts as compared with that of the transition state.

Intramolecular general acid catalysis of sulfate transfer — Nucleophilic attack by oxyanions on the SO3– group

2005 ◽  
Vol 83 (9) ◽  
pp. 1629-1636 ◽  
Author(s):  
Anthony J Kirby ◽  
José Carlos Gesser ◽  
Florian Hollfelder ◽  
Jacks P Priebe ◽  
Faruk Nome
Keyword(s):  
Hydrogen Bond ◽  
Transition State ◽  
Acid Catalysis ◽  
Nucleophilic Attack ◽  
General Acid ◽  
State Transfer ◽  
Carboxylate Anions ◽  
Mechanism Of Hydrolysis ◽  
Intramolecular Hydrogen ◽  
Hydrolysis Of

The mechanism of hydrolysis of 8-N,N-dimethylaminonaphthyl sulfate closely resembles that of the corresponding phosphate monoester. Nucleophilic attack by water on the sulfate group of the zwitterion is catalyzed by the neighbouring dimethylammonium group, acting as a particularly efficient general acid through the intramolecular hydrogen bond. This hydrogen bond is present in both reactant and product, but is strongest in the transition state. Transfer of the sulfuryl group to oxygen nucleophiles, including water and carboxylate anions, shows steric and electrostatic effects, and a sensitivity to basicity which is low, but significantly higher than expected for uncatalyzed transfer of the SO3– group.Key words: sulfate, sulfatase, intramolecular, general acid catalysis, promiscuity.

Zur Theorie des kinetischen Lösungsmittel-Isotopeneffektes auf säurekatalytische organische Reaktionen II

1964 ◽  
Vol 19 (6) ◽  
pp. 461-467 ◽  
Author(s):  
Alfred V. Willi
Keyword(s):  
Partition Function ◽  
Transition State ◽  
Isotope Effect ◽  
Acid Catalysis ◽  
Free Energies ◽  
Solvent Isotope Effect ◽  
General Acid ◽  
Solvent Isotope ◽  
Hydrolysis Of ◽  
Limiting Value

From the known difference of free energies of liquid H2O and D2O at 25°, the following partition function ratio may be calculated: QD2O/QH2O = 1416. By combination with the constant L=11.0 for the equilibrium 2 D3O⊕ +3 H2O ⇆ 2H3O⊕ + 3D2O, one obtains QD3O/QH3O = 16100. The partition function ratio of an organic acid ROH is approximated by QROD/QROH ≈ (QD2O/QH2O)½. KH/KD≈ 3.3 is calculated for the solvent isotope effect on the acidity of ROH. Correspondingly, the approximation QSD/QSH ≈ (QD3O/QΗ3O)⅓ is introduced for conjugate acids (SH®) of ethers and carbonyl compounds, leading to the solvent isotope effect: KH/KD ≈ 2.2. - In the acid-catalysed hydrolysis of acetals, the formation of the transition state involves the complete transformation from the H3O⊕ to the H2O state of 2 OH bonds and the partial transformation of one more OH bond. Consequently, the limits 1/3.3 < kH/kD < 1/2.2 are calculated for the kinetic solvent isotope effect. The following equation is valid for the general acid catalysis by HA of the enolisation of acetone: kHA/kDA = (KHA/KDA) / (KSH/KSD). Theoretical values are calculated which agree well with experimental data. - On the basis of a simple model for the transition state, the solvent isotope effect on rate determining H® transfer from acetic acid to a substrate (general acid catalysis) may be calculated. (kHA/kDA) min = 5.7 is obtained as a limiting value.

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