Second change in rate-determining step and a nonlinear Broensted relation for general base catalysis of 2-methylthiosemicarbazone formation

1972 ◽  
Vol 94 (9) ◽  
pp. 3262-3263 ◽  
Author(s):  
Jane M. Sayer ◽  
William P. Jencks
Keyword(s):  
Base Catalysis ◽  
General Base ◽  
Rate Determining Step

Multiple changes in rate-determining step in the acid and base catalyzed cyclizations of ethyl N-(p-nitrophenyl)hydantoates caused by methyl substitution

1999 ◽  
Vol 77 (5-6) ◽  
pp. 849-859
Author(s):  
Iva B Blagoeva ◽  
Anthony J Kirby ◽  
Asen H Koedjikov ◽  
Ivan G Pojarlieff
Keyword(s):  
Steric Hindrance ◽  
Acid Catalysis ◽  
Isotope Effects ◽  
Base Catalysis ◽  
Ethoxy Group ◽  
First Order ◽  
Tetrahedral Intermediate ◽  
General Base ◽  
Rate Determining Step ◽  
Acid And Base

The slopes of the pH-rate profiles for the cyclization of 2-methyl- and 2,3-dimethyl hydantoates 1-NPU and 2-NPU between pH 1 and 7 change from 1 to 0 and then back to 1. A reaction first order in H+ was observed with the latter compound. The 2,2,3-trimethyl derivative 3-NPU showed only one reaction first order in OH-, but complex acid catalysis is described by slopes 0, -1, 0, and finally -1 again. The cyclizations were general base catalyzed, with Brønsted β values of 0.5-0.6. The OH- catalysis at higher pH for 1-NPU and 2-NPU showed inverse solvent kinetic isotope effects and deviated from the Brønsted relationships, while that for 3-NPU showed a normal effect and complied with the Brønsted relationship. The accelerations due to the gem-dimethyl effect were lost with the OH- and general base-catalyzed reactions of 3-NPU. This behaviour is due to a change from the rate-determining formation of the tetrahedral intermediate with 1-NPU and 2-NPU to the rate-determining breakdown with 3-NPU, due to steric hindrance to protonation of the leaving ethoxy group. The OH- reaction at higher pH involves attack of the ureide anion with 1-NPU and 2- NPU, becoming concerted with deprotonation when catalyzed by general bases and changing to acid inhibition of the anion of the tetrahedral intermediate at low pH. With 3-NPU at higher pH, T- is in equilibrium and the conjugate acids of the general bases accelerate its breakdown by protonating the ethoxy group. Acid catalysis of the cyclization of 3-NPU at higher pH is also protonation of the leaving group from T0 changing to the rate-determining formation of T at lower pH. The latter mechanism is preferred for the cyclization of 2-NPU.Key words: gem-dimethyl effect, mechanism, general base catalysis, proton transfer, steric hindrance.

General base catalysis in the iodination of adrenochrome

1967 ◽  
Vol 45 (21) ◽  
pp. 2473-2476 ◽  
Author(s):  
G. L. Mattok ◽  
D. L. Wilson
Keyword(s):  
Dissociation Constants ◽  
Base Catalysis ◽  
Hydrogen Phthalate ◽  
Nucleophilic Catalysis ◽  
Catalytic Activities ◽  
General Base ◽  
Rate Determining Step ◽  
Iodination Reaction

The iodination of adrenochrome by iodine is subject to general base catalysis. This is in accord with the view that the rate-determining step in this reaction is the removal of the C-7 proton in the adrenochrome. The order of catalytic activities of the various ions is phosphate > bicarbonate > propionate > acetate, which is also the sequence of the pKa values of the corresponding acids. However, the catalytic efficiencies of the hydrogen phthalate and phthalate anions are about 10 times greater than those predicted from the dissociation constants of the acid. This is rationalized in terms of an alternative electrophilic–nucleophilic catalysis of the iodination reaction.

The Oxidation of Benzyldimethylamine by Bromine

1973 ◽  
Vol 51 (15) ◽  
pp. 2546-2554 ◽  
Author(s):  
Donald G. Lee ◽  
R. Srinivasan
Keyword(s):  
Acetic Acid ◽  
Hydrogen Atom ◽  
Base Catalysis ◽  
Slow Step ◽  
Aqueous Acetic Acid ◽  
General Base ◽  
Rate Determining Step ◽  
Rate Of Reaction ◽  
Electron Withdrawing Substituents

The rates of oxidation of several substituted benzyldimethylamines by bromine in 50% aqueous acetic acid have been determined spectrophotometrically. Electron-withdrawing substituents decrease the rate of reaction with the Hammett ρ value being −0.95. The reaction is subject to general base catalysis and substitution of deuterium in the α-position decreases the rate of reaction by approximately 30%, thus indicating that the α-C—H bond is cleaved in the slow step of the reaction. All results are consistent with a mechanism which involves, in the rate determining step, loss of an α-hydrogen atom as a proton with concomitant transfer of electrons from nitrogen to the oxidant.

Mutarotation of D(+)-glucose. III. Evidence for cooperative and competitive nucleophilic catalysis by molybdenum(VI) and tungsten(VI) anions

1984 ◽  
Vol 37 (7) ◽  
pp. 1411 ◽  
Author(s):  
CJ O'Conner ◽  
AAT Bailey
Keyword(s):  
Activation Parameters ◽  
Ion Pair ◽  
Pair Formation ◽  
Base Catalysis ◽  
Nucleophilic Catalysis ◽  
General Base ◽  
Rate Determining Step ◽  
Conjugate Acid ◽  
Oxo Anions ◽  
Charged Ions

The rate of mutarotation of α-D(+)-glucose is subject to general base catalysis by a variety of oxo anions. The relative magnitudes of the second-order rate constants are B4O72->W7O246->Mo7O246- > HCO3->WO42->MoO42- Catalysis by W7O246- and Mo7O246- is competitive but that by WO42- and MoO42- is cooperative. A mechanism for catalysis by the polymeric anions is proposed which involves ion-pair formation between the anion and the conjugate acid of glucose. Specifically oriented aquation sheaths about the charged ions cause them to act as enhanced nucleophiles in the rate-determining step to form the aldehyde intermediate. Activation parameters support this model.

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