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.

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.

Stereo‐dynamical ion‐pair formation in collisions of highly‐charged ions with argon dimers

2019 ◽  
Vol 49 (1) ◽  
pp. 120-124
Author(s):  
Tomoko Ohyama‐Yamaguchi ◽  
Atsushi Ichimura
Keyword(s):  
Ion Pair ◽  
Pair Formation ◽  
Highly Charged Ions ◽  
Ion Pair Formation ◽  
Charged Ions

Ion-Pair Formation of Sodium Salts of Several Oxo Anions and Cadmium Halides in Water and the Distribution of Monovalent Sulfate Ion Pairs in Nitrobenzene

2010 ◽  
Vol 55 (7) ◽  
pp. 2463-2469 ◽  
Author(s):  
Yoshihiro Kudo ◽  
Daisuke Todoroki ◽  
Naoki Horiuchi ◽  
Shoichi Katsuta ◽  
Yasuyuki Takeda
Keyword(s):  
Ion Pairs ◽  
Ion Pair ◽  
Pair Formation ◽  
Sulfate Ion ◽  
Sodium Salts ◽  
Ion Pair Formation ◽  
Oxo Anions

Octahedral cobalt(III) complexes in dipolar aprotic solvents. XIII. The isomerization of cis- and trans-Dibromobis(ethylenediamine)cobalt(III) ions, [CoBr2 em2]+, in anhydrous sulpholane

1968 ◽  
Vol 21 (3) ◽  
pp. 595 ◽  
Author(s):  
WR Fitzgerald ◽  
DW Watts
Keyword(s):  
Equilibrium Constants ◽  
Activation Parameters ◽  
Ion Pair ◽  
Pair Formation ◽  
Isomerization Reaction ◽  
State Entropy ◽  
Enthalpy Changes ◽  
Solvent Molecules ◽  
Cis And Trans ◽  
Dipolar Aprotic Solvents

The isomerization of cis- and trans-[CoBr2 en2]+ has been studied in anhydrous sulpholane (tetramethylene sulphone). No evidence has been found for the presence of species containing coordinated solvent molecules, and the mechanism of isomerization of both isomers is interpreted as SNl the rates being influenced by ion-pair formation. Activation parameters have been measured for the isomerization of both species at various bromide concentrations. Equilibrium constants and standard state entropy and enthalpy changes for the isomerization reaction and for the formation of the cis-[CoBr2 en2]+ .......Br- ion pair have been measured.

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.

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