Intramolecular general base catalysis and the rate-determining step in the nucleophilic cleavage of ionized phenyl salicylate with primary and secondary amines

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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Studies on formaldehyde resin. XXIII. Mechanism of general base catalysis of hydroxymethylation of melamine with formaldehyde. Evidence for the change of rate-determining step.

KOBUNSHI RONBUNSHU ◽

10.1295/koron.43.77 ◽

1986 ◽

Vol 43 (2) ◽

pp. 77-81

Author(s):

Toshio YOSHII ◽

Takeo KONAKAHARA ◽

Kenji SATO

Keyword(s):

Base Catalysis ◽

Formaldehyde Resin ◽

General Base ◽

Rate Determining Step

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General base catalysis in the iodination of adrenochrome

Canadian Journal of Chemistry ◽

10.1139/v67-406 ◽

1967 ◽

Vol 45 (21) ◽

pp. 2473-2476 ◽

Cited By ~ 2

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.

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