Proton Transfer from the C5-proR/proSPositions ofl-Dihydroorotate:  General-Base Catalysis, Isotope Effects, and Internal Return1

1999 ◽  
Vol 121 (51) ◽  
pp. 12054-12062 ◽  
Author(s):  
Argyrides Argyrou ◽  
Michael W. Washabaugh
Keyword(s):  
Proton Transfer ◽  
Isotope Effects ◽  
Base Catalysis ◽  
General Base

The oxidation of aldehydes with bromine. II. Study of the general base catalysis

1964 ◽  
Vol 17 (11) ◽  
pp. 1210 ◽  
Author(s):  
BG Cox ◽  
PT McTigue
Keyword(s):  
Proton Transfer ◽  
Carboxylic Acids ◽  
Base Catalysis ◽  
Aliphatic Aldehydes ◽  
General Base

It is shown that the oxidation with bromine of four aliphatic aldehydes to the corresponding carboxylic acids involves a rate-determining proton transfer. It is concluded that the aldehyde hydrate is the reactive entity in these oxidations.

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.

Sign in / Sign up

Export Citation Format

Share Document