The relative importance of ground-state conformation and orbital orientation in intramolecular catalysis: hydrolysis of aryl hydrogen biphenyl-2,2′-dicarboxylates

1983 ◽  
pp. 421-424 ◽  
Author(s):  
Ramamurthy Chandrasekar ◽  
Nagaswami Venkatasubramanian
Keyword(s):  
Ground State ◽  
Relative Importance ◽  
Intramolecular Catalysis ◽  
Hydrolysis Of ◽  
Orbital Orientation

Effect of Medium on Reactivity for Alkaline Hydrolysis of p-Nitrophenyl Acetate and S-p-Nitrophenyl Thioacetate in DMSO-H2O Mixtures of Varying Compositions: Ground State and Transition State Contributions

2021 ◽  
Author(s):  
Ik-Hwan Um ◽  
Seungjae Kim
Keyword(s):  
Transition State ◽  
Alkaline Hydrolysis ◽  
Ground State ◽  
Rate Constants ◽  
Kinetic Data ◽  
Reaction Medium ◽  
Stepwise Mechanism ◽  
Rate Determining Step ◽  
Leaving Group ◽  
Hydrolysis Of

Second-order rate constants (kN) for reactions of p-nitrophenyl acetate (1) and S-p-nitrophenyl thioacetate (2) with OH‒ have been measured spectrophotometrically in DMSO-H2O mixtures of varying compositions at 25.0 ± 0.1 oC. The kN value increases from 11.6 to 32,800 M‒1s‒1 for the reactions of 1 and from 5.90 to 190,000 M‒1s‒1 for those of 2 as the reaction medium changes from H2O to 80 mol % DMSO, indicating that the effect of medium on reactivity is more remarkable for the reactions of 2 than for those of 1. Although 2 possesses a better leaving group than 1, the former is less reactive than the latter by a factor of 2 in H2O. This implies that expulsion of the leaving group is not advanced in the rate-determining transition state (TS), i.e., the reactions of 1 and 2 with OH‒ proceed through a stepwise mechanism, in which expulsion of the leaving group from the addition intermediate occurs after the rate-determining step (RDS). Addition of DMSO to H2O would destabilize OH‒ through electronic repulsion between the anion and the negative-dipole end in DMSO. However, destabilization of OH‒ in the ground state (GS) is not solely responsible for the remarkably enhanced reactivity upon addition of DMSO to the medium. The effect of medium on reactivity has been dissected into the GS and TS contributions through combination of the kinetic data with the transfer enthalpies (ΔΔHtr) from H2O to DMSO-H2O mixtures for OH‒ ion.

scholarly journals Hydrolysis of the phytate of wheat flour during breadmaking

1981 ◽  
Vol 46 (1) ◽  
pp. 1-6 ◽  
Author(s):  
D. G. H. Daniels ◽  
N. Fisher
Keyword(s):  
Wheat Flour ◽  
Molar Ratio ◽  
Relative Importance ◽  
Inositol Hexaphosphate ◽  
Processing Times ◽  
Hydrolysis Of

1. Differences in the extent of breakdown of phytate in wholemeal and white flours prepared from three wheats when the flours were made into bread using the three main UK commercial breadmaking processes were investigated.2. The extent of breakdown (31–46% for wholemeal breads, 88–99% for white breads) was not proportional to the relative processing times involved (1–4 h). The importance of destruction of phytate in the oven is stressed.3. The phytase (myo-inositol hexaphosphate phosphohydrolase. EC 3.1.3.8) activities of the wholemeal flours and of the yeast were determined. Re-examination of some information in the literature enabled the relative importance of these activities, and of the various stages of breadmaking, in determining the extent of hydrolysis of phytate to be assessed.4. Average values for the molar ratio, phytate: zinc, of 22:1 and 0.8:1 were calculated for wholemeal and white breads respectively. The nutritional significance of these results is discussed.

Intramolecular Catalysis of the Hydrolysis of N-Dimethylaminomaleamic Acid

Science ◽  
1963 ◽  
Vol 140 (3566) ◽  
pp. 485-486 ◽  
Author(s):  
G. Dahlgren ◽  
N. L. Simmerman
Keyword(s):  
Intramolecular Catalysis ◽  
Hydrolysis Of

Hydrolysis of aryl hydrogen maleate esters mediated by cyclodextrins — Effect on the intramolecular catalysis

2005 ◽  
Vol 83 (9) ◽  
pp. 1281-1286 ◽  
Author(s):  
Gabriel O Andrés ◽  
O Fernando Silva ◽  
Rita H de Rossi
Keyword(s):  
Maleic Anhydride ◽  
Transition State ◽  
Rate Constant ◽  
Association Constant ◽  
Kinetic Studies ◽  
Association Constants ◽  
Bulk Solution ◽  
Intramolecular Catalysis ◽  
Hydrolysis Of ◽  
Nonlinear Fashion

Kinetic studies of the hydrolysis of Z-aryl hydrogen maleates (Z = H, p-CH3, m-CH3, p-Cl, m-Cl) were carried out in the presence and absence of hydroxypropyl-β-cyclodextrin (HPCD) at variable pH from 1.00 to 3.00. The reaction involves the formation of maleic anhydride as an intermediate and the rate of its formation is strongly dependent on the pH. This is because the neighboring carboxylate group is a better catalyst than the carboxylic group. The rate constant for the formation of maleic anhydride decreases as the HPCD concentration increases in a nonlinear fashion. The results were interpreted in terms of the formation of a 1:1 inclusion complex of the esters with HPCD. The neutral (HA) and anionic (A) species of the substrate have different association constants (K[Formula: see text] and K[Formula: see text]). In all cases studied, K[Formula: see text] is higher than K[Formula: see text] for the same substrate. This difference is responsible for a decrease in the amount of the anionic substrate (reactive species) in the presence of HPCD, which results in a diminution of the observed rate constant. Besides, the rate constant for the reaction of the complexed substrate is smaller than that in the bulk solution indicating that the transition state of the cyclodextrin mediated reaction is less stabilized than the anionic substrate. The values of ΔΔG‡ are almost independent of the substituent on the aryl ring and range within 0.48 and 1.05 kcal mol–1 (1 cal = 4.184 J). There is no correlation between KTS and the association constant of the substrate indicating that the factors stabilizing the transition state are different from those that stabilize the substrate. Key words: cyclodextrins, intramolecular catalysis, hydrolysis, inhibition.

Sign in / Sign up

Export Citation Format

Share Document