Remarkably large solvolytic rate enhancement due to relief of ground state leaving group strain

1974 ◽  
Vol 96 (6) ◽  
pp. 1969-1970 ◽  
Author(s):  
Joel Slutsky ◽  
Richard C. Bingham ◽  
Paul V. R. Schleyer ◽  
W. C. Dickason ◽  
H. C. Brown
Keyword(s):  
Ground State ◽  
Rate Enhancement ◽  
Leaving Group ◽  
Group Strain

ChemInform Abstract: A REMARKABLY LARGE SOLVOLYTIC RATE ENHANCEMENT DUE TO RELIEF OF GROUND STATE LEAVING GROUP STRAIN

1974 ◽  
Vol 5 (22) ◽  
pp. no-no
Author(s):  
JOEL SLUTSKY ◽  
RICHARD C. BINGHAM ◽  
PAUL VON R. SCHLEYER ◽  
W. C. DICKASON ◽  
H. C. BROWN
Keyword(s):  
Ground State ◽  
Rate Enhancement ◽  
Leaving Group ◽  
Group Strain

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.

Effect of modification of the electrophilic center on the α effect

2005 ◽  
Vol 83 (9) ◽  
pp. 1365-1371 ◽  
Author(s):  
Ik-Hwan Um ◽  
Ji-Youn Lee ◽  
Sun-Young Bae ◽  
Erwin Buncel
Keyword(s):  
Ground State ◽  
Strong Dependence ◽  
Solvation Energy ◽  
The Other ◽  
Substrate Structure ◽  
Leaving Group ◽  
The Common ◽  
The Difference ◽  
The Impact

We report on a nucleophilic study of esters R-C(=X)-Y-Ar in which the electrophilic center has been modified by replacing O by S in the leaving group or carbonyl center: 4-nitrophenyl acetate (1), S-4-nitrophenyl thioacetate (2), 4-nitrophenyl benzoate (3), and O-4-nitrophenyl thionobenzoate (4). The studies include O– and S– nucleophiles as well as α nucleophiles in H2O at 25.0 ± 0.1 °C. The sulfur nucleophile (4-chlorothiophenoxide, 4-ClPhS–) exhibits significant enhanced reactivity for the reactions with thiol and thione esters 2 and 4 compared with their oxygen analogues 1 and 3. On the contrary, the common nucleophile OH– is much less reactive towards 2 and 4 compared with 1 and 3. The effect of changing both the electrophilic center and the nucleofugic center on the reactivity of the other oxygen nucleophiles is not so significant: 4-chlorophenoxide (4-ClPhO–) is four to six times more reactive in the reactions with thiol and thione esters 2 and 4 compared with their oxygen analogues 1 and 3. The α effects exhibited by butan-2,3-dione monoximate (Ox–) and HOO– are strongly dependent on the nature of the electrophilic center of the substrates, indicating that the difference in the ground-state solvation energy cannot be fully responsible for the α effect. Our results clearly emphasize the strong dependence of the α effect on the substrate structure, notably, the nature of the electrophilic center. The impact of change in the nucleofuge (1→2) and the electrophilic center (3→4) on reactivity indicates that α nucleophiles will need to be “purpose built” for decontamination and nucleophilic degradation of specific biocides.Key words: α effect, nucleophilicity, nucleofuge effect, electrophilicity, polarizability.

scholarly journals An Ancestral Tryptophanyl-tRNA Synthetase Precursor Achieves High Catalytic Rate Enhancement without Ordered Ground-State Tertiary Structures

2016 ◽  
Vol 11 (6) ◽  
pp. 1661-1668 ◽  
Author(s):  
Paul J. Sapienza ◽  
Li Li ◽  
Tishan Williams ◽  
Andrew L. Lee ◽  
Charles W. Carter
Keyword(s):  
Ground State ◽  
Trna Synthetase ◽  
Tertiary Structures ◽  
Rate Enhancement ◽  
Catalytic Rate

An unusual ground-state stabilization effect and origins of the alpha-effect in aminolyses of Y-substituted phenyl X-substituted benzoates

1998 ◽  
Vol 76 (6) ◽  
pp. 729-737 ◽  
Author(s):  
Ik-Hwan Um ◽  
Eun-Kyung Chung ◽  
So-Mi Lee
Keyword(s):  
Ground State ◽  
Negative Charge ◽  
Reaction Rates ◽  
Primary Amines ◽  
Bonding Interaction ◽  
Leaving Group ◽  
Hammett Correlation ◽  
Alpha Effect ◽  
Stabilization Effect ◽  
Electron Withdrawing Group

Second-order rate constants have been measured spectrophotometrically for the reactions of X-C6H4CO2C6H4-Y with a series of primary amines in H2O containing 20 mol% DMSO at 25.0 ± 0.1°C. The reactivity increases as the substituent (X and Y) becomes a stronger electron-withdrawing group. The sigma + constants give better Hammett correlation than sigma constants for the reactions of 4-nitrophenyl X-substituted benzoates with glycylglycine (glygly) and hydrazine (NH2NH2), indicating that the ground-state stabilization effect is unusually significant on the reaction rates. The reactions of X-C6H4CO2C6H4-Y with glygly and NH2NH2 appear to proceed through the same mechanism, but the degree of leaving-group departure and the negative charge developed in the acyl moiety at the rate-determining TS is considered to be more significant for the glygly system than the NH2NH2 system based on ßlg and rho X values. The magnitude of the alpha -effect is observed to be not always dependent on the ßnuc value but dependent on the electronic nature of the substituent X and Y, i.e., an electron-donating substituent increases the alpha -effect, while an electron-withdrawing one decreases the alpha -effect. The present study has led to the conclusion that the ground-state effect is important for the reaction rates but it is not solely responsible for the alpha -effect, and the intramolecular H-bonding interactions (4) are proposed for the cause of the increasing or decreasing alpha -effect trends observed in the present system.Key words: alpha -effect, intramolecular H-bonding interaction, ground-state stabilization effect.

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