The acidities of 4-arylsulfonylmethylpyridines and N-methyl and N-benzyl 4-arylsulfonylmethylpyridinium cations in aqueous solution

1992 ◽  
Vol 70 (10) ◽  
pp. 2635-2644 ◽  
Author(s):  
Stefan Wodzinski ◽  
John W. Bunting
Keyword(s):  
Aqueous Solution ◽  
Substituent Effects ◽  
Linear Free Energy Relationship ◽  
Acidity Function ◽  
Linear Free Energy ◽  
Phenyl Sulfone ◽  
Carbon Acids ◽  
Pyridinium Cations ◽  
Conjugate Bases ◽  
Insight Into

The pKa values for the deprotonation of a series of 4-(X-phenylsulfonylmethyl)pyridines (6) (pKa = 19.89 (X = H); ρ = 3.0) were determined in aqueous dimethyl sulfoxide solutions at 25 °C using the H0q acidity function. The pKa values were also measured for the corresponding series of N-mefhyl 4-(X-phenylsulfonylmethyl)pyridinium cations (2) (pKa = 11.27 (X = H); ρ = 1.45) and also for a series of N-(X-benzyl) 4-phenylsulfonylmethylpyridinium cations (7) (pKa = 10.70 (X = H); ρ = 0.65) in aqueous solution (ionic strength 0.1 at 25 °C). Comparison of the substituent effects upon the pKa values of the sulfonyl-activated carbon acids 2, 6, and 7 with the substituent effects upon the pKa values of the corresponding three series of ketones gives insight into the electron-density distribution in the carbanionic conjugate bases of sulfone and ketone carbon acids. Extrapolation of a linear free energy relationship between the pKa values of neutral sulfones and ketones allows the estimation of pKa = 28.7 for the deprotonation of methyl phenyl sulfone in aqueous solution.

Vanadium(V) oxyanions. The dependence of vanadate alkyl ester formation on the pKa of the parent alcohols

1988 ◽  
Vol 66 (9) ◽  
pp. 2294-2298 ◽  
Author(s):  
Alan S. Tracey ◽  
Bruno Galeffi ◽  
Soroush Mahjour
Keyword(s):  
Aqueous Solution ◽  
Nuclear Magnetic Resonance ◽  
Free Energy ◽  
Resonance Spectroscopy ◽  
Linear Free Energy Relationship ◽  
Alkyl Esters ◽  
Linear Free Energy ◽  
Ester Formation ◽  
Interesting Correlation ◽  
Extra Electron

The interaction of vanadate (VO4H21−) with a variety of alkyl alcohol's (ROH) covering about seven pKa units of acidity have been studied in aqueous solution by 51V nuclear magnetic resonance spectroscopy. A linear free energy relationship between the equilibrium constant for the formation of the vanadate monoester (HOVO3R1−) and the acidity of the alcohol was established at 23 °C. The correlation was found to be [Formula: see text] where K = [HOVO3R1−]/[VO4H21−][ROH]. This correlation was shown not to extend to ligands with π-bonding capability such as phenols or phosphates, where the products are highly favoured relative to the alkyl esters. An interesting correlation between pKa values of the product esters and the pKa of the parent alcohols was also observed. It was found that below a [Formula: see text] of about 15 the pKa values of the esters were essentially constant. However, above this value of 15 the pKa of the ester was found to increase rapidly with an increase in pKa of the alcohol. This result may indicate that the electron accepting ability of the metal is exhausted with the higher pKa alcohols, and the extra electron density is transferred to the oxygens, thus causing an increase in pKa of the ester.

scholarly journals Substituent Effects on Carbon Acidity in Aqueous Solution and at Enzyme Active Sites

Synlett ◽  
2017 ◽  
Vol 28 (12) ◽  
pp. 1407-1421 ◽  
Author(s):  
John Richard ◽  
Tina Amyes
Keyword(s):  
Aqueous Solution ◽  
Amino Acids ◽  
Active Site ◽  
Active Sites ◽  
Substituent Effects ◽  
Imidazolium Cations ◽  
Enzyme Active Sites ◽  
Iminium Cations ◽  
Carbon Acids

Methods are described for the determination of pK as for weak carbon acids in water. The application of these methods to the determination of the pK as for a variety of carbon acids including nitriles, imidazolium cations, amino acids, peptides and their derivatives and, α-iminium cations is presented. The substituent effects on the acidity of these different classes of carbon acids are discussed, and the relevance of these results to catalysis of the deprotonation of amino acids by enzymes and by pyridoxal 5′-phosphate is reviewed. The procedure for estimating the pK a of uridine 5′-phosphate for C-6 deprotonation at the active site of orotidine 5′-phosphate decarboxylase is described, and the effect of a 5-F substituent on carbon acidity of the enzyme-bound substrate is discussed.1 Introduction2 The Carbon Acidity of Ethyl Thioacetate3 The Carbon Acidity of Carboxylic Acid Derivatives4 The Carbon Acidity of Imidazolium Cations5 The α-Carbon Acidity of Amino Acids, Peptides and Their Derivatives6 Electrophilic Catalysis of Deprotonation of Amino Acids: The α-Carbon Acidity of Iminium Cations7 pK as for Carbon Acids at Enzyme Active Sites8 Concluding Remarks

Substituent Effects on the Chymotrypsin-Catalyzed Hydrolysis of Specific Ester Substrates

1971 ◽  
Vol 49 (2) ◽  
pp. 210-217 ◽  
Author(s):  
R. E. Williams ◽  
M. L. Bender
Keyword(s):  
Free Energy ◽  
Rate Constants ◽  
Substituent Effects ◽  
Phenyl Group ◽  
Linear Free Energy Relationship ◽  
Acylation Reaction ◽  
General Base ◽  
Linear Free Energy ◽  
Base Mechanism ◽  
Hydrolysis Of

The substituent effect on the chymotrypsin-catalyzed hydrolysis of several phenyl esters of specific substrates has been studied. The second-order acylation rate constants (kcat/Km(app)) obey a linear free energy relationship with ρ = +0.63 for phenyl hippurates and ρ = +0.46 for phenyl N-benzyloxycarbonyl-L-tryptophanates when substituents are introduced into the phenyl group of the ester function. These results further support the previously proposed general acid – general base mechanism for the acylation reaction and the formation of a tetrahedral intermediate in the course of the reaction.

scholarly journals Prediction of Adsorbability of Organic Compounds from Aqueous Solution on Activated Carbon by Means of the Linear Free-energy Relationship

1983 ◽  
Vol 56 (4) ◽  
pp. 1002-1005 ◽  
Author(s):  
Ikuo Abe ◽  
Katsumi Hayashi ◽  
Mutsuo Kitagawa ◽  
Tsuneaki Hirashima
Keyword(s):  
Aqueous Solution ◽  
Free Energy ◽  
Activated Carbon ◽  
Organic Compounds ◽  
Linear Free Energy Relationship ◽  
Linear Free Energy

Substituent effects in naphthalene. III. Saponification of 6- and 7-substituted Methyl 2-naphthoates and application of the Hammett equation

1966 ◽  
Vol 19 (2) ◽  
pp. 221 ◽  
Author(s):  
PR Wells ◽  
W Adcock
Keyword(s):  
Free Energy ◽  
Alkaline Hydrolysis ◽  
General Feature ◽  
Substituent Effects ◽  
Linear Free Energy Relationship ◽  
Reaction Centre ◽  
Hammett Equation ◽  
P Values ◽  
Linear Free Energy ◽  
Aromatic Systems

The rates of alkaline hydrolysis in 70% v/v aqueous dioxan at 25� have been determined for the 6-and 7-NO2, 6- and 7-CN, 6- and 7-F, 6- and 7-Cl, 6- and 7-NMe2, 6-Me and 6-OMe substituted methyl 2-naphthoates, methyl 2- naphthoate, m-NO2, p-NO2, p-Me substituted methyl benzoates, and methyl benzoate. An examination of the application of the Hammett equation reveals that this linear free energy relationship applies with high precision but that it yields different p values for different substituent-reaction centre dispositions. This may be a general feature for all aromatic systems although in many cases the differences are not larger than the experimental uncertainties.

Thermodynamic and kinetic acidities of N-(substituted benzyl) 4-phenylacetylpyridinium cations in aqueous solution

1991 ◽  
Vol 69 (6) ◽  
pp. 945-948 ◽  
Author(s):  
John W. Bunting ◽  
P. Philippe Aubin
Keyword(s):  
Aqueous Solutions ◽  
Substituent Effects ◽  
Order Rate Constant ◽  
Hydroxide Ion ◽  
Subsequent Transformation ◽  
Order Rate ◽  
Carbon Acids ◽  
Pyridinium Cations ◽  
Saturation Effects ◽  
Conjugate Base

The pKa values for the deprotonation of a series of eight 1-(X-benzyl)-4-phenylacetylpyridinium cations (6) have been measured in aqueous solutions of ionic strength 0.1 at 25 °C: pKa = −0.18σ + 8.91. The pseudo-first-order rate constants for deprotonation of these carbon acids have been measured over the range pH = 11–13, and have been found to display kinetic saturation effects that are consistent with the addition of hydroxide ion to the carbonyl group (pKz) as the product of kinetic control upon basification of neutral aqueous solutions of these pyridinium cations, with the subsequent transformation of this anionic hydrate to the thermodynamically more stable enolate conjugate base. Analysis of the pH–rate profiles gives substituent effects upon pKz (ρ = −0.19) and upon the second-order rate constant (kOH (ρ = 0.09)) for deprotonation of 6 by hydroxide ion. Key words: carbon acids, deprotonation, pKa, kinetics, substituent effects.

Molecular orbital treatment of substituent effects. I. Structures of some carbon acids and their conjugate bases

1983 ◽  
Vol 61 (1) ◽  
pp. 45-49 ◽  
Author(s):  
Kenneth E. Edgecombe ◽  
Russell J. Boyd
Keyword(s):  
Molecular Orbital ◽  
Substituent Effects ◽  
Stabilization Energy ◽  
Basis Set ◽  
Carbon Acids ◽  
Resonance Stabilization ◽  
Stabilization Energies ◽  
Conjugate Bases ◽  
Equilibrium Geometries

The STO-3G optimized structures of three series of carbon acids, CHnX4−n, where n = 1, 2, or 3 and X = F, CN, or NO2 and their corresponding conjugate bases are compared with the limited number of experimentally determined structures. The 6-31G* equilibrium geometries of [Formula: see text] are included as a check on the reliability of the STO-3G structures of the anions. Although the effects of successive substitutions on the STO-3G structures are generally systematic, a few apparent anomalies are readily explained in terms of resonance stabilization. Calculations at the 3-21G level, on the STO-3G optimized structures, indicate that the stabilization energy associated with increasing the number of orbitals in the basis set is approximately additive. The stabilization energies are 0.865 ± 0.007 au per fluorine substituent, 0.661 ± 0.008 au per cyano substituent, and 1.561 ± 0.027 au per nitro substituent.

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