Kinetics and mechanism of the formation of N-vinyl pyridinium cations in elimination reactions in aqueous base

1992 ◽  
Vol 70 (4) ◽  
pp. 1195-1203 ◽  
Author(s):  
John W. Bunting ◽  
Andrea Toth ◽  
James P. Kanter
Keyword(s):  
Reaction Mechanisms ◽  
Substituent Effects ◽  
Hydroxide Ion ◽  
First Order ◽  
Pyridinium Cation ◽  
Rate Determining Step ◽  
Bromide Ion ◽  
Aqueous Base ◽  
Elimination Reactions ◽  
Pyridinium Cations

The rates of the elimination reactions of N-(2-bromoethyl) pyridinium cations (1) and N,N′-ethylene bispyridinium dications (3) to give the corresponding N-vinyl pyridinium cations (2) have been measured spectrophotometrically in basic aqueous solutions (ionic strength 0.1, 25 °C) for a variety of substituents in the pyridine rings of each of these classes of pyridinium cation. The reaction kinetics are first order in 1 or 3 and first order in hydroxide ion. Brønsted-type plots of the second-order rate constants (kOH) as a function of the basicity (as pKBH) of the corresponding substituted pyridine are nonlinear for each of 1 and 3 and can be interpreted in terms of E1cb reaction mechanisms. For 1, the Brønsted-type plot displays two distinct "concave down" linear regions; rate-determining deprotonation for pKBH < 5.16 (slope = −0.30), and a change in rate-determining step to bromide ion departure for pKBH > 5.16 (slope −0.58). For 3, the Brønsted-type plot appears to be smoothly curved for symmetrically disubstituted bispyridinium dications, as a consequence of the multiple substituent effects upon each step of the E1cb reactions of these dications. However, log kOH for 3 is a smooth linear function of the previously reported log kOH for the E1cb reactions of N-(2-cyanoethyl) pyridinium cations over a range in which a change in rate-determining step has been directly demonstrated for these latter cations. Thus a change in rate-determining step as a function of pyridine basicity is also required within the E1cb mechanism for 3. The E1cb reactions of 1 are approximately 104-fold faster than the corresponding hydroxide ion catalyzed E2 eliminations from 2-phenylethyl bromides that are isoelectronic with 1.

Rate and equilibrium studies of the deprotonation of benzylic ketones bearing pyridinium cations

1989 ◽  
Vol 67 (3) ◽  
pp. 428-432 ◽  
Author(s):  
John W. Bunting ◽  
Dimitrios Stefanidis
Keyword(s):  
Activation Parameters ◽  
Kinetic Studies ◽  
Similar Data ◽  
Equilibrium Studies ◽  
Hydroxide Ion ◽  
Pyridinium Cation ◽  
Carbon Acids ◽  
Entropy Of Activation ◽  
Pyridinium Cations ◽  
Conjugate Base

Rates and equilibria for the deprotonation of four benzylic ketones containing pyridinium substituents (1, 2, 5, and 6) have been investigated in basic aqueous solution (ionic strength 0.1) over the range 15–45 °C, and thermodynamic and activation parameters have been evaluated. Similar data are also reported for the deprotonation of nitroethane. The kinetic preference for hydroxide ion addition to the carbonyl group in competition with the thermodynamically preferred enolate ion formation, which was previously reported for the 1-methyl-4-phenylacetylpyridinium cation (1) and its 3-phenylacetyl isomer (2), is also found for the 1-(1-methyl-2-oxo-2-phenylethyl)pyridinium cation (6). Rates of equilibration of the 1-(2-oxo-2-phenylethyl)-pyridinium cation (5) with its enolate ion conjugate base are too rapid to allow investigation by stopped-flow spectrophotometry. For the hydroxide ion catalyzed deprotonation of each of 1, 2, 6, and nitroethane, [Formula: see text] is more negative than ΔS0. This difference, which represents the entropy of activation for protonation of the conjugate base of each of these carbon acids by water, is approximately constant at [Formula: see text] = −4.9 ± 0.5 cal deg−1 mol−1. Keywords: carbon acids, kinetic studies, deprotonation, activation parameters, pKa values.

Base hydrolysis of trans-Halogenoamminebis(dimethylglyoximato) (III). Complexes

1969 ◽  
Vol 22 (12) ◽  
pp. 2569 ◽  
Author(s):  
SC Chan ◽  
PY Leung
Keyword(s):  
Equilibrium Constants ◽  
Base Hydrolysis ◽  
Hydroxide Ion ◽  
First Order ◽  
Rate Determining Step ◽  
Rapid Formation ◽  
Pseudo First Order ◽  
Conjugate Bases ◽  
Hydrolysis Of ◽  
Conjugate Base

The disappearance of trans-[Co(LH)2(NH3)X] (LH = dimethylglyoximate ion, X = chloride or bromide) has been studied in aqueous solutions over a range of alkali concentrations at various temperatures. The kinetics were done with excess of hydroxide ion at a constant ionic strength so that pseudo first-order rate constants were obtained in all the runs. The results were interpreted in terms of the rapid formation of a pre- equilibrium species which then reacts in a rate-determining step to give products. The relatively large equilibrium constants support a conjugate-base pre-equilibrium, in which the proton is lost from oxygen, while the relatively low reactivities of the conjugate-bases are consistent with the absence of electropositive electromeric effects. The similarity in the reactivities of the chloro and the bromo conjugate-bases suggests the possibility of an SN2CB mechanism.

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.

Hydrolysis and aminolysis of alkyl xanthate esters and cellulose analogues

1999 ◽  
Vol 77 (5-6) ◽  
pp. 1050-1056 ◽  
Author(s):  
Eduardo Humeres ◽  
Valdir Soldi ◽  
Marilene Klug ◽  
Mauricéa Nunes ◽  
Célia MS Oliveira ◽  
...  
Keyword(s):  
Cellulose Xanthate ◽  
Covalent Bond ◽  
Hydroxide Ion ◽  
First Order ◽  
Tetrahedral Intermediate ◽  
Rate Determining Step ◽  
First Order Kinetics ◽  
Elimination Mechanism ◽  
Catalyzed Reactions ◽  
C Nmr

The hydrolysis and aminolysis of a series of S-substituted O-alkylxanthate esters was studied in 20% v/v aqueous methanol at 35°C. The pH-rate profiles of the hydrolyses showed water and hydroxide-ion-catalyzed reactions. The reaction of 2,4-dinitrophenyl cellulose xanthate (CelXDNP) and p-nitrobenzyl cellulose xanthate (CelXNB) with polyalanine and lysozyme produced a covalent bond between the polypeptide and the cellulose matrix, as shown by solid-state 13C NMR. However, the nature of the bonding could not be identified. The reaction of nucleophiles (H2O, OH-, RNH2) and xanthic esters was consistent with an addition-elimination mechanism through a tetrahedral intermediate. Brønsted plots against the pKa of the nucleophile (βnu) or the nucleofuge of the substrate (βlg) were used to characterize the rate-determining step. The pKa values of the nucleophiles ranged between -1.74 and 15.74, and for the nucleofuges, they were in the range of 10.50-0.92. For nucleophiles with pKa values up to about 10, βlg was 0.10-0.15, and βnu changed from 0.48 to 0.35 for the strongest electron-withdrawing nucleofuge. It was concluded that the water-catalyzed hydrolyses, and also aminolyses with moderately basic amines, occur with rate-determining formation of the tetrahedral intermediate. For strong bases such as hydroxide ion, the disappearance of the intermediate becomes the slowest step. The reaction of cellulose xanthic esters with external nucleophiles as hydroxide ion and amines shows simple first-order kinetics and is slower than alkyl or sugar xanthates, probably due to the diffusion effect through the tight cybotactic region of cellulose. Key words: hydrolysis, aminolysis, alkyl xanthic esters, cellulose xanthic esters, sugar xanthic esters.

ChemInform Abstract: Kinetics and Mechanism of the Formation of N-Vinyl Pyridinium Cations in Elimination Reactions in Aqueous Base.

ChemInform ◽  
2010 ◽  
Vol 23 (50) ◽  
pp. no-no
Author(s):  
J. W. BUNTING ◽  
A. TOTH ◽  
J. P. KANTER
Keyword(s):  
Kinetics And Mechanism ◽  
Aqueous Base ◽  
Elimination Reactions ◽  
Pyridinium Cations

Substituent effects of rate constants for thermal [4π + 2π] cycloreversion of spiro-fused β-lactam oxadiazolines

1993 ◽  
Vol 71 (6) ◽  
pp. 907-911 ◽  
Author(s):  
Michel Zoghbi ◽  
John Warkentin
Keyword(s):  
Thermal Decomposition ◽  
Rate Constant ◽  
Rate Constants ◽  
Transition States ◽  
Substituent Effects ◽  
Ground States ◽  
Phenyl Substituent ◽  
First Order ◽  
Average Value ◽  
Carbonyl Ylide

Twelve Δ3-1,3,4-oxadiazolines in which C-2 is also C-4 of a β-lactam moiety (spiro-fused β-lactam oxadiazoline system) were thermolyzed as solutions in benzene. Substituents in the β-lactam portion affect the rate constant for thermal decomposition of the oxadiazolines to N2, acetone, and a β-lactam-4-ylidene. The total spread of first-order rate constants at 100 °C was 47-fold and the average value was 6.7 × 10−4 s−1. A phenyl substituent at N-1 or at C-3 was found to be rate enhancing, relative to methyl. At C-3, H and Cl were also rate enhancing, relative to methyl. The data are interpreted in terms of the differential effects of substituents on the stabilities of the ground states, and on the stabilities of corresponding transition states for concerted, suprafacial, [4π + 2π] cycloreversion. The first products, presumably formed irreversibly, are N2 and a carbonyl ylide. The latter subsequently fragments to form acetone (quantitative) and a β-lactam-4-ylidene.

Organic chemistry students’ challenges with coherence formation between reactions and reaction coordinate diagrams

2018 ◽  
Vol 19 (3) ◽  
pp. 732-745 ◽  
Author(s):  
Maia Popova ◽  
Stacey Lowery Bretz
Keyword(s):  
Organic Chemistry ◽  
Reaction Mechanisms ◽  
Reaction Coordinate ◽  
Structured Interviews ◽  
Chemistry Students ◽  
Student Difficulties ◽  
Need Support ◽  
Elimination Reactions

The purpose of this study was to elucidate and describe students’ thinking when making connections between substitution and elimination reactions and their corresponding reaction coordinate diagrams. Thirty-six students enrolled in organic chemistry II participated in individual, semi-structured interviews. Three major themes were identified that characterize students’ difficulties with integrating the information from the reactions and the reaction coordinate diagrams: incorrect ideas about the meanings of the reaction coordinate diagrams’ features, errors when examining reaction mechanisms, and an inability to assess the relative energies of reaction species. These findings suggest that students need support for coherence formation between reactions and reaction coordinate diagrams. Implications for teaching to address these student difficulties are suggested.

Substituent Effects on the Hydrolysis of p-Substituted Benzonitriles in Sulfuric Acid Solutions at (25.0± 0.1) °C

2008 ◽  
Vol 63 (9) ◽  
pp. 603-608 ◽  
Author(s):  
Khamis A. Abbas
Keyword(s):  
Sulfuric Acid ◽  
Rate Constants ◽  
Inductive Effect ◽  
Substituent Effects ◽  
Acid Solutions ◽  
Rate Determining Step ◽  
Inductive Effects ◽  
High Concentrations ◽  
Sulfuric Acid Solutions ◽  
Hydrolysis Of

The rate constants of the hydrolysis of p-substituted benzonitriles with sulfuric acid solutions (18.2 M to 10 M) have been determined spectrophotometrically at (25.1±0.1) °C. It was found that the catalytic activity of sulfuric acid was strongly inhibited by water. The logarithms of the observed rate constants were correlated with different substituent inductive (localized) and resonance (delocalized) constants. The results of the correlation studies indicated that the rate-determining step of the hydrolysis of benzonitriles in 18.2 M sulfuric acid was the addition of a nucleophile, and the hydrolysis was clearly enhanced by the electron-withdrawing inductive effect, while the rate-determining step of the hydrolysis of p-substituted benzonitriles in 10.0 M sulfuric acid was most probably the protonation of benzonitriles, and the rate constants increased by both electron-donating resonance and inductive effects. A mixture of the two mechanisms most probably occurred in 15.3 to 17.0 M sulfuric acid. HSO4 − rather thanwater most probably acted as nucleophile in the hydrolysis of benzonitriles especially at high concentrations of sulfuric acid solutions.

Reduction processes in the fast atom bombardment mass spectra of pyridinium salts. The effect of reduction potential and concentration

1989 ◽  
Vol 67 (2) ◽  
pp. 183-186 ◽  
Author(s):  
Clement W. Kazakoff ◽  
Robin T. B. Rye ◽  
Oswald S. Tee
Keyword(s):  
Concentration Dependence ◽  
Mass Spectra ◽  
Fast Atom Bombardment ◽  
Reduction Potential ◽  
Pyridinium Salts ◽  
Pyridinium Cation ◽  
Reduction Processes ◽  
Pyridinium Cations ◽  
Bombardment Energy ◽  
Matrix Free

The enhancement of the (C + 1)/C ratio in the fast atom bombardment mass spectra of seven pyridinium cations has been measured. No dependence of the enhancement on the cation reduction potential could be identified. The N-methyl pyridinium cation, which showed no enchancement under matrix-free conditions, exhibited an increase in the (C + 1)/C ratio with decreasing concentration. This concentration dependence was eliminated when the bombardment energy was reduced from 9 to 5 keV. Possible mechanisms for the concentration dependence and the variation with bombardment energy are proposed. Keywords: fast atom bombardment, reduction, pyridinium salts.

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