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.

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.

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.

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.

Nitrogen removal characteristics of nitrification and denitrification filters

1994 ◽  
Vol 29 (10-11) ◽  
pp. 409-416 ◽  
Author(s):  
F. Çeçen ◽  
I. E. Gönenç
Keyword(s):  
Nitrogen Removal ◽  
Rate Constants ◽  
Order Rate Constant ◽  
Zero Order ◽  
Ammonia Removal ◽  
Nitrite Accumulation ◽  
Order Kinetic ◽  
Order Rate ◽  
In Series ◽  
Nitrification And Denitrification

The kinetics of nitrogen removal was studied in upflow submerged nitrification and denitrification filters in series. Nitrification followed first-, half-, and zero-order kinetics. For the half-order range the half-order rate constant was about 0.9gNH4-N1/2m−1/2d−1. The zero-order rate constants for the DO ranges of 2-3 mg/L and 4-5 mg/L were found as 0.47 gNH4-Nm−2d−1 and 1.82 gNH4-Nm−2d−1, respectively. In the zero-order region ammonia removal proceeded as a half-order reaction in oxygen concentration and the half-order rate constants were about 1.4-2.7 gO21/2m−1/2d−1. Nitrite accumulation reached a considerable degree at bulk oxygen to bulk ammonia ratios lower than 5 since the formation of nitrate was inhibited. Similar to nitrification half- and zero-order kinetic regions were also observed in denitrification. The half- and zero-order rate constants for carbon unlimited cases (influent COD/NOx-N&gt;5) were about 0.23 gNOx-N1/2m−1/2d−1 and 1.9 gNOx-Nm−2d−1, respectively. The nitrite produced in the nitrification stage could be reduced in denitrification. The removal kinetics in the presence of nitrite was found to be similar to the kinetics when the influent consisted of nitrate only.

scholarly journals The reactivities and ionization properties of the active-site dithiol groups of mammalian protein disulphide-isomerase

1991 ◽  
Vol 275 (2) ◽  
pp. 335-339 ◽  
Author(s):  
H C Hawkins ◽  
R B Freedman
Keyword(s):  
Rate Constant ◽  
Ph Dependence ◽  
Order Rate Constant ◽  
Second Order ◽  
Native Enzyme ◽  
Liver Protein ◽  
Thiol Groups ◽  
Protein Disulphide Isomerase ◽  
Order Rate ◽  
Reactive Groups

1. The number of reactive thiol groups in mammalian liver protein disulphide-isomerase (PDI) in various conditions was investigated by alkylation with iodo[14C]acetate. 2. Both the native enzyme, as isolated, and the urea-denatured enzyme contained negligible reactive thiol groups; the enzyme reduced with dithiothreitol contained two groups reactive towards iodoacetic acid at pH 7.5, and up to five reactive groups were detectable in the reduced denatured enzyme. 3. Modification of the two reactive groups in the reduced native enzyme led to complete inactivation, and the relationship between the loss of activity and the extent of modification was approximately linear. 4. Inactivation of PDI by alkylation of the reduced enzyme followed pseudo-first-order kinetics; a plot of the pH-dependence of the second-order rate constant for inactivation indicated that the essential reactive groups had a pK of 6.7 and a limiting second-order rate constant at high pH of 11 M-1.s-1. 5. Since sequence data on PDI show the presence within the polypeptide of two regions closely similar to thioredoxin, the data strongly indicate that these regions are chemically and functionally equivalent to thioredoxin. 6. The activity of PDI in thiol/disulphide interchange derives from the presence of vicinal dithiol groups in which one thiol group of each pair has an unusually low pK and high nucleophilic reactivity at physiological pH.

scholarly journals Compounds of 6, 13-Diamino-6, 13-Dimethyl-1, 4, 8, 11 - Tetraazacyclotetradecane

2021 ◽  
Author(s):  
◽  
Asokamali Siriwardena
Keyword(s):  
Rate Constants ◽  
Magnetic Measurements ◽  
Order Rate Constant ◽  
Acidic Solutions ◽  
First Order ◽  
Order Rate ◽  
Pendant Arm ◽  
Nickel Copper ◽  
13C Nuclear Magnetic Resonance ◽  
Kinetics Of

<p>The reaction of bis-(diaminoethane)nickel(II) chloride, ([Ni(en)2]Cl2 in methanol with formaldehyde and nitroethane in the presence of triethylamine proceeds readily to produce (6, 13-dimethyl-6, 13-dinitro-1, 4, 8, 11-tetraazacyclotetradecane)nickel(II) chloride, [Ni(dini)] - Cl2. Reduction of the nitro groups of this compound by catalytic hydrogenation yields three isomers of the pendant arm macrocyclic complex (6, 13-diamino-6, 13-dimethyl-1, 4, 8, 11-tetraazachyclotetradecane)nickel(II) chloride, designated a-, b- and c-[Ni(diam)]Cl2. These were separated by fractional crystallization. The aisomer was observed to isomerizes slowly in solution to the b- form. A parallel dissociation reaction of the a- isomer was also observed. The demetallation of a- and b- isomers of the diam complex of nickel by reaction with cyanide or concentrated acid at 140 degrees C produces the macrocycle meso-(6, 13-diamino-6, 13-dimethyl-1, 4, 8, 11-tetraazacyclotetra-decane), diam. A variety of hexamine, pentamine and tetramine complexes of diam with nickel(II), copper(II), cobalt(II) and (III), chromium(III), palladium(II), rhodium(III), zinc(II) and cadmium(II) were prepared. Hexamine and tetramine forms of labile metal complexes could be rapidly and reversibly interconverted by altering the pH. The hexamine cobalt(III) cation, [Co(diam)]3+ was by far the most inert of the prepared cobalt(III) complexes, remaining unaffected in hot acidic solutions. In contrast, a single pendant arm of the hexamine [Cr(diam)]3+ cation could be dissociated in acid. (Two possibly triamine complexes of lead were also prepared). These compounds were characterized by elemental analysis, magnetic measurements, electronic, infrared, 1H and 13C nuclear magnetic resonance spectra. The pendant arm protonation constants (log K) of diam and selected complexes of nickel, copper and palladium were calculated from potentiometric titration measurements at 25 degrees C. The log K values for diam at 25 degrees C (I = 0.1 M NaclO4) were 11.15, 9.7, 6.2 and 5.3. Kinetics of the parallel isomerization and dissociation of a-[Ni(dimH2)]4+ in HCl/NaCl solutions were monitored spectrophotometrically at 50 degrees C. The rate of reaction in acidic solutions showed a non-linear dependency on acid concentration. The observed first order rate constant (kobs) for disappearance of a-[Ni(diamH2)]4+ (by isomerization and dissociation) in 2.0 M HCl, 0.1 M NaOH and 2.0 M NaCl were 3.05 x 10-4, 2.0(3) x 10-2 and 5.0 x 10-5 s-1 respectively. The rate of the dissociation component of the reaction of a-[Ni(diamH2)]4+ in 2.0 M HCl at 50 degrees C was 1.82 x 10-7 s-1. Acid bydrolysis kinetics of (Cu[diamH2])(ClO4)4 in hydrochloric acid and perchloric acid at 50 and 70 degrees C were studied spectrophotometrically. The reactions were slow and the observed first order rate constants were to a first approximation independent of the particular acid or its concentration. The observed first order rate constants were 1 x 10-9 and 8 x 10-9 s-1 at 50 and 70 degrees C respectively. Questions about the nature of the reaction being followed have been raised.</p>

Substituent effects of cis-dioxobis(dithiocarbamato) molybdenum(VI) on redox properties: redox potentials for one-electron reduction and second-order rate constants for oxygen atom transfer

1998 ◽  
Vol 269 (2) ◽  
pp. 260-268 ◽  
Author(s):  
Kei Unoura ◽  
Akira Yamazaki ◽  
Akira Nagasawa ◽  
Yoshikiyo Kato ◽  
Hiroki Itoh ◽  
...  
Keyword(s):  
Oxygen Atom ◽  
Rate Constants ◽  
Substituent Effects ◽  
Second Order ◽  
Redox Properties ◽  
Atom Transfer ◽  
Redox Potentials ◽  
Oxygen Atom Transfer ◽  
Order Rate ◽  
Electron Reduction

A method of determining electrical potential gradient across mitochondrial membrane in perfused rat hearts

1993 ◽  
Vol 265 (2) ◽  
pp. H445-H452 ◽  
Author(s):  
B. Wan ◽  
C. Doumen ◽  
J. Duszynski ◽  
G. Salama ◽  
K. F. LaNoue
Keyword(s):  
Mitochondrial Membrane ◽  
Electrical Potential ◽  
Potential Gradient ◽  
Order Rate Constant ◽  
Isolated Rat Heart ◽  
First Order ◽  
Order Rate ◽  
Rat Hearts ◽  
Perfused Rat Hearts ◽  
Electrical Potential Gradient

The electrical potential gradient across the mitochondrial membrane (delta psi m) in perfused rat hearts was estimated by calculating the equilibrium distribution of the lipophilic cation tetraphenylphosphonium (TPP+), using measured kinetic constants of uptake and release of TPP+. First-order rate constants of TPP+ uptake were measured during 30-min perfusions of intact rat hearts with tracer amounts (5.0 nM) of tritium-labeled TPP+ ([3H]TPP+) in the perfusate. This was followed by a 30-min washout, during which the first-order rate constant of efflux was estimated. Values of [3H]TPP+ outside the heart and total [3H]TPP+ inside the heart at equilibrium were calculated. From this information and separately estimated time-averaged plasma membrane potentials (delta psi c) it was possible to calculate free cytosolic [3H]TPP+ at equilibrium. It was also possible to calculate free intramitochondrial [3H]TPP+ at equilibrium as the difference between total tissue [3H]TPP+ minus free cytosolic TPP+ and the sum of all the bound [3H]TPP+. Bound [3H]TPP+ was determined from [3H]TPP+ binding constants measured in separate experiments, using both isolated mitochondria and isolated cardiac myocytes under conditions where both delta psi m and delta psi c were zero. Delta psi m was calculated from the intramitochondrial and cytosolic free TPP+ concentrations using the Nernst equation. Values of delta psi m were 144.9 +/- 2.0 mV in hearts perfused with 5 mM pyruvate and 118.2 +/- 1.4 mV in hearts perfused with 11 mM glucose, in good agreement with delta psi m obtained from isolated rat heart mitochondria.(ABSTRACT TRUNCATED AT 250 WORDS)

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