Studies in solvolysis. Part II. Some comments on the ion-pair mechanism for displacements at a primary carbon atom

1970 ◽  
Vol 48 (24) ◽  
pp. 3807-3818 ◽  
Author(s):  
John M. W. Scott
Keyword(s):  
Carbon Atom ◽  
Equilibrium Constant ◽  
Rate Constant ◽  
Rate Constants ◽  
Ion Pairs ◽  
Ion Pair ◽  
Experimental Basis ◽  
Methyl Halides ◽  
Bimolecular Rate Constant ◽  
Substitution Process

The implication of Sneen and co-workers (2–5) that all SN2 substitutions at a primary carbon atom probably proceed via the intermediate production of an intimate ion-pair is examined with respect to the reactions of the methyl halides (MeX; X = F, Cl, Br, I) with various nucleophiles (H2O, OH−, F−, Cl−, Br−, I−, CN−, CSN−, S2O32−) in water. By establishing certain rules concerning the behavior of derived reactivity scales (essentially ρ values) as contrasted with absolute reactivity scales (observed rate constants), it is concluded that Sneen and Larsen's mechanistic description is consistent with the experimental facts, and that in such cases the substitution process involves a pre-equilibrium constant, Ke, which is independent of the attacking nucleophile. This is followed by a rate determining bimolecular rate constant, kn, which depends on the ion pair and the nature of the nucleophile. The observed rate (k°) is given by k = Kekn. A method of calculating Ke is described and values of kn for nine nucleophiles attacking the four methyl halide ion-pairs are reported along with a number of confirmatory calculations. It is concluded that the classical Hughes–Ingold SN2 Heitler–London description of these reactions is inadequate. Some further suggestions to place the new mechanistic description on a firmer experimental basis are made.

Isotope effects and activation parameters for the proton transfer reaction from 1-(4-nitrophenyl)-1-nitroethane to free anions and ion pairs of cesium n-propoxide in n-propanol solvent

1986 ◽  
Vol 64 (6) ◽  
pp. 1021-1025 ◽  
Author(s):  
Arnold Jarczewski ◽  
Grzegorz Schroeder ◽  
Przemyslaw Pruszynski ◽  
Kenneth T. Leffek
Keyword(s):  
Proton Transfer ◽  
Rate Constants ◽  
Isotope Effects ◽  
Ion Pairs ◽  
Activation Parameters ◽  
Solvation Shell ◽  
Ion Pair ◽  
Initial State ◽  
Free Ion ◽  
Deuteron Transfer

Rate constants for the proton and deuteron transfer from 1-(4-nitrophenyl)-1-nitroethane to cesium n-propoxide in n-propanol have been measured under pseudo-first-order conditions with an excess of base for four temperatures between 5 and 35 °C. Using literature values of the fraction of cesium n-propoxide ion pairs that are dissociated into free ions, separate second-order rate constants for the proton and deuteron transfer to the ion pair and to the free ion have been calculated. The cesium n-propoxide ion pair is about 2.8 times more reactive than the free n-propoxide ion. The primary kinetic isotope effects for the two reactions are the same (kH/kD = 6.1–6.3 at 25 °C) within experimental error. The enthalpy of activation is smaller for the ion-pair reaction and the entropy of activation more negative than for the free-ion reaction. For proton transfer, ΔH±ion pair = 8.3 ± 0.2 kcal mol−1, ΔH±ion = 9.6 ± 1.0 kcal mol−1, ΔS±ion pair = −12.3 ± 0.6 cal mol−1 deg−1, ΔS±ion = −10.1 ± 3.4 cal mol−1 deg−1. The greater reactivity of the ion pair relative to the free ion is interpreted in terms of the weaker solvation shell of the ion pair in the initial state.

The Kinetics of Isotope Effects in a Model Solvolysis System Involving One Optically Active Ion Pair Intermediate

1974 ◽  
Vol 52 (10) ◽  
pp. 1937-1941 ◽  
Author(s):  
P. Christian Vogel
Keyword(s):  
Rate Constant ◽  
Rate Constants ◽  
Isotope Effects ◽  
Kinetic Scheme ◽  
Ion Pair ◽  
Rate Equations ◽  
Differential Analysis ◽  
First Order ◽  
Order Rate ◽  
Special Cases

The derivation of the observed first-order rate constants from the "exact" integrated rate equations for the kinetic scheme of reaction 1 is presented. It is shown that the solvolytic exponential first-order rate constant is a special case of the polarimetric rate constant and that the optical activity of the product is determined by a multiplicative ratio of rate constants for the optically important reactions of the ion pair intermediate. A form of the integrated first-order polarimetric rate equation with a linearly independent parameter set is presented. The functions for the first-order rate constants derived using the steady state approximation are special cases of the functions derived from the exact equations, as are the functions for the first-order rate constants for two systems which involve pre-equilibria followed by a slow product forming step. These functions cannot all be derived one from the other. A differential analysis of observed isotope effects as functions of isotope effects on the rate constants for reactions involving the intermediates is presented.

ION PAIR EFFECTS IN THE REACTION BETWEEN POTASSIUM FERROCYANIDE AND POTASSIUM PERSULFATE

1966 ◽  
Vol 44 (4) ◽  
pp. 437-445 ◽  
Author(s):  
R. W. Chlebek ◽  
M. W. Lister
Keyword(s):  
Ionic Strength ◽  
Rate Constants ◽  
Equilibrium Constants ◽  
Ion Pairs ◽  
Ion Pair ◽  
Glass Electrode ◽  
Potassium Ferrocyanide ◽  
Potassium Persulfate ◽  
Ion Concentration ◽  
True Rate

The rate of the reaction between potassium ferrocyanide and potassium persulfate has been measured over a range of conditions. The rate is dependent on the potassium ion concentration, and it is shown that this is explained if it is assumed that KFe(CN)63− and KS2O8− are the reacting species. The equilibrium constants governing the formation of these ion pairs were measured with a cation-sensitive glass electrode. Similar constants for the products KFeCCN6)2− and KSO4−, and also for KNO3, were measured. From these equilibrium constants, the true rate constants of the reaction can be obtained, and it is shown that these vary with ionic strength in the manner predicted by Brönsted's equation.

Electron-Transfer Reactions in Non-Aqueous Media. IV. A Temperature-Dependence Study of the Reduction of CoCl(NH3)52+ by Iron(II) in N,N-Dimethylformamide

1979 ◽  
Vol 32 (7) ◽  
pp. 1425 ◽  
Author(s):  
KR Beckham ◽  
DW Watts
Keyword(s):  
Electron Transfer ◽  
Temperature Dependence ◽  
Equilibrium Constant ◽  
Rate Constant ◽  
Iron Atom ◽  
Rate Constants ◽  
Aqueous Media ◽  
Transfer Reactions ◽  
Complex Functions ◽  
Temperature Dependences

A detailed study has been made of the temperature dependence of the rate of reduction of CoCl-(NH3)52+ by iron(II) in N,N-dimethylformamide. The observed rate constants (kobs) for this reaction are complex functions of an equilibrium constant (K) for the formation of a bridged intermediate, the rate constant for electron transfer in this bridged intermediate (k), and the iron(II) concentration. From studies of the dependence of kobs on iron(II) concentration at five temperatures the temperature dependences of both K and k have been resolved, yielding respectively ΔH� -20k�12 kJ mol-1, ΔS� -44�40 J K-1 mol-1 and ΔH* 107�4 kJ mol-1, ΔS* 57�16 J K-1 mol-1. The results are interpreted in terms of a bridged intermediate in which the iron atom is tetrahedrally coordinated.

Kinetics and mechanisms of the reaction of 2,4,6-trinitrocumene and 2,4,6-trinitroethylbenzene with 1,1,3,3-tetramethylguanidine in N,N-dimethylformamide solvent

1987 ◽  
Vol 65 (5) ◽  
pp. 1007-1011 ◽  
Author(s):  
Mihir K. Biswas ◽  
Arnold Jarczewski ◽  
Kenneth T. Leffek
Keyword(s):  
Carbon Atom ◽  
Complex Formation ◽  
Proton Transfer ◽  
Equilibrium Constant ◽  
Rate Constants ◽  
Equilibrium Constants ◽  
Activation Parameters ◽  
Reaction Parameters ◽  
Deuterium Isotope Effect ◽  
Constant Rate

The reaction of tetramethylguanidine (TMG) with trinitrocumene (TNC) and trinitroethylbenzene (TNEB) in dimethylformamide solvent has been studied with respect to products and kinetics. For TNC only σ-complex formation with the benzene ring was observed, for which the equilibrium constant, rate constants, and activation parameters were measured. For TNEB, both σ-complex formation and proton transfer from the σ-carbon atom of the ethyl group were observed. The equilibrium constants, rate constants, and activation parameters were separately determined for each reaction and a primary deuterium isotope effect, kH/kD = 13.6 (at 20 °C), was found for the proton transfer. The reaction parameters are compared to those for proton transfer from TNT to tetramethylguanidine in DMF solvent.

Spectroscopic studies of the interaction of cobalt(II) N,N',N″,N‴-tetramethyltetra-3,4-pyridinoporphyrazine with amino acids and nitrogen oxides

2001 ◽  
Vol 05 (12) ◽  
pp. 839-845 ◽  
Author(s):  
MAMOTHIBE A. THAMAE ◽  
TEBELLO NYOKONG
Keyword(s):  
Nitric Oxide ◽  
Amino Acids ◽  
Nitrogen Oxides ◽  
Equilibrium Constant ◽  
Rate Constant ◽  
Rate Constants ◽  
Kinetic Data ◽  
Spectroscopic Studies

The interaction of histidine, cysteine, NO and nitrite with cobalt(II) N,N',N″,N‴-tetramethyltetra-3,4-tetrapyridinoporphyrazine ([ Co II tmtppa ]4+) is reported. Metal-based autoreduction of [ Co II tmtppa ]4+ occurs with the formation of the [ Co I tmtppa (-2)]3+ species in the presence of histidine and cysteine. Kinetic data for the auto reduction of [ Co II tmtppa ]4+ in the presence of these amino acids gave the rate constants k f = 2.1 × 101 and 2.8 dm3 mol-1 s-1, for cysteine and histidine, respectively. One molecule of NO or nitrite was found to coordinate to the [ Co II tmtppa ]4+ species. The equilibrium and rate constants for the coordination of the nitric oxide were K = 2.3 × 104 dm 3 mol -1 and k f = 7.5 dm 3 mol -1 s -1, respectively. The coordination of nitrite to [ Co II tmtppa ]4+ occurred with an equilibrium constant of K = 2.0 × 102 dm 3 mol -1 and a rate constant of k f = 4.0 × 10-3 dm 3 mol -1 s -1. There was no evidence for the coordination of two molecules of nitrite to the [ Co II tmtppa ]4+ species.

Kinetics of oxidation of Cu(I) complexes of cysteine and penicillamine: nature of intermediates and reactants at pH 10.0

1989 ◽  
Vol 67 (4) ◽  
pp. 736-745 ◽  
Author(s):  
Stephen P. Mezyk ◽  
David A. Armstrong
Keyword(s):  
Equilibrium Constant ◽  
Rate Constant ◽  
Rate Constants ◽  
Slow Component ◽  
Mixed Valence ◽  
Planar Geometry ◽  
Published Data ◽  
Diffusion Controlled ◽  
Square Planar ◽  
Thiolate Ligand

The Cu(I)•L2 complex with cysteine ligands at total Cu(I) concentrations of 10–30 μM was shown to be oxidised by cysteinyl radicals (RS•) with a diffusion-controlled rate constant k11a = 1.8 × 109 M−1 s−1. The corresponding reaction with the cysteine disulphide anion (RS•—SR−) proceeded at a slower rate, k11b = 2.7 × 108 M−1s−1. At higher Cu(I) concentrations, a slow and a fast component of absorption growth was observed. The slow component rate was independent of Cu(I) concentration, but it became more intense as the Cu(I) concentration rose. The yields and kinetic data were shown to be consistent with the presence of an equilibrium between the Cu(I)•L2 species and a second Cu(I) complex, Cu(I)2•L3, with an equilibrium constant of K1 = 162.[Formula: see text]This finding is consistent with the earlier work of Bagiyan etal. The rate constant of the oxidation of Cu(I)2•L3 by the cysteinyl radical was k12 = 1.0 × 109 M−1 s−1. Similar results were obtained with penicillamine, except the rate constants and equilibrium constant were smaller, (k11a = 4.5 × 108 M−1 s−1, k11b < 2 × 108 M−1 s−1, k12 = 5.5 × 108 M−1 s−1 and K1 = 113). This was attributed to the presence of the β-methyl groups in penicillamine, which exert a large steric effect.The ultraviolet spectra of the long-lived products, which are stable on a millisecond timescale, was consistent with a Cu(II)•L2 structure with a square planar geometry. The oxidation of the Cu(I)2.L3 species proceeded via intermediates, which relaxed to the final product spectra with rate constants of k13b = 2.6 and 1.1 × 104 s−1 for cysteine and penicillamine, respectively. Comparison of the spectra of the intermediates with published data showed that they were consistent with the presence of a bridging thiolate ligand between Cu(I) and Cu(II). Keywords: oxidation, copper, mixed valence, cysteine, penicillamine, complexation.

ROLE OF THE HIGH-AFFINITY PENTASACCHARIDE IN HEPARIN ACCELERATION OF ANTITHROMBIN III INHIBITION OF THROMBIN AND FACTOR Xa

1987 ◽  
Author(s):  
Steven T Olson ◽  
Ingemar Bjork ◽  
Paul A Craig ◽  
Joseph D Shore ◽  
Jean Choay
Keyword(s):  
Ionic Strength ◽  
Conformational Change ◽  
Rate Constant ◽  
Rate Constants ◽  
Antithrombin Iii ◽  
Factor Xa ◽  
Order Conditions ◽  
Bimolecular Rate Constant ◽  
High Affinity ◽  
Inhibition Of Thrombin

The high-affinity heparin pentasaccharide (H5) and an 8000 Mr high-affinity heparin (H26) have been compared with respect to their interaction with antithrombin III (AT) and their accelerating effect on AT inhibition of thrombin (T) and factor Xa by rapid kinetic and equilibrium binding studies at pH 7.4, 25°C. Kds of .068 μM at I 0.15 and 0.57 μM at I 0.3 were determined for tne AT-H5 interaction, which were 5 and 2.5-fold weaker, respectively, than affinities determined for H26. Comparison of the kinetics of binding of H5 and H26 to AT at I 0.15 under pseudofirst order conditions ([H]o>> [AT]o) demonstrated a saturable dependence of the observed rate constant for both reaction with indistinguishable limiting rate constants of 700 +/-120 s-1 and 520 +/-90 s-1 , but somewhat different Kds for the initial binding interaction of 20 and 29 μM for H5 and H26, respectively. These results indicate that H5 induces the same conformational change in AT as the larger heparin, but that the rate of reversal of this conformational change is greater for H5 which is the basis for its weaker AT affinity. Bimolecular rate constants for neutralization of factor Xa and thrombin by AT-H5 and AT-H26 complexes were determined by p-aminobenzamidine displacement under pseudo-first order conditions([AT-H] >> [T]o or [Xa]o). I-in-dependent values of .62 μM-1 s-1 were obtained for Xa inhibition by AT-H5 at I 0.15 and 0.3, compared to I-dependent values of 1.4 and 0.91 μM-1 s-1 for AT-H26. For thrombin inhibition by AT-H5, and I-independent enhancement of 1.6-fold in the bimolecular rate constant from .0098 to .016 μM-1 s-1 was observed, in sharp contrast to the marked I-independent enhancement by AT-H26 of the bimolecular rate constant ranging from 4000 to 200-fold at I 0.15 and 0.3, respectively. These results are consistent with a primary ionic strength-independent contribution of the AT conformational change to heparin enhancement of factor Xa but not thrombin neutralization by AT, with an ionic strength-dependent component for both reactions, compatible with a differential role for a protease-heparin interaction. Supported by grant HL-30237

The ion-pair mechanism and bimolecular displacement at saturated carbon. VII. Racemization of substituted 1-phenylbromoethanes; ionic strength effects

1989 ◽  
Vol 67 (2) ◽  
pp. 297-304
Author(s):  
Allan R. Stein
Keyword(s):  
Ionic Strength ◽  
Rate Constant ◽  
Salt Effect ◽  
Ion Pairs ◽  
Tetrabutylammonium Bromide ◽  
Ion Pair ◽  
Nucleophilic Attack ◽  
Unimolecular Reaction ◽  
Salt Effects ◽  
Bromide Ion

No, or at most very small, salt effects have been detected for second-order racemizations of various 1-phenylbromoethanes, confirming that the "mixed kinetics", previously reported for 4-methyl- and 3,4-dimethylphenylbromoethanes with tetrabutylammonium bromide in acetonitrile and nitromethane, did not reflect a salt effect on a unimolecular reaction. Thus the uni- and bimolecular rate components can be evaluated using the equation:[Formula: see text]where k1 = first-order or unimolecular rate constant, the intercept, and [Formula: see text], the latter being the rate constant for the bimolecular substitution. A "special salt effect" was found for the unimolecular component; it was especially pronounced for 3,4-dimethyl substrate in acetonitrile with Bu4NClO4 as the electrolyte. In acetone, where no unimolecular reaction component was detected even with those substrates giving the most stable carbocations, the only salt effect was a common ion, Bu4N+, effect on the incomplete dissociation of Bu4NBr. From an iterative best fit for plots of observed rate versus calculated bromide ion activity for unsubstituted, 4-methyl, and 3,4-dimethyl substrates, Kassoc ~ 15 ± 1 at 40 °C. The results are interpreted as additional support for a progression of mechanism with nucleophilic attack possible at any stage of the series of equilibria: substrate [Formula: see text] contact ion pair [Formula: see text] various solvated ion pairs [Formula: see text] dissociated ions. Keywords: special salt effects, ionic strength effects, racemization of 1-phenylbromoethanes, mechanism of nucleophilic substitution, ion pair mechanism.

Effect of the acyl substituent on the equilibrium constant for hydration of esters

1980 ◽  
Vol 58 (13) ◽  
pp. 1281-1294 ◽  
Author(s):  
J. Peter Guthrie ◽  
Patricia A. Cullimore
Keyword(s):  
Aqueous Solution ◽  
Equilibrium Constant ◽  
Rate Constant ◽  
Rate Constants ◽  
Acid Catalysis ◽  
Methyl Esters ◽  
Equilibrium Constants ◽  
Free Energies ◽  
General Acid ◽  
Aldehydes And Ketones

Heats of hydrolysis have been measured for the trimethyl orthoesters of isobutyric, propionic, benzoic, methoxyacetic, chloroacetic, and cyanoacetic acids using aqueous acid with an organic cosolvent where necessary, and of the corresponding esters in alkaline solution. Solubilities or free energies of transfer from gas to aqueous solution have been measured, permitting calculation of the free energies of formation of the aqueous orthoesters, and by methods which we have published previously, calculation of the free energies of formation of the covalent hydrates of the esters, and the free energy changes for hydration of these esters.Using estimated pKa values equilibrium constants were calculated for the addition of hydroxide to the esters. The data are in good agreement with the appropriate Marcus equation relating rate and equilibrium constants with a value for b of 8.99 ± 0.17. This line was used to estimate the equilibrium constant for addition of hydroxide, and thence of water, to some additional esters where only the rate constant was available. Rate constants for hydrolysis of methyl esters in aqueous solution at 25 °C were calculated from literature data, correcting for the effect of other conditions as necessary. From the equilibrium constants for addition of water we could estimate the rate constants for uncatalyzed hydrolysis; for the cases where this rate constant has been measured, the agreement was satisfactory. For acid catalyzed hydrolysis the data permit a test of the two alternative mechanisms considered previously, namely specific acid catalysis and general acid catalysis with hydronium ion acting as a general acid. For esters the mechanism is clearly specific acid catalysis, but for aldehydes and ketones it appears very likely that the mechanism is general acid catalysis.

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