Plots Of Kobs Research Articles

Kinetics of halide exchange in cyclopentadienylruthenium bis(triarylphosphine)halides

The kinetics of halide exchange in cyclopentadienylruthenium bis(triarylphosphine) halide complexes CpRu(PAr3)2X (1a X = Cl, 1b X = Br, 1c X = I; X′= Cl, Br, Ar = C6H5, 2 Ar = p-CH3OC6H4, X = Cl, X′= Br and 3 Ar = p-FC6H4, X = Cl, X′ = Br) have been measured and the results interpreted with respect to two possible mechanisms: halide dissociation vs phosphine dissociation. Halide exchange between nBu4NX′ and 1–3 in fluorobenzene under pseudo-first order conditions is found to be first order in 1–3. Relative rates of halide exchange follow the order: 1a > 1b > 1c > 2 ≈ 3 with activation parameters ΔH† = 117 ± 6 kJ/mol and ΔS† = 44 ± 18 J/mol-K (1a, X′ = Br) and ΔH† = 120 ± 14 kJ/mol ΔS† = 50 ± 45 J/mol-K (1b, X′ = Cl). Plots of kobs vs [X′–] for 1 and 2 are consistent with a two-term rate law where kobs = (k1 + k2[X′–]) while reactions of 3 are independent of the halide concentration. The rate data are consistent with phosphine dissociation as the rate determining step followed by a slightly faster reaction with halide ions where the dissociated phosphine competes effectively with halide for the open coordination site in a CpRu(PAr3)X intermediate.

Kinetics and mechanism of the one‐step, two‐electron oxidation of diaquadichloro(1,10‐phenanthroline)chromium(III) chloride dihydrate by periodate to chromium(V) in aqueous acidic solutions

AbstractThe kinetics of oxidation of [CrIIICl2(phen)(H2O)2]Cl⋅2H2O (phen = 1,10‐phenanthroline) by periodate {I(VII)} have been studied in aqueous acidic solutions. The oxidation product has been isolated and characterized by elemental analysis, IR, and ESR as [CrVCl2(phen)(O)]Cl⋅1.5H2O. Under pseudo–first‐order conditions, the reaction showed first‐order dependence on the initial Cr(III) complex concentration. At fixed reaction conditions, the dependence of kobs on [I(VII)] showed saturation kinetics. The rate constant, kobs, increased with increasing pH, indicating that the hydroxo form of the chromium(III) complex is the most reactive species. A plot of kobs([H+] + K1) versus [H+] is linear with an intercept, indicating that the reaction proceeds via two parallel pathways. One pathway is independent of [H+] and the other on [H+]–1. Since water is known to be a very weak bridging ligand, the pathway involving the aqua form of the complex, almost certainly, is bridged to periodate via the chloro ligand of the chromium(III) complex. The thermodynamic parameters of the processes involved are reported. A one‐step two‐electron transfer via an inner‐sphere mechanism has been assigned to the two pathways based on the form of the rate law

Photochemical pump and NMR probe to monitor the formation and kinetics of hyperpolarized metal dihydrides.

On reaction of IrI(CO)(PPh3)21 with para-hydrogen (p-H2), Ir(H)2I(CO)(PPh3)22 is formed which exhibits strongly enhanced 1H NMR signals for its hydride resonances. Complex 2 also shows similar enhancement of its NMR spectra when it is irradiated under p-H2. We report the use of this photochemical reactivity to measure the kinetics of H2 addition by laser-synchronized reactions in conjunction with NMR. The single laser pulse promotes the reductive elimination of H2 from Ir(H)2I(CO)(PPh3)22 in C6D6 solution to form the 16-electron precursor 1, back reaction with p-H2 then reforms 2 in a well-defined nuclear spin-state. The build up of this product can be followed by incrementing a precisely controlled delay (τ), in millisecond steps, between the laser and the NMR pulse. The resulting signal vs. time profile shows a dependence on p-H2 pressure. The plot of kobs against p-H2 pressure is linear and yields the second order rate constant, k2, for H2 addition to 1 of (3.26 ± 0.42) × 102 M-1 s-1. Validation was achieved by transient-UV-vis absorption spectroscopy which gives k2 of (3.06 ± 0.40) × 102 M-1 s-1. Furthermore, irradiation of a C6D6 solution of 2 with multiple laser shots, in conjunction with p-H2 derived hyperpolarization, allows the detection and characterisation of two minor reaction products, 2a and 3, which are produced in such low yields that they are not detected without hyperpolarization. Complex 2a is a configurational isomer of 2, while 3 is formed by substitution of CO by PPh3.

A kinetic study of the oxidations of 2-mercaptoethanol and 2-mercaptoethylamine by heteropoly 11-tungsto-1- vanadophosphate in aqueous acidic medium

The kinetics of oxidation of 2-mercaptoethanol and 2-mercaptoethylamine by the heteropoly 11-tungsto-1- vanadophosphate anion, [PVVW11O40]4−, have been studied spectrophotometrically in aqueous perchloric acid at 25 °C. EPR and optical studies show that [PVVW11O40]4− is reduced to the one-electron reduced heteropoly blue, [PVIVW11O40]5−, whilst the thiols are oxidized to the corresponding disulphides, RSSR. Spectrophotometric titrations show that the stoichiometry of both reactions is 1:1. At constant pH, the reactions show simple second-order kinetics with first-order dependence of rate on both [oxidant] and [thiol]. At constant [thiol], the rate of the reaction increases with increasing pH. Plots of kobs/[thiol]t versus 1/[H+] are linear with finite intercepts, showing that both the undissociated thiol (RSH) and thiolate ion (RS−) are reactive species. Generation of RS· from RSH proceeds via a separated-concerted proton–electron transfer mechanism. The reaction of thiolate ion is a simple outer-sphere electron transfer reaction. By applying the Marcus theory, the self-exchange rate constants for the couples HOCH2CH2S·/HOCH2CH2S− and H3N+CH2CH2S·/H3N+CH2CH2S− were evaluated as 3 × 109 and 2.2 × 108 M−1 s−1, respectively, at 25 °C.

Donor property of cobalt(II) Schiff base complexes toward triphenyltin(IV)-chloride: A kinetic study

In this study, some cobalt(II)tetraaza Schiff base complexes were used as donors in coordinating to triphenyltin(IV)chloride as acceptors; the kinetics and mechanism of the adduct formation were studied spectrophotometrically. Co(II)tetraaza Schiff base complexes used were [Co(amaen)][N,N′-ethylene-bis-(o-amino-α-methylbenzylideneiminato)cobalt(II)] (1), [Co(appn)] [N,N′-1,2-propylene-bis-(o-amino-α-phenylbenzylideneiminato)cobalt(II)] (2), [Co(ampen)] [N,N′-ethylene-bis-(o-amino-α-phenylbenzylideneiminato)cobalt-(II)] (3), [Co(cappn)][N,N′-1,2-proylene-bis-(5-chloro-o-amino-α-phenylbenzylideneiminato)cobalt(II)] (4), and [Co(campen)] [N,N′-ethylene-bis-(5-chloro-o-amino-α-phenylbenzylid-eneiminato)cobalt(II)] (5). The reactivity trend of the complexes in interaction with triphenyltin(IV)chloride was Co(amaen) > Co(appn) > Co(ampen) > Co(cappn) > Co(campen). The linear plots of kobs versus the molar concentration of the triphenyltin(IV)chloride, a high span of the second-order rate constant k2 values, and large negative values of ΔS≠ and low ΔH≠ values suggest an associative (A) mechanism for the acceptor–donor adduct formation. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 635–640, 2012

Phenolysis and aminolysis of 4‐nitrophenyl and 2,4‐dinitrophenyl S‐methyl thiocarbonates in aqueous ethanol

AbstractThe reactions of S‐methyl O‐(4‐nitrophenyl) thiocarbonate (1) and S‐methyl O‐(2,4‐dinitrophenyl) thiocarbonate (2) with a series of secondary alicyclic (SA) amines and phenols are subjected to a kinetic investigation. Under nucleophile excess, pseudo‐first‐order rate coefficients (kobs) are obtained. Plots of kobs against the free nucleophile concentration at constant pH are linear with slopes kN. The Brønsted plots (log kN vs. nucleophile pKa) for the reactions are linear with slope (β) values in the 0.5–0.7 range, in accordance with concerted mechanisms. Comparison of the SA aminolysis of 1 with the same one carried out in water shows that the change of solvent from water to aqueous ethanol destabilizes the zwitterionic tetrahedral intermediate, changing the mechanism from stepwise to concerted. This destabilization is greater than that due to the change from SA amines to quinuclidines. For the phenolysis reactions, the kN values in aqueous ethanol are smaller than those for the same reactions in water. Considering that the nucleophile is an anion, this result is unexpected because the anion should be more stabilized in the more polar solvent. This result is explained by the facts that the phenoxide reactant has a negative charge that is delocalized in the aromatic ring and the transition state is highly polar. © 2011 Wiley Peiodicals, Inc. Int J Chem Kinet 43: 353–358, 2011

Synthesis, Characterization, and Kinetic Study of Complexation of Vanadyl Acetylacetone with Some Unsymmetrical N3O Schiff Bases

This article reports the synthesis and characterization of some new vanadyl Schiff base complexes, ((N-salicylideneN′-pyrrolidene)-1,2-ethylenediaminato)oxovanadium(IV) [VO (Salpyren)], ((7-methyl-N-salicylidene-N′-pyrrolidene)-1,2-ethylenediaminato) oxovanadium(IV) [VO(Mesalpyren)], ((7-phenylN-salicylidene-N′-pyrrolidene)-1,2-ethylenediaminato) oxovanadium(IV) [VO(Phsalpyren)], ((N-salicylidene-N′-pyrrolidene)-1,3propylenediaminato)oxovanadium(IV) [VO(Salpyrpd)], ((7methyl-N-salicylidene-N′-pyrrolidene)-1,3-propylenediaminato) oxovanadium(IV) [VO(Mesalpyrpd)], ((7-phenyl-N-salicylidene-N′-pyrrolidene)-1,3-propylenediaminato)oxovanadium(IV) [VO (Phsalpyrpd)]. These complexes were characterized by elemental analyses, IR, UV-Vis, and mass spectra. The kinetics and mechanism of the complex formation in methanol were studied spectrophotometrically. The second order k2 rate constants show the following trend: Mesalpyren > Salpyren > Phsalpyren > Mesalpyrpd > Salpyrpd > Phsalpyrpd The linear plots of kobs vs. the molar concentration of VO(acac)2, the large negative values of Δ S ≠ and the low values of Δ H ≠ suggest an associative (A) mechanism.

Mechanism of the oxidation of hydrazoic acid by tetrachloroaurate(III) ion

The oxidation of hydrazoic acid in perchloric acid in the absence of added chloride under pseudo first-order conditions ([HN3] » [AuCl4−]) is first order in [Au(III)]. Michaelis–Menten type of dependence (linear plots of kobs−1 vs [HN3]−1) is observed with respect to [HN3]. The kobs is independent of ionic strength and the plot between kobs−1 and [H+] is linear. The inner-sphere mechanism is consistent with the formation of an axial complex (K = 25 dm3 mol−1) between AuCl3(HO)− ion and HN3 prior to its rate determining decomposition (k = 0.0182 s−1). It is inferred that the free radicals N3• do not oxidise Au(II). The reaction becomes outer-sphere in the presence of added Cl− ions which are inferred to form a cage around the hydronium ion surrounding the AuCl4− ions. The penetration of N3− through the cage is rate controlling and within the cage, the electron transfer from N3− ion to AuCl4− is fast. The value of the rate determining constant k2 is 0.547 dm3 mol−1 s−1 and the equilibrium constant KCl for the cage formation is 5 dm3 mol−1 at 25 °C. It is calculated that the minimum HN3 concentration required before the reaction exhibits zero-order dependence in HN3 is 0.31 mol dm−3 when [H+] = 0.18 mol dm−3 at 25 °C.

Aminolysis and pyridinolysis of O‐aryl S‐(4‐nitrophenyl) thiocarbonates in aqueous ethanol. Kinetics and mechanism

AbstractThe reactions of O‐phenyl S‐(4‐nitrophenyl) thiocarbonate (PNPTC) and O‐(4‐chlorophenyl) S‐(4‐nitrophenyl) thiocarbonate (ClPNPTC) with a series of secondary alicyclic (SA) amines and pyridines are subjected to a kinetic investigation in 44 wt% ethanol–water, at 25.0°C and an ionic strength of 0.2 M. The reactions are followed spectrophotometrically at 420 nm (4‐nitrobenzenethiolate anion appearance). Under amine excess, pseudo‐first‐order rate coefficients (kobs) are found. For all these reactions, plots of kobs vs free amine concentration at constant pH are linear, the slope (kN) being independent of pH. The Brønsted‐type plots (log kN vs pKa of the conjugate acid of the amines) for the reactions of the series of SA amines with PNPTC and ClPNPTC are linear with slopes 0.59 and 0.54, respectively. The values of these slopes are in accordance with a concerted mechanism. The Brønsted‐type plots for the pyridinolysis reactions are biphasic, suggesting a stepwise mechanism with a change in the rate‐determining step, from breakdown to formation of a tetrahedral intermediate, as the basicity of the pyridines increases. Copyright © 2008 John Wiley & Sons, Ltd.

Electron transfer reactions (ETR) of tert‐butyl perbenzoates with dimethyl sulfide: the rates controlled by translational entropy

AbstractVarious tert‐butyl perbenzoates were prepared according to the known methods. The mixtures of tert‐butyl perbenzoate and dimethyl sulfide underwent thermolysis at several temperatures (T °C: 80, 90, 100, and 110). The rates of consumption of the peresters (kobs) were obtained from ln C0/Ct = kobs × t where C0 and Ct are the concentration of the perester at time 0 and t, respectively. For example: The decrease of the perester satisfied pseudo first order kinetics, and kobswere determined. kHom. and kET were obtained from kobs = kHom. + kET[CH3SCH3] where [CH3SCH3]/[perester] > 10. kET and its relative rates are produced from plot of kobs against [CH3SCH3]. Hammett correlation are plotted against σ and σ+ in which σ is better correlated than σ+. Hammett ρ could be considered selectivity which stays constant irrespective of temperature. The phenomenon is not consistent with reactivity/selectivity principle. The differential activation entropies linearly increase with substituent constant σ to give slope of 2.63 while the similar slope for differential enthalpies shows much lower value of 0.892. The dominance of differential entropic terms clearly indicates the entropy control of the rates. Copyright © 2006 John Wiley & Sons, Ltd.

Kinetics and mechanism of the reactions of S‐4‐nitrophenyl 4‐methylthiobenzoate with secondary alicyclic amines and pyridines

AbstractThe reactions of the title substrate with a series of six secondary alicyclic amines and a series of eight pyridines are subjected to a kinetic investigation in 44 wt% ethanol‐water, at 25.0°C, and an ionic strength of 0.2 mol · dm−3. Under amine excess pseudo first‐order rate coefficients (kobs) are obtained. Plots of kobs against free amine concentration at constant pH are linear, with the slope (kN) independent of pH. The Brønsted‐type plots (log kN against pKa of the conjugate acids of the amines) are non‐linear, with the curvature center located at pKa (pK) 9.7 and 9.4, for the reactions of secondary alicyclic amines and pyridines, respectively. The plots are consistent with a zwitterionic tetrahedral intermediate (T±) on the reaction path and a change in rate‐determining step. The greater pK value for secondary alicyclic amines than pyridines is explained by the greater nucleofugality from T± of secondary alicyclic amines compared to isobasic pyridines. These pK values are lower than those found for the reactions of S‐4‐nitrophenyl 4‐Y‐substituted thiolbenzoates (Y = H, Cl, NO2) with the corresponding amine series. These results indicate that electron donation from the non‐leaving group in T± favors leaving group expulsion from T± relative to amine expulsion. Copyright © 2006 John Wiley & Sons, Ltd.

The Reaction of (Bipyridyl)palladium(II) Complexes with Thiourea − Influence of DNA and Other Polyanions on the Rate of Reaction

Abstract[Pd(bipy)(py)2](PF6)2 reacts stepwise with excess thiourea to give [Pd(tu)4](PF6)2. The kinetics of the second step, which refers to the replacement of bipyridyl in [Pd(bipy)(tu)2](PF6)2, have been studied in water and in the presence of calf thymus DNA, sodium polyriboadenylate, sodium polyvinylsulfonate or sodium polymetaphosphate at 25 °C and pH = 7 and a fixed sodium chloride concentration. The reaction follows a first order course and a plot of kobs against [thiourea]2 affords a straight line with a small intercept. DNA inhibits the process without altering the rate law. The kobs values decrease systematically on increasing the DNA concentration eventually tending to a limiting value. The values are larger at higher ionic strengths and the other polyanions show similar behaviour. The influence of DNA on the kinetics can be related to steric inhibition caused by noncovalent binding with the complex. Upon interaction with DNA, [Pd(bipy)(tu)2]2+ gives rise to immediate spectroscopic changes in the UV/Vis region as well as induced circular dichroism suggesting that the complex, like similar platinum(II) and palladium(II) species of bipyridyl, intercalates with the double helix. Such a type of interaction hampers the attack of the nucleophile at the metal centre inhibiting the reaction. The decrease in the rate of ligand substitution upon decreasing salt concentration but at a given DNA concentration is due to the influence of ionic strength on the complex−DNA interaction. The reactivity inhibition by single‐stranded poly(A), polyvinylsulfonate or polymetaphosphate can be accounted for in terms of self‐aggregation of the complex induced by the polyanion. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Study of the kinetics and activation parameters of reduction of Mn(III) to Mn(II) by SO3 2− ion in (MnSiW11O40H2)5- heteropoly ion

In order to gain an insight into the mechanism of reduction of Mn(III) heteropoly ions, and also to establish the conditions for use of some of these ions as oxidizing agent, following measurements have been made. The pseudo-first order rate constants, kobs, have been determined and specific rate constants, k, were calculated from the plots of kobs against SO3 2− concentrations. A plot of ln(k2/T) against inverse temperature gives enthalpy of activation as 10.67 kJ mol−1 and entropy of activation as −237.90 J K−1 mol−1. Effects of ionic strength and pH have also been studied over a limited range.

Isomerization of the uncomplexed actinidin molecule: kinetic accessibility of additional steps in enzyme catalysis provided by solvent perturbation

The effects of increasing the content of the aprotic dipolar organic co-solvent acetonitrile on the observed first-order rate constant (k(obs)) of the pre-steady state acylation phases of the hydrolysis of N-acetyl-Phe-Gly methyl thionester catalysed by the cysteine proteinase variants actinidin and papain in sodium acetate buffer, pH 5.3, were investigated by stopped-flow spectral analysis. With low acetonitrile content, plots of k(obs) against [S]0 for the actinidin reaction are linear with an ordinate intercept of magnitude consistent with a five-step mechanism involving a post-acylation conformational change. Increasing the acetonitrile content results in marked deviations of the plots from linearity with a rate minimum around [S]0=150 microM. The unusual negative dependence of k(obs) on [S]0 in the range 25-150 microM is characteristic of a rate-determining isomerization of the free enzyme before substrate binding, additional to the five-step mechanism. There was no evidence for this phenomenon nor for the post-acylation conformational change in the analogous reaction with papain. For this enzyme, however, acetonitrile acts as an inhibitor with approximately uncompetitive characteristics. Possible mechanistic consequences of the differential solvent-perturbed kinetics are indicated. The free enzyme isomerization of actinidin may provide an explanation for the marked difference in sensitivity between this enzyme and papain of binding site-catalytic site signalling in reactions of substrate-derived 2-pyridyl disulphide reactivity probes.

Kinetics and mechanism of water substitution in the low-spin Fe(II) complex of 4-octasulfophenylpyrazinoporphyrazine.

The substitution reaction of the axial-coordinated water by pyridine, pyrazine and 4-CN-pyridine in the low-spin Fe(II) complex of octasulfophenyltetrapyrazinoporphyrazine was studied. Kinetic and thermodynamic parameters for the different reaction steps of the process were determined. On the basis of NMR data and spectrophotometric titrations, a pronounced non-equivalence of the two coordinated N-donor ligands was observed. The substitution of water by pyridine and 4-CN-pyridine is shown to include the formation of a precursor outer-sphere complex, whereas substitution by pyrazine follows a limiting dissociative mechanism.

Effect of Amine Nature on Rates and Mechanism: Pyridinolyses of 4-Nitrophenyl Benzoate

Pseudo-first-order rate constants (<TEX>$k_{obs}$</TEX>) have been measured spectrophotometrically for the reactions of 4-nitrophenyl benzoate with a series of pyridines in <TEX>$H_2O$</TEX> containing 20 mol % DMSO. The plots of kobs vs pyridine concentration are linear up to ca. 0.1 M pyridines, indicating that the effect of self-association of pyridines with their conjugate acids are insignificant in this concentration range. The Bronsted-type plot has been obtained to be linear with a <TEX>${\beta}_{nuc}$</TEX> value 1.11, suggesting that the pyridinolyses proceed through a rate-determining breakdown of the zwiterionic addition intermediate. The pyridines studied have shown higher reactivity than isobasic primary and secondary amines toward the substrate. 1-Benzoyl-4-dimethylaminopyridinium ion, a possible intermediate, has not been detected since the rate of its hydrolysis in the reaction condition is comparable with or even faster than its formation.

The effect of medium on rate and mechanism: aminolysis of O-4-nitrophenyl thionobenzoate in MeCN and H2O.

Pseudo-first-order rate constants (kobs) have been measured spectrophotometrically for reactions of O-4-nitrophenyl thionobenzoate (1) with a series of alicyclic secondary amines in MeCN and H2O at 25.0 +/- 0.1 degrees C. The plot of kobs vs amine concentration exhibits an upward curvature in all cases, indicating that the reactions proceed through two tetrahedral intermediates (a zwitterionic T(+/-) and its deprotonated anionic T-) regardless of the amine basicity and the nature of the reaction medium. However, all the amines investigated have been found to be much less reactive in MeCN than in H2O, although the amines are more basic in the former medium by 7-9 pKa units.

Europium Ion Catalyzed Methanolysis of Esters at Neutral pH and Ambient Temperature. Catalytic Involvement of Eu3+(CH3O-)(CH3OH)x

The Eu(3+)-promoted methanolysis of three esters, p-nitrophenyl acetate (1), phenyl acetate (2), and ethyl acetate (3) is reported, as well as the potentiometric titration of Eu(3+) in MeOH at various [Eu(SO(3)CF(3))(3)] (SO(3)CF(3) = OTf). The titration data are analyzed in terms of two ionizations corresponding to macroscopic and values, which are respectively defined as the values at which the [CH(3)O(-)]/[Eu(3+)] = 0.5 and 1.5. As a function of increasing [Eu(OTf)(3)], increases slightly due to a proposed Eu(3+)/(-)OTf ion pairing effect, which tends to reduce the acidity of the metal-coordinated CH(3)OH, while decreases due to the formation of Eu(3+) dimers and oligomers which stabilize the (Eu(3+)(CH(3)O(-))(2))(n)forms through bridging of the methoxides between two or more metal ions. For ester 1, a detailed kinetic analysis of the reaction rates as a function of both [Eu(OTf)(3)] and in buffered methanol reveals that the /second-order rate constant (k(2)) plot for the catalyzed reaction follows a bell-shaped profile, suggesting that the active form is a Eu(3+)(CH(3)O(-)) monomer with a kinetic of 6.33 +/- 0.06 for formation and a of 8.02 +/- 0.10 for its conversion into the inactive (Eu(3+)(CH(3)O(-))(2))(n)oligomeric form. At higher values, plots of k(obs) vs [Eu(OTf)(3)] are linear at low metal concentration and plateau at higher metal concentration due to the formation of inactive higher order aggregates. The Eu(3+)(CH(3)O(-)) catalysis of the methanolysis of esters 1, 2, and 3 is substantial. Solutions of 10(-2) M of the catalyst at 7.12 accelerate the reaction relative to the methoxide reaction at that by 8 530 000-, 195 000 000- and 7 813 000-fold, respectively.

Effect of amine nature on reaction mechanism: aminolyses of o-4-nitrophenyl thionobenzoate with primary and secondary amines.

Pseudo-first-order rate constants (k(obs)) have been measured spectrophotometrically for reactions of O-4-nitrophenyl thionobenzoate (2) with a series of primary and acyclic secondary amines. The plots of k(obs) vs amine concentration are linear for the reaction of 2 with primary amines. The slope of the Brønsted-type plot for the reaction of 2 with primary amines decreases from 0.77 to 0.17 as the amine basicity increases, indicating that the reaction proceeds through a zwitterionic addition intermediate in which the rate-determining step changes from the breakdown of the intermediate to the reaction products to the formation of the intermediate as the amine basicity increases. On the other hand, for reactions with all the acyclic secondary amines studied, the plot of k(obs) vs amine concentration exhibits an upward curvature, suggesting that the reaction proceeds through two intermediates, e.g., a zwitterionic addition intermediate and an anionic intermediate. The microscopic rate constants (k(1), k(-)(1), k(2), and k(3) where available) have been determined for the reactions of 2 with all the primary and secondary amines studied. The k(1) value is larger for the reaction with the primary amine than for the reaction with the isobasic acyclic secondary amines, while the k(-)(1) value is much larger for the latter reaction than for the former reaction. The k(3) value for the reaction with secondary amine is independent of the amine basicity. The small k(2)/k(-)(1) ratio is proposed to be responsible for the deprotonation process observed in aminolyses of carbonyl or thiocarbonyl derivatives.

Kinetics and mechanism of the aminolysis of 4‐nitrophenyl and 2,4‐dinitrophenyl 4‐methylphenyl carbonates in aqueous ethanol

AbstractThe reactions of 4‐methylphenyl 4‐nitrophenyl carbonate (MPNPC) and 4‐methylphenyl 2,4‐dinitrophenyl carbonate (MPDNPC) with a series of secondary alicyclic amines are subjected to a kinetic investigation in 44 wt% ethanol–water, at 25.0°C, ionic strength 0.2 M (KCl). Under amine excess over the substrate, pseudo‐first‐order rate coefficients (kobs) are obtained. Plots of kobs against [amine] are linear, with kN as slopes. A biphasic Brönsted‐type plot for kN is obtained for the aminolysis of MPNPC, with slopes β1 = 0.2 (high pKa) and β2 = 0.9 (low pKa). This is in accordance with a stepwise mechanism, through a zwitterionic tetrahedral intermediate (T±), and a change in the rate‐determining step, from formation to breakdown of T± as the amine pKa decreases. For the aminolysis of MPDNPC, a slightly curved Brönsted‐type plot for kN is obtained, with β1 = 0.1 (low pKa) and β2 = 0.55 (high pKa). This is consistent with a concerted mechanism. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 309–315, 2002