Rate Law Kobs Research Articles

The Synthesis, Separation and Structures of Three [Co(cyclen)((S)-AlaO)]2+ Isomers. The Alkaline Hydrolysis of [Co(cyclen)((S)-AlaO)]2+

The reaction of [Co(cyclen)(OH2)OH]2+ (cyclen = 1,4,7,10-tetraazacyclododecane) with (S)-alanine at pH 7·2 gives a mixture of three [Co(cyclen)((S)-AlaO)]2+ isomers (1)–(3). These have been isolated by using both cation ion-exchange chromatography (Dowex 50 W×2, HCl eluent) and reversed phase ion-pair chromatography (C18, p-toluenephosphate in MeOH/H2O eluent). By using a combination of 1H n.m.r. techniques (n.O.e. and COSY) for solutions in (CD3)2SO the syn(N),anti(O) (1), syn(O), anti(N) (2) and syn(N), syn(O) (3) configurations have been assigned to these isomers. These have been confirmed by single-crystal X-ray analysis: [Co(cyclen)((S)-AlaO)] I2.H2O, isomer (1), P43212, a = b = 8·55150(10), c 51·8693(11) Å, Z 8, R 0·0343; [Co(cyclen)((S)-AlaO)] (ClO4)2.H2O, isomer (2), P212121, a 8·499(3), b 14·538(5), c 16·592(4) Å, Z 4, R 0·0388; [Co(cyclen)((S/R)-AlaO)] ZnBr4, a 1 : 1 mixture containing both (S)-alanine (isomer (3)) and (R)-alanine, P21/c, a 7·618(2), b 13·806(4), c 19·094(7) Å, Z 4, R 0·0726. In alkaline solution (0·1–1·0 M NaOH, 25·0°C, I = 1·0 M (NaClO4)), equilibration between (1), (2) and (3) is faster than hydrolysis to give cis-[Co(cyclen)(OH)2]++(S)-AlaO-. Time zero spectroscopic observation (300 nm) allowed the equilibrium constant, K, for the reaction [Co(cyclen)((S)-AlaO)]2+ + OH- ↔ [Co(cyclen – H)((S)-AlaO)]+ +H2O to be determined as 1·05 M-1 at 25·0°C and I = 1·0 M. The hydrolysis reaction follows the rate law kobs = kKK1K2[OH-]2/(1+K[OH-]+KK1K2[OH-]2) with k = 1·0 s-1 corresponding to rate-determining loss of (S)-AlaO- from the ring-opened complex, [Co(cyclen – H)((S)-AlaO)OH].

Aminolysis of sulfamate esters in chloroform—nonlinear kinetics

Aminolysis in chloroform of sulfamate esters RNHSO2OC6H4NO2-4 principally with a series of imidazoles gives good kobs first-order rate constants under pseudo-first-order conditions. Plots of kobsvs. [amine] display downward curvature leading to saturation and these plots can be fitted to the rate law kobs = Kmk1[amine]/{1 + Km[amine]} where Km and k1 are defined by eqn. (i). The Hammett ρ value for the formation of the complex [S·Amine] is +1.64. Using data derived for the rate-determining step, the breakup of the intermediate is shown to be unimolecular with a ρacyl (substituents in the R part of S) of –1.78 consistent with an E1cB process. Steric effects of the bound-amine in the complex are negligible.

Kinetics and mechanism of reactions of bis(biguanide)copper(II) ion with glycine and ?-alanine in aqueous media

The kinetics of the reaction of [Cu(bigH)2]2+ (bigH = biguanide) with an excess of amino acid (LH), namely glycine or α-alanine, in aqueous solution in the 7.6–9.0 pH range at different temperatures (30–40° C) have been followed by stopped-flow spectrophotometry. The ligand replacement process has been found to pass through intermediate formation of a ternary complex, [Cu(bigH)L]+ at the slower step, followed by rapid transformation into the binary complex, [CuL2]. The overall ligand replacement process has a ligand dependent (k1) path which is first order with respect to the incoming ligand (L-), and a ligand independent (k0) path. Under pseudo-first order conditions containing excess amino acid, the experimental observations conform to the rate law kobs = k0 + k1Ka[L]T/([H=] + Ka), where [L]T stands for the total concentration of amino acid and Ka gives the deprotonation constant of LH. The solvent assisted dissociation (i.e. k0 path) leads to a copper(II) mono-biguanide complex followed by rapid nucleophilic substitution; the k1 path is in agreement with an associative mechanism. The activation parameters (ΔH≠, ΔS≠) for each step have been determined.

Nucleophilic Catalysis by HPO42- in the Hydrolysis of Cr2O72-. Formation and Decay of HO3POCrO32-

The initial reaction of Cr2O72- in phosphate buffer (pH 6.03-8.54, 25.0°C, I = 1.0 M NaClO4) follows the rate law kobs = kK [HPO42]/(1+K[HPO42-]). This is interpreted as arising from the reversible and rapid formation of a chromium(VI)- phosphato intermediate of increased coordination number (K = 5.5�1.3M-1), and rate-determining loss of CrO42- from this species (k = 4.4�0.5 s-1) to give HO3POCrO32- ( pKc a= 6.96). This appears to be the first clear demonstration of an addition-elimination (stepwise) mechanism for substitution at chromium (VI). Subsequent equilibration of HO3POCrO32- to give HCrO4- and H2PO4- (K = 5.95�1.90 M-1) is seen as a separate process which is subject to specific H+ and OH-, and general base (HPO42-) catalysis, in addition to a spontaneous reaction.

ChemInform Abstract: Substitution Reactions on Sterically Hindered Square‐planar trans‐(NiBr(R)(PR′3)2) (R: aryl) Complexes. Effects of the Substituents of the Aryl Ligand.

Substitution reactions of sterically hindered trans-[NiBr(R)(PPh3)2] complexes (R = C6Cl5, C6F5,C6H2Me3-2,4,6, C6HCl4-2,3,5,6, C6HCl4-2,3,4,6, C6HCl4-2,3,4,5, C6H2Cl3-2,3,4, C6H3Cl2-2,4, or C6H4Cl-2) and of trans-[NiBr(R)(PR′3)2](PR′3= PEtPh2 or PMePh2; R = C6Cl5 C6F5, C6H2Me3-2,4,6, or C6H2Cl3-2,3,4) have been studied in acetone solution. The reaction rates obey the well established two-term rate law kobs=kS+kan[anion] and seem to be solely controlled by steric factors; namely, the existence of one or two substituents on the R ligand, in an ortho position with respect to the Ni–C bond. The different donor ability of the ligand might be considered only for R = C6H2Me3-2,4,6. The effects of the para and meta chloro substituents of the R ligand on the reaction rates are negligible. Steric strain in the molecule, as a whole, is also considered by studying the effect of a decrease in the phosphine size on the reaction rate.

Substitution reactions on sterically hindered square-planar trans-[NiBr(R)(PR′3)2](R = aryl) complexes. Effects of the substituents of the aryl ligand

Substitution reactions of sterically hindered trans-[NiBr(R)(PPh3)2] complexes (R = C6Cl5, C6F5,C6H2Me3-2,4,6, C6HCl4-2,3,5,6, C6HCl4-2,3,4,6, C6HCl4-2,3,4,5, C6H2Cl3-2,3,4, C6H3Cl2-2,4, or C6H4Cl-2) and of trans-[NiBr(R)(PR′3)2](PR′3= PEtPh2 or PMePh2; R = C6Cl5 C6F5, C6H2Me3-2,4,6, or C6H2Cl3-2,3,4) have been studied in acetone solution. The reaction rates obey the well established two-term rate law kobs=kS+kan[anion] and seem to be solely controlled by steric factors; namely, the existence of one or two substituents on the R ligand, in an ortho position with respect to the Ni–C bond. The different donor ability of the ligand might be considered only for R = C6H2Me3-2,4,6. The effects of the para and meta chloro substituents of the R ligand on the reaction rates are negligible. Steric strain in the molecule, as a whole, is also considered by studying the effect of a decrease in the phosphine size on the reaction rate.

ChemInform Abstract: Kinetics of the Displacement of Cyclobutane‐1,1‐dicarboxylate from Diammine(cyclobutane‐1,1‐dicarboxylato)platinum(II) in Aqueous Solution.

The displacement of 1,1-cyclobutanedicarboxylate (cbdca2–) from [Pt(NH3)2(cbdca)] has been studied in aqueous solution. In the presence of acid the process resembles the successive displacement of two monodentate carboxylates. The first (ring-opening) stage follows the rate law kobs.=(k0+k1k0[H+])(1 +k0[H+])–1, k0= 8 × 10–5 s–1, K0= 0.6 dm3 mol–1, k1= 8.0 × 10–4 s–1 at 25 °C, while the second follows the simple relationship kobs.=k[H+], k= 1.61 × 10–4 dm3 mol–1 s–1 at 25 °C. In the absence of acid and other nucleophiles the complex is inert and in the presence of chloride the displacement of ligand follows a first-order dependence on [Cl–], kobs.=kCl[Cl–]. At 80 °C, kCl= 1.32 × 10–4 dm3 mol–1 s–1. The chelate differs from the bis-monodentate carboxylate species in the great importance of the reverse, ring-closing process, which can be prevented in the presence of acid.

Kinetics of the displacement of cyclobutane-1,1-dicarboxylate from diammine(cyclobutane-1,1-dicarboxylato)platinum(II) in aqueous solution

The displacement of 1,1-cyclobutanedicarboxylate (cbdca2–) from [Pt(NH3)2(cbdca)] has been studied in aqueous solution. In the presence of acid the process resembles the successive displacement of two monodentate carboxylates. The first (ring-opening) stage follows the rate law kobs.=(k0+k1k0[H+])(1 +k0[H+])–1, k0= 8 × 10–5 s–1, K0= 0.6 dm3 mol–1, k1= 8.0 × 10–4 s–1 at 25 °C, while the second follows the simple relationship kobs.=k[H+], k= 1.61 × 10–4 dm3 mol–1 s–1 at 25 °C. In the absence of acid and other nucleophiles the complex is inert and in the presence of chloride the displacement of ligand follows a first-order dependence on [Cl–], kobs.=kCl[Cl–]. At 80 °C, kCl= 1.32 × 10–4 dm3 mol–1 s–1. The chelate differs from the bis-monodentate carboxylate species in the great importance of the reverse, ring-closing process, which can be prevented in the presence of acid.

Kinetics of nucleophilic attack on co-ordinated organic moieties. Part 25. A nucleophilic order for attack upon the [Fe(CO)3(1–5-η-C7H9)]+cation

Kinetic studies of the addition of a range of phosphine and phosphite nucleophiles (PR3) to the cation [Fe(CO)3(1–5-η-C7H9)]+ to give [Fe(CO)3(1–4-η-R3P·C7H9)]+ adducts reveal the general rate law kobs=k1(PR3]. Combination of these results with analogous data for amine and anionic nucleophiles provides the first comprehensive nucleophilicity order for attack on this cycloheptadienyl substrate. For the neutral phosphorus and nitrogen nucleophiles, this nucleophilicity order quantitatively parallels that found for the related but much more reactive [Fe(CO)3(1–5-η-C6H7)]+ substrate, indicating departure from the reactivity-selectivity principle. However, with N3– the C7H9 cation reveals exceptional reactivity, suggesting a change in mechanism for this anionic nucleophile.

Formulation of vaccine adjuvant muramyldipeptides (MDP). 2. The thermal reactivity and pH of maximum stability of MDP compounds in aqueous solution.

The degradation of muramyldipeptides (MDPs) in aqueous solution obeys the rate law kobs = kH+aH+ + ko + kHO-aHO- and the Arrhenius equation. For example, the rate constants for degradation of N-acetylmuramyl-L-threonyl-D-isoglutamine, 3, at 25 degrees C are kH+ = 2.3 X 10(-6) M-1 sec-1, ko = 8.2 X 10(-10) sec-1, and kHO- = 0.19 M-1 sec-1. The degradation rates are dependent on the side-chain substituents; it is predicted that sterically hindered MDP compounds will show an extended shelf life in aqueous solution. Product studies in the weakly acid pH region (where the pH of maximum stability occurs) show that MDP compounds degrade largely by hydrolysis of the dipeptide side chain. These data show that MDP 3 exhibits a shelf life (t90) of greater than 2 years in aqueous solutions of pH 4-4.5, the pH of maximum stability.

Malonylcoenzyme A models. Part 2. The methylene deprotonation step of the E1cB acyl transfer of malonic acid thiolmonoesters

The deprotonation of the α-methylene site in S-aryl hydrogenthiomalonates (HO2CCH2COSR) was general-base catalysed. Nitrogen bases fit a single Bronsted correlation for this deprotonation step with slope 0.59 for primary, secondary, and tertiary amines. In general, amines followed saturation kinetics, but hydroxylamine followed the rate law kobs=k1[NH2OH]+k2[NH2OH]2; nevertheless, the k1 parameter fitted the Bronsted correlation for nitrogen bases. Oxygen-based buffers fitted an independent Bronsted correlation with β 0.42 and were slightly less reactive than nitrogen bases for a given catalyst pKa. Saturation kinetics were observed for thiolysis of S-4-chlorophenyl hydrogenthiomalonate, implying a ketenoid (E1cB) pathway for malonyl transfer. The primary isotope effect (H/D) for N-methylmorpholine catalysed deprotonation (dedeuteriation) of HO2C. CH2COSPh and DO2C.CD2COSPh was ca. 3.6. The second-order rate constant for ethylamine attack on S-4-chlorophenyl hydrogen-(2,2-dimethyl)thiomalonate was 600-fold smaller than that estimated at lower buffer concentrations for S-4-chlorophenyl hydrogenthiomalonate, in accord with a general-base catalysed deprotonation mechanism for the latter. The 2,2-dimethylmalonate ester showed a marked rate of attack of water on the monoanion probably reflecting intramolecular general-base catalysis by the –CO2– group. Second-order rate constants for hydroxide ion attack on S-phenyl and S-4-chlorophenyl hydrogenthiomalonates were measured.

Base hydrolysis of acidopentaaminecobalt(III) ions and the question of common intermediates. Hydrolysis of some t-[Co(tren)(NH3)X]2+/3+ ions (X = NO3-, MeSO3-, Cl-, Me2SO, N3-)

The preparations, and characterizations, of the complex cations t-[Co(tren)(NH3)X]2+/3+* (X = NO3-, Me2SO, H2O, Cl-, N3-) and p-[Co(tren)(NH3)X]2+/3+ (X = N3-, Cl-, H2O) are described. Base hydrolysis of t-[Co(tren)(NH3)X]2+/3+ (X = MeSO3-, Me2SO, NO3-, N3-) and of p- [Co(tren)(NH3)N3]2+ follows the rate law kobs = k[OH-] over the [OH-] range 1.0-10-4 mol dm-3 except for the 3+ ion t-[Co(tren)(NH3)Me2SO]3+ where some curvature is observed at [OH-] > 0.025 mol dm-3. The products for the t-[Co(tren)(NH3)X]2+/3+ ions (X = Cl-, NO3-, MeSO3-, Me2SO) depend on X, and on the presence or absence of supporting electrolyte (1.0 mol dm-3 NaClO4 or NaN3) varying from 12.1 % retention of the t-configuration for X = NO3- in the absence of electrolyte to c. 26% for X = Me2SO in 1.0 mol dm-3 NaN3. Significantly more retention is observed in the azido products (75-80% t-[Co(tren)(NH3)N3]2+). The results are interpreted in terms of important contributions to the mechanism from pre-formed ion pairs.

Oxygen exchange and glycinate ring opening in [Co(en)2(gly)]2+

Over the pH range 0-2 chelated glycine in [Co(en)2(gly)2+ exchanges one of its oxygens according to the rate law kobs = k(1),[H+] with k(1), (1.0+0.1) × 10-5 mol-1 dm3 s-1. The second oxygen exchanges much more slowly with k(2) c. 7 × 10-8 s-1 in 0.1 mol dm-3 acid. In alkaline solution, [OH-] being 1.0-10-3 mol dm-3, the two oxygen atoms exchange according to the expression kobs = k(1),(2)[OH-] with k(1) 3.0 × 10-3mol-1 dm3 s-1 and k(2) c. 1.7 × 10-6 mol-1 dm3 s-1 both at 25.0°C and ionic strength, μ 1.0 (NaClO4). The latter process occurs with C-O bond fission to form cis-[Co(en)2(gly)(OH)]+, with kd 1.8 × 10-5 mol-1 dm3 s-1, which subsequently undergoes a more rapid OH--catalysed loss of glycine, with k'd, 6 × 10-4 mol-1 dm3 s-1, to form cobalt(III) and cobalt(II) products.

On the mechanism of the Hg2+ and base induced hydrolysis reactions of the β2‐(RR, SS)‐Co (trien) (glyOR)Cl2+ions (R = H, C2H5), evidence for the site of deprotonation in the reactive conjugate base

AbstractAcid hydrolysis of the ester function in Δ‐(−)589‐β2‐(RR)‐[Co (trien) (glyOEt) Cl]2+ ((−)‐1) produces optically pure Δ‐(−)589‐(RR)‐[Co (trien) (glyOH)Cl]2+ ((−)‐4). Hg2+ induced removal of chloride in (−)‐4 follows the rate law kobs = kHg [Hg2+] with kHg = (1.36 ± 0.03) × 10−2 M−1s−1, 25°, μ=1.0, and produces optically pure Δ‐(−)589‐β2‐(RR)‐[Co(trien) (glyO)]2+ ((−)‐2). Competition by NO occurs in this reaction ([NO], 1M, 3%) indicating a path whereby external nucleophiles (YNO, H2O) compete with the intramolecular carboxylate function for an intermediate of reduced coordination number. Rapid ring closure to 2 must ensue for Y  H2O. Base hydrolysis of chloride in (±)‐1 produces (±)‐2 together with its diastereoisomer β2‐(RS, SR)‐[Co(trien) (glyO)]2+, ((±)‐3), in which one secondary amine function has an inverted configuration. 2 and 3 incorporate 18O‐labelled solvent into the Co‐O position of the coordinated carboxylate moiety (2: 9.0%; 3: 12.3%) indicating that at least part of the product arises via intramolecular hydrolysis in β2‐hydroxo ethylglycinate intermediates (Fig. 4). Base hydrolysis of (−)‐4 follows the rate law Kobs = kOH[OH−] with kOH = (6.3 ± 0.6) × 10−4M−1 S−1, 25°, μ = 1.0 producing (−)‐2 (37‐45%) and (−)‐3 (63‐55%), the ratio being somewhat medium dependent. Competition by added N (1M) occurs using (±) ‐4 forming β2‐(RR, SS)‐[Co (trien) (glyO)N3]+ (∼2%) and β2‐(RS, SR)‐[Co (trien) (glyO)N3]+ (∼ 13%). Mutarotation at the secondary nitrogen centre is shown to occur after the rate determining loss of Cl− in 1 and 4 and before the formation of 2 and 3. It is concluded that this secondary nitrogen is the site of deprotonation in the reactive conjugate bases of 1 and 4, and possible mechanisms for the mutarotation process are considered.

The acid dependent dissociation of glycinatocopper(II) ion: Evidence for zwitterion reactivity

The dissociation of glycinatocopper(II) ion has been observed by stopped-flow spectrophotometry at 675 nm between pH 1.5 and 2.9 to follow the rate law kobs = k−1+kH[H+] with k−1 = 41±19 s−1 and kH = (7.6±1.1)×103M−1, s−1 at 25°C. The results indicate, Through application of the principle of microscopic reversibility, that the zwitterionic form of glycine cannot be ignored in the kinetics of the complex formation reaction.

Kinetics of Ligand-binding and Oxidation-Reduction Reactions of Cytochrome c from Horse Heart and Candida krusei

Abstract Reactions of cytochromes c from horse heart and Candida krusei (a yeast) were studied at 25° under pseudo-first order conditions by a flow technique. The rate law for the reduction of C. krusei by Cr(II) (pH 6.0, µ = 1.0 m) in chloride is kobs = a[Cr(II)]/(l + b[Cr(II)]), where a and b are 1.0 x 104 m-1 s-1 and 167 ± 38 m-1, respectively. The binding of imidazole (Im) to C. krusei conforms to the rate law kobs = kr + kf[Im] (kr = 1.65 ± 0.15 s-1, kf = 16.4 ± 0.3 m-1 s-1 at 695 nm, pH 7.1, µ = 1.00 m) which is consistent with the reaction C. krusei + Im (kf)/rlarr2;/(k↑) C. krusei-Im. Under the same conditions the equilibrium constant for formation of the C. krusei-imidazole complex determined from absorbance changes is 11.0 ± 0.6 m-1 which, within experimental error, is identical with the ratio kf/kr = 9.9 ± 0.9 m-1. For reduction of ferricytochrome c by pentaamminebenzimidazoleruthenium(II) (Ru(II)) and oxidation of ferrocytochrome c by ferricyanide (Fe(III)), the rate laws are of the form kobs = k[X], which is consistent with the simple process, cyt + X (k)/→ products. For C. krusei and horse heart, respectively, the rate constants (k) measured are as follows: X = Ru(II), k = (1.0 ± 0.3) x 106 m-1 s-1, 4.7 x 1O5 m-1 s-1 (pH 6.1, µ = 1.00 m); X = Fe(III), k = (2.1 ± 0.2) x 107 m-1 s-1, 1.2 x 107 m-1 s-1 (pH 7.2, µ = 0.10 m). The rate law found for the reduction of cyanoferricytochrome c by dithionite (pH 6.4, µ = 1.00 m) is kobs = k'[S2O42-]1/2, where k' = 9.2 m-1/2 s-1 (C. krusei) and 25.9 m-1/2 s-1 (horse heart). This rate law is consistent with a rate-determining reduction by SO2- formed in a rapidly established preequilibrium dissociation of S2O42- into SO2- radicals. The immediate products of the dithionite reduction of the cyanoderivatives are the cyanoferrocytochromes c. Their conversion to the native ferrocytochromes c, monitored by conventional techniques, was found to be a first order process (pH 6.4, µ = 1.0 m) with rate constants 6.8 x 10-3 s-1 (C. krusei) and 5.0 x 10-3 s-1 (horse heart). As might have been predicted from their structural similarities, the cytochromes from the two species exhibit no major reactivity differences.

The release of halide ions from some primary haloalkylamines

The kinetics for the release of halide ions from some haloalkylamines and the corresponding haloalkylammonium ions have been investigated in aqueous solutions under different conditions. In basic solutions, the free amines react according to a two-term rate law ����������������������� kobs = k1+k2[OH-] and their reactivities are in the order: 2-chloroethylamine ≈ 2-chloro- n-propylamine � 3-chloro-n-propylamine. The products formed are the corresponding iminoalkanes and the possible mechanisms are briefly discussed. In neutral solutions, the haloalkylammonium ions release halide ions by a considerably slower first-order reaction, which is retarded by acid. Since here there is not much difference in reactivity between 2-chloro-n-propylamine and 3-chloro-n-propylamine, the reaction probably involves a single-stage bimolecular attack by the solvent, leading to the formation of aminoalkanols. In both cases, the reactivities of 2-bromoethylamine me greater than those of the corresponding chloro compound.