Kinetics and mechanism of the aminolysis of O‐phenyl 4‐nitrophenyl dithiocarbonate in aqueous ethanol

The reactions of the title substrate (1) 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 (k_obs) are obtained. Plots of k_obs against [NH], where NH is the free amine, are nonlinear upwards, except the reactions of piperidine, which show linear plots. According to the kinetic results and the analysis of products, a reaction scheme is proposed with two tetrahedral intermediates, one zwitterionic (T^±) and another anionic (T⁻), with a kinetically significant proton transfer from T^± to an amine to yield T⁻ (k₃ step). By nonlinear least‐squares fitting of an equation derived from the scheme to the experimental points, the rate microcoefficients involved in the reactions are determined. Comparison of the kinetics of the title reactions with the linear k_obs vs. [NH] plots found in the same aminolysis of O‐ethyl 4‐nitrophenyl dithiocarbonate (2) in the same solvent shows that the rate coefficient for leaving group expulsion from T^± (k₂) is larger for 2 due to a stronger push by EtO than PhO. The k₃ value is the same for both reactions since both proton transfers are diffusion controlled. Comparison of the title reactions with the same aminolysis of phenyl 4‐nitrophenyl thionocarbonate (3) in water indicates that (i) the k₂ value is larger for the aminolysis of 1 due to the less basic nucleofuge involved and the small solvent effect on k₂, (ii) the k₃ value is smaller for the reactions of 1 due to the more viscous solvent, (iii) the rate coefficient for amine expulsion from T^± (k₋₁) is larger for the aminolysis of 1 than that of 3 due to a solvent effect, and (iv) the value of the rate coefficient for amine attack (k₁) is smaller for the aminolysis of 1 in aqueous ethanol, which can be explained by a predominant solvent effect relative to the electron‐withdrawing effect from the nucleofuge. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 839–845, 1999     

Anilinolysis of reactive aryl 2,4‐dinitrophenyl carbonates: Kinetics and mechanism

The reactions of a series of anilines with phenyl 2,4‐dinitrophenyl ( 1 ), 4‐nitrophenyl 2,4‐dinitrophenyl ( 2 ), and bis(2,4‐dinitrophenyl) ( 3 ) carbonates are subjected to a kinetic investigation in 44 wt% ethanol–water, at 25.0 ± 0.1°C and an ionic strength of 0.2 M. Under amine excess pseudo‐first‐order rate coefficients (k_obs) are obtained. Plots of k_obs against free amine concentration at constant pH are linear, with slopes k_N. The Brønsted plots (log k_N vs. anilinium pK_a) for the anilinolysis of 1 – 3 are linear, with slope (β) values of 0.52, 0.61, and 0.63, respectively. The values of these slopes and other considerations suggest that these reactions are ruled by a concerted mechanism. For these reactions, the k_N values follow the reactivity sequence: 3 > 2 > 1 . Namely, the reactivity increases as the number of nitro groups attached to the nonleaving group increases. Comparison of the reactions of this work with the stepwise pyridinolysis of carbonates 1 – 3 indicates that the zwitterionic tetrahedral intermediate (T^±) formed in the pyridinolysis reactions is destabilized by the change of its pyridino moiety by an isobasic anilino group. This is attributed to the superior leaving ability from the T^± intermediate of anilines, relative to isobasic pyridines, which destabilize kinetically this intermediate. The k_N values for the anilinolysis of carbonates 1 – 3 are similar to those found in the reactions of these carbonates with secondary alicyclic amines. With the kinetic data for the anilinolysis of the title substrates and 4‐methylphenyl and 4‐chlorophenyl 2,4‐dinitrophenyl carbonates, a multiparametric equation is derived for log k_N as a function of the pK_a of the conjugate acids of anilines and nonleaving groups. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 191–197, 2011     

Steric factors in the gas phase elimination kinetics of ethyl N‐benzyl‐N‐cyclopropylcarbamate and ethyl diphenylcarbamate

The elimination kinetics of ethyl N‐benzyl‐N‐cyclopropylcarbamate and ethyl diphenylcarbamate were investigated over the temperature range of 349.9–440.0°C and the pressure range of 31–106 Torr. These reactions have been found to be homogeneous, unimolecular, and obey a first‐order rate law. The products are ethylene, carbon monoxide, and the corresponding secondary amine. The rate coefficient is expressed by the following Arrhenius equations: For ethyl N‐benzyl‐N‐cyclopropylcarbamate log k₁ (s^?1) = (12.94 ± 0.09) ? (198.5 ± 0.9) kJ mol^?1 (2.303RT)^?1 For ethyl diphenylcarbamate log k₁ (s^?1) = (12.91 ± 0.18) ? (208.2 ± 2.4) kJ mol^?1 (2.303RT)^?1 The presence of phenyl and bulky groups at the nitrogen atom of the ethylcarbamate showed a decrease in the rate of elimination. Steric factor may be operating during the process of decomposition of these substrates. These reactions appear to undergo a semipolar six‐membered cyclic transition type of mechanism.© 2001 John Wiley & Sons, Inc. Int J Chem Kinet 34: 67–71, 2002     

An FTIR spectroscopic study of the kinetics and mechanism for the NO2-initiated oxidation of tetramethyl ethylene at 298 K

The kinetics and mechansim for the NO₂-initiated oxidation of tetramethyl ethylene (TME) have been studied using the FTIR spectroscopic method in mixtures containing NO₂ and TME (0.1?1.0 Torr) and N₂? O₂ (700 Torr) at 298 ± 2 K. While TME decayed according to -d[TME]/dt = k_obs[NO₂][TME], NO₂ exhibited a complex kinetic behavior. Furthermore, values of k_obs were dependent on [O₂]. Among the products were (CH₃)₂CO and at least three NO₂-containing compounds. These results indicate the formation of a nitro-alkylperoxy radical via reactions (1), (?1), and (2), and its subsequent reactions leading to the observed products. The [O₂]-dependence of k_obs yielded k₁ = (1.07 ± 0.15) × 10^?20 cm³ molecule^?1 S^?1 and k_?1/k₂ = (3.54 ± 0.61) × 10¹⁸ molecule cm^?3.     

Kinetics and mechanism of tertiary amine‐catalyzed cleavage of N′‐morpholino‐N‐(2′‐methoxyphenyl)phthalamide: Kinetic evidence for the presence of a reactive intermediate on the reaction path

Pseudo‐first‐order rate constants (k_obs) for tertiary amine (DABCO and Me₃N) buffer‐catalyzed cyclization of N′‐morpholino‐N‐(2′‐methoxyphenyl)phthalamide ( 1 ) to N‐(2′‐methoxyphenyl)phthalimide ( 2 ) reveal saturation (nonlinear) plots of k_obs versus [Buf]_T (total tertiary amine buffer concentration) at a constant pH. Such plots at different pH have been attributed to the presence of a reactive intermediate (T^?) formed by tertiary amine buffer‐catalyzed intramolecular nucleophilic addition of the secondary amide nitrogen to the carbonyl carbon of the tertiary amide group of 1 . © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 263–272, 2010     

Kinetics of the Reductions of (Ethylenediaminetetraacetato)cobaltate(III) by 4,4-Dipyridylamine Complexes of Pentacyanoferrate(II) and Pentaammineruthenium(II)

The reactions of Fe(CN)₅dpa^3? and Ru(NH₃)₅dpa²⁺ (dpa = 4,4′-dipyridylamine) with Co(edta)^? have been investigated kinetically. For Fe(CN)₅dpa^3? complex, a linear relationship was observed between the pseudo-First-order rate constants and the concentrations of Co(edta) which leads to a specific rate 0.876 ± 0.006 M^?1S^?1 at T = 25°C., μ = 0.10 M and pH = 8.0. For the Ru(NH₃)₅dpa²⁺ system, the plots k_obs vs [Co(edta)^?] become nonlinear at concentrations of Co(edta) greater than 0.01 M and the reaction is interpreted on the basis of a mechanism involving the formation of an ion pair between Ru(NH₃)₅dpa²⁺ and Co(edta)^? followed by electron transfer from Ru(II) to Co(III). The nonlinear least squares fit of the kinetic results shows that Q_ip = 10.6 ± 0.7 M^?1 and k_et = 93.9 ± 0.7 s^?1 at pH = 8.0,μ = 0.10 M and T = 25°C.     

Temperature‐dependent kinetics study of the gas‐phase reactions of atomic chlorine with acetone, 2‐butanone,and 3‐pentanone

A laser flash photolysis–resonance fluorescence technique has been employed to study the kinetics of the reactions of atomic chlorine with acetone (CH₃C(O)CH₃; k₁), 2‐butanone (C₂H₅C(O)CH₃; k₂), and 3‐pentanone (C₂H₅C(O)C₂H₅; k₃) as a function of temperature (210–440 K) and pressure (30–300 Torr N₂). No significant pressure dependence is observed for any of the reactions studied. Arrhenius expressions (units are 10^?11 cm³ molecule^?1 s^?1) obtained from the data are k₁(T) = (1.53 ± 0.19) exp[(?594 ± 33)/T], k₂(T) = (2.77 ± 0.33) exp[(+76 ± 33)/T], and k₃(T) = (5.66 ± 0.41) exp[(+87 ± 22)/T], where uncertainties are 2σ and represent precision only. The accuracy of reported rate coefficients is estimated to be ±15% over the entire range of pressure and temperature investigated. The room temperature rate coefficients reported in this study are in good agreement with a majority of literature values. However, the activation energies reported in this study are in poor agreement with the literature values, particularly for 2‐butanone and 3‐pentanone. Possible explanations for discrepancies in published kinetic parameters are proposed, and the potential role of Cl + ketone reactions in atmospheric chemistry is discussed. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 259–267, 2008     

Catalytic effect of anionic micelles on alkaline hydrolysis of N-hydroxyphthalimide. Evidence for the probable occurrence of the reaction in between Gouy-Chapman and exterior boundary of Stern layers of the micelles

The nucleophilic second-order rate constant (k_OH) for the reaction of ōH with ionized N-hydroxyphthalimide (S^?) appears to follow a reaction mechanism similar to that for reactions of ōH with neutral phthalimide and its N-substituted derivatives. Kinetically indistinguishable terms, k′_w[H₂O][S^?] and k_ōH[ōH][SH] (SH represents nonionized N-hydroxyphthalimide), which constitute the pH-independent rate region of the pH-rate profile, are resolved qualitatively. It is shown that the term k_ōH[ōH][SH], rather than k′_w[H₂O][S^?], is important in these reactions. The rates of ōH-catalyzed cleavage of S^? were studied at 32° in the presence of micelles of sodium dodecyl sulphate (SDS). At a constant [ōH], the observed pseudo first-order rate constants (k_obs) increase linearly with [SDS]_T (total SDS concentration). These data are explained in terms of the pseudophase model of micellar effects on reactivity. The linear dependence of k_obs with [SDS]_T (within [SDS]_T range of 0.0–0.2 or 0.3 M) is attributed to the occurrence of the reaction between the exterior boundary of Stern layer and Gouy-Chapman layer.     

Hydroxyl radical reactions with halogenated ethanols in aqueous solution: Kinetics and thermochemistry

Laser flash photolysis combined with competition kinetics with SCN^? as the reference substance has been used to determine the rate constants of OH radicals with three fluorinated and three chlorinated ethanols in water as a function of temperature. The following Arrhenius expressions have been obtained for the reactions of OH radicals with (1) 2‐fluoroethanol, k₁(T) = (5.7 ± 0.8) × 10¹¹ exp((?2047 ± 1202)/T) M^?1 s^?1, (2) 2,2‐difluoroethanol, k₂(T) = (4.5 ± 0.5) × 10⁹ exp((?855 ± 796)/T) M^?1 s^?1, (3) 2,2,2‐trifluoroethanol, k₃(T) = (2.0 ± 0.1) × 10¹¹ exp((?2400 ± 790)/T) M^?1 s^?1, (4) 2‐chloroethanol, k₄(T) = (3.0 ± 0.2) × 10¹⁰ exp((?1067 ± 440)/T) M^?1 s^?1, (5) 2, 2‐dichloroethanol, k₅(T) = (2.1 ± 0.2) × 10¹⁰ exp((?1179 ± 517)/T) M^?1 s^?1, and (6) 2,2,2‐trichloroethanol, k₆(T) = (1.6 ± 0.1) × 10¹⁰ exp((?1237 ± 550)/T) M^?1 s^?1. All experiments were carried out at temperatures between 288 and 328 K and at pH = 5.5–6.5. This set of compounds has been chosen for a detailed study because of their possible environmental impact as alternatives to chlorofluorocarbon and hydrogen‐containing chlorofluorocarbon compounds in the case of the fluorinated alcohols and due to the demonstrated toxicity when chlorinated alcohols are considered. The observed rate constants and derived activation energies of the reactions are correlated with the corresponding bond dissociation energy (BDE) and ionization potential (IP), where the BDEs and IPs of the chlorinated ethanols have been calculated using quantum mechanical calculations. The errors stated in this study are statistical errors for a confidence interval of 95%. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 174–188, 2008     

Effects of pure AcOH and mixed AcOH–CS (CS?=?CH3CN and THF) solvents on the rate of cleavage of phthalic anhydride (PAn)

The values of pseudo-first-order rate constants (k _obs) for the acetolysis of phthalic anhydride (PAn) increase from 6.60?×?10^?7 to 31.5?×?10^?7?s^?1 with the increase in temperature from 30 to 50?°C. These values of k _obs give activation parameters ?H* and ?S* as 14.4?±?0.4?kcal?mol^?1 and ?39.1?±?1.3?cal?K^?1?mol^?1, respectively. The values of k _obs remain essentially unchanged with the increase in the content of CS (CS?=?CH₃CN or THF) from 0 to 40?% v/v in mixed AcOH?CCS solvents. These observations have been explained qualitatively.     

Temperature‐dependent rate coefficients and mechanism for the gas‐phase reaction of chlorine atoms with acetone

The rate coefficient for the gas‐phase reaction of chlorine atoms with acetone was determined as a function of temperature (273–363 K) and pressure (0.002–700 Torr) using complementary absolute and relative rate methods. Absolute rate measurements were performed at the low‐pressure regime (～2 mTorr), employing the very low pressure reactor coupled with quadrupole mass spectrometry (VLPR/QMS) technique. The absolute rate coefficient was given by the Arrhenius expression k(T) = (1.68 ± 0.27) × 10^?11 exp[?(608 ± 16)/T] cm³ molecule^?1 s^?1 and k(298 K) = (2.17 ± 0.19) × 10^?12 cm³ molecule^?1 s^?1. The quoted uncertainties are the 2σ (95% level of confidence), including estimated systematic uncertainties. The hydrogen abstraction pathway leading to HCl was the predominant pathway, whereas the reaction channel of acetyl chloride formation (CH₃C(O)Cl) was determined to be less than 0.1%. In addition, relative rate measurements were performed by employing a static thermostated photochemical reactor coupled with FTIR spectroscopy (TPCR/FTIR) technique. The reactions of Cl atoms with CHF₂CH₂OH (3) and ClCH₂CH₂Cl (4) were used as reference reactions with k₃(T) = (2.61 ± 0.49) × 10^?11 exp[?(662 ± 60)/T] and k₄(T) = (4.93 ± 0.96) × 10^?11 exp[?(1087 ± 68)/T] cm³ molecule^?1 s^?1, respectively. The relative rate coefficients were independent of pressure over the range 30–700 Torr, and the temperature dependence was given by the expression k(T) = (3.43 ± 0.75) × 10^?11 exp[?(830 ± 68)/T] cm³ molecule^?1 s^?1 and k(298 K) = (2.18 ± 0.03) × 10^?12 cm³ molecule^?1 s^?1. The quoted errors limits (2σ) are at the 95% level of confidence and do not include systematic uncertainties. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 724–734, 2010     

Structure-reactivity correlation in the reactions of pyrrolidine with O-ethyl S-aryl dithiocarbonates in aqueous ethanol

The reactions of pyrrolidine with O-ethyl S-(X-phenyl) dithiocarbonates (X = 4-methyl, 4-methoxy, H, 4-chloro, 4-nitro, 2,4-dinitro, and 2,4,6-trinitro) are subjected to a kinetic study in 44 wt% aqueous ethanol, 25.0°C, and ionic strength 0.2 M (maintained with KCl). Pseudo-first-order kinetics are found under amine excess. Linear plots of the pseudo-first-order rate coefficient against concentration of free-base pyrrolidine are obtained for all the reactions, the nucleophilic rate coefficient (k_N) being the slope of such plots. The Bronsted-type plot (log k_N vs. pK_a for the leaving group) is linear with slope β_lg = − 0.2, which is consistent with a mechanism through a tetrahedral intermediate (T^±) where its formation is rate determining. The β_lg value is very similar to that found in the same reactions in water. There is a great difference in the mechanism of the reactions of O-ethyl S-phenyl dithiocarbonate with pyrrolidine (order one in amine) and piperidine (complex order in amine) in aqueous ethanol, and this is attributed to a greater nucleofugality from T^± of piperidine rather than pyrrolidine. © 1997 John Wiley & Sons, Inc.     

Kinetics of the reactions of the hydroxyl radical with dimethyl ether and diethyl ether

Absolute rate coefficients for the reactions of the hydroxyl radical with dimethyl ether (k₁) and diethyl ether (k₂) were measured over the temperature range 295–442 K. The rate coefficient data, in the units cm³ molecule^?1 s^?1, were fitted to the Arrhenius equations k₁ (T) = (1.04 ± 0.10) × 10^?11 exp[?(739 ± 67 cal mol^?1)/RT] and k₂(T) = (9.13 ± 0.35) × 10^?12 exp[+(228 ± 27 kcal mol^?1)/RT], respectively, in which the stated error limits are 2σ values. Our results are compared with those of previous studies of hydrogen-atom abstraction from saturated hydrocarbons by OH. Correlations between measured reaction-rate coefficients and C? H bond-dissociation energies are discussed.     

Kinetics of OH reactions with N2H4, CH3NHNH2 and (CH3)2NNH2 in the gas phase

The gas phase reaction kinetics of OH with three di‐amine rocket fuels—N₂H₄, CH₃NHNH₂, and (CH₃)₂NNH₂—was studied in a discharge flow tube apparatus and a pulsed photolysis reactor under pseudo‐first‐order conditions in [OH]. Direct laser‐induced fluorescence monitoring of the [OH] temporal profiles in a known excess of the [diamine] yielded the following absolute second‐order OH rate coefficient expressions; k₁ = (2.17 ± 0.39) × 10^?11 e^(160±30)/T, k₂ = (4.59 ± 0.83) × 10^?11 e^(85±35)/T and k₃ = (3.35 ± 0.60) × 10^?11 e^(175±25)/T cm³ molec^?1 s^?1, respectively, for reactions with N₂H₄, CH₃NHNH₂ and (CH₃)₂NNH₂ in the temperature range 232–637 K. All three reactions did not show any discernable pressure dependence on He or N₂ buffer gas pressure of up to 530 torr. The magnitude of the weak temperature and the lack of pressure effects of the OH + N₂H₄ reaction rate coefficient suggest that a simple direct metathesis of H‐atom may not be important compared to addition of the OH to one of the N‐centers of the diamine skeleton, followed by rapid dissociation of the intermediate into products. Our findings on this reaction are qualitatively consistent with a previous ab initio study [ 3 ]. However, in the alkylated diamines, direct H‐abstraction from the methyl moiety cannot be completely ruled out. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 354–362, 2001     

Shock tube studies of the N2O/Ar and N2O/H2/Ar systems

N₂O decay has been monitored via infrared emission for a series of mixtures containing N₂O/Ar and N₂O/H₂/Ar. These mixtures were studied behind reflected shock waves in the temperature interval of 1950–3075°K with total concentrations ranging from 1.2 to 2.5 × 10¹⁸ molec/cm³. In all cases the N₂O decayed exponentially, and a rate constant k_obs was obtained. Runs without added H₂ could be described by the following Arrhenius parameters: log A = ?9.72 ± 0.08 (in units of cm³/molec · sec) and E_A = 203.5 ± 3.6 kJ/mole. Addition of 0.01% and 0.1% H₂ was observed to increase the decay rate; the largest increase occurred between 2250 and 2500°K with 0.1% H₂, where k_obs doubled. Mixtures with no added H₂ were analyzed by numerical integration of the following reactions: Quantitative agreement between calculations and observations were obtained with both high and low choices for k₂ and k₃. The additional reactions were included in the analysis of the mixtures containing H₂. Here agreement was obtained only when low values were assigned to k₂ and k₃. The combinations of k₁ ← k₃ which agreed with all the data were k₁ = 3.25 × 10^?10 exp (?215 kJ/RT) and k₂ = k₃ = 1.91 × 10^?11 exp (-105 kJ/RT).     

Kinetics of the gas‐phase reactions of chlorine atoms with CH2F2, CH3CCl3, and CF3CFH2 over the temperature range 253–553 K

Relative rate techniques were used to study the title reactions in 930–1200 mbar of N₂ diluent. The reaction rate coefficients measured in the present work are summarized by the expressions k(Cl + CH₂F₂) = 1.19 × 10^?17 T² exp(?1023/T) cm³ molecule^?1 s^?1 (253–553 K), k(Cl + CH₃CCl₃) = 2.41 × 10^?12 exp(?1630/T) cm³ molecule^?1 s^?1 (253–313 K), and k(Cl + CF₃CFH₂) = 1.27 × 10^?12 exp(?2019/T) cm³ molecule^?1 s^?1 (253–313 K). Results are discussed with respect to the literature data. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 401–406, 2009     

Kinetics of the gas‐phase reactions of CHXCFX (X = H,F) with OH (253–328 K) and NO3 (298 K) radicals and O3 (236–308 K)

Rate constants for the reactions of OH and NO₃ radicals with CH₂?CHF (k₁ and k₄), CH₂?CF₂ (k₂ and k₅), and CHF?CF₂ (k₃ and k₆) were determined by means of a relative rate method. The rate constants for OH radical reactions at 253–328 K were k₁ = (1.20 ± 0.37) × 10^?12 exp[(410 ± 90)/T], k₂ = (1.51 ± 0.37) × 10^?12 exp[(190 ± 70)/T], and k₃ = (2.53 ± 0.60) × 10^?12 exp[(340 ± 70)/T] cm³ molecule^?1 s^?1. The rate constants for NO₃ radical reactions at 298 K were k₄ = (1.78 ± 0.12) × 10^?16 (CH₂?CHF), k₅ = (1.23 ± 0.02) × 10^?16 (CH₂?CF₂), and k₆ = (1.86 ± 0.09) × 10^?16 (CHF?CF₂) cm³ molecule^?1 s^?1. The rate constants for O₃ reactions with CH₂?CHF (k₇), CH₂?CF₂ (k₈), and CHF?CF₂ (k₉) were determined by means of an absolute rate method: k₇ = (1.52 ± 0.22) × 10^?15 exp[?(2280 ± 40)/T], k₈ = (4.91 ± 2.30) × 10^?16 exp[?(3360 ± 130)/T], and k₉ = (5.70 ± 4.04) × 10^?16 exp[?(2580 ± 200)/T] cm³ molecule^?1 s^?1 at 236–308 K. The errors reported are ±2 standard deviations and represent precision only. The tropospheric lifetimes of CH₂?CHF, CH₂?CF₂, and CHF?CF₂ with respect to reaction with OH radicals, NO₃ radicals, and O₃ were calculated to be 2.3, 4.4, and 1.6 days, respectively. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 619–628, 2010     

The elimination kinetics and mechanisms of ethyl piperidine‐3‐carboxylate,ethyl 1‐methylpiperidine‐3‐carboxylate,and ethyl 3‐(piperidin‐1‐yl)propionate in the gas phase

The gas‐phase elimination kinetics of the above‐mentioned compounds were determined in a static reaction system over the temperature range of 369–450.3°C and pressure range of 29–103.5 Torr. The reactions are homogeneous, unimolecular, and obey a first‐order rate law. The rate coefficients are given by the following Arrhenius expressions: ethyl 3‐(piperidin‐1‐yl) propionate, log k₁(s^?1) = (12.79 ± 0.16) ? (199.7 ± 2.0) kJ mol^?1 (2.303 RT)^?1; ethyl 1‐methylpiperidine‐3‐carboxylate, log k₁(s^?1) = (13.07 ± 0.12)–(212.8 ± 1.6) kJ mol^?1 (2.303 RT)^?1; ethyl piperidine‐3‐carboxylate, log k₁(s^?1) = (13.12 ± 0.13) ? (210.4 ± 1.7) kJ mol^?1 (2.303 RT)^?1; and 3‐piperidine carboxylic acid, log k₁(s^?1) = (14.24 ± 0.17) ? (234.4 ± 2.2) kJ mol^?1 (2.303 RT)^?1. The first step of decomposition of these esters is the formation of the corresponding carboxylic acids and ethylene through a concerted six‐membered cyclic transition state type of mechanism. The intermediate β‐amino acids decarboxylate as the α‐amino acids but in terms of a semipolar six‐membered cyclic transition state mechanism. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 106–114, 2006     

KINETICS OF FORMATION AND STABILITY CONSTANTS OF MONO AND BIS l-TYROSINE COMPLEXES OF Cu(II), Co(II), AND Ni(II)

Abstract

The kinetics and stability constants of l-tyrosine complexation with copper(II), cobalt(II) and nickel(II) have been studied in aqueous solution at 25° and ionic strength 0.1 M. The reactions are of the type M(HL)^(3-n)+ _n-1 + HL- ? M(HL)^(2-n)+_n(k_n, forward rate constant; k_-n, reverse rate constant); where M=Cu, Co or Ni, HL^? refers to the anionic form of the ligand in which the hydroxyl group is protonated, and n=1 or 2. The stability constants (K_n=k_n/k_-n) of the mono and bis complexes of Cu²⁺, Co²⁺ and Ni²⁺ with l-tyrosine, determined by potentiometric pH titration are: Cu²⁺, log K₁=7.90 ± 0.02, log K₂=7.27 ± 0.03; Co²⁺, log K₁=4.05 ± 0.02, log K₂=3.78 ± 0.04; Ni²⁺, log K₁=5.14 ± 0.02, log K₂=4.41 ± 0.01. Kinetic measurements were made using the temperature-jump relaxation technique. The rate constants are: Cu²⁺, k₁=(1.1 ± 0.1) × 10⁹ M ^?1 sec^?1, k_-1=(14 ± 3) sec^?1, k₂=(3.1 ± 0.6) × 10⁸ M ^?1 sec^?1, k^?2=(16 ± 4) sec^?1; Co²⁺, k₁=(1.3 ± 0.2) × 10⁶ M ^?1 sec^?1, k_-1=(1.1 ± 0.2) × 10² sec^?1, k₂=(1.5 ± 0.2) × 10⁶ M ^?1 sec^?1, k_-2=(2.5 ± 0.6) × 10² sec^?1; Ni²⁺, k₁=(1.4 ± 0.2) × 10⁴ M ^?1 sec^?1, k_-1=(0.10 ± 0.02) sec^?1, k₂=(2.4 ± 0.3) × 10⁴ M ^?1 sec^?1, k_-2=(0.94 ± 0.17) sec^?1. It is concluded that l-tyrosine substitution reactions are normal. The presence of the phenyl hydroxyl group in l-tyrosine has no primary detectable influence on the forward rate constant, while its influence on the reverse rate constant is partially attributed to substituent effects on the basicity of the amine terminus.     

Trans ligand influence in cobalt(III) Schiff base complexes: Electronic and steric effects on the kinetics of hydrolysis

Rate constants for the hydrolysis (k_h) of six different amines in trans‐[Co((BA)₂en)(amine)₂]ClO₄ complexes (amine = aniline 1a , para‐toluidine 1b , benzylamine 1c (primary amines), pyrrolidine 2a , piperidine 2b , morpholine 2c (secondary amines), and (BA)₂en = Bisbenzoylacetoneethylenediiminato) in mixed methanol/water (1:1) solvent have been determined between 30 and 55°C. The hydrolysis product of 2c , trans‐[Co((BA)₂en)(morpholine)(H₂O)]ClO₄, has been separately prepared and characterized by UV–vis and ¹H NMR spectroscopy. Depending on the nature of the axial amine ligand the limiting first‐order rate constants for the amine hydrolysis at 40°C range from (3.42 ± 0.10) × 10^?5 to (5.32 ± 0.13) × 10^?5 s^?1. At the first glance, a reasonable trend cannot be established between k_h and the basicity or the inductive trans effect of the amine ligands. However, when the complexes are classified into two groups, based on the type of the amine (primary and secondary), the values of k_h correlate well with the basicity or inductive effect of the amine in each group. The observed trend in k_h values for the complexes with primary amines is 1a (5.32 ± 0.13) × 10^?5 s^?1 > 1b (3.51 ± 0.14) × 10^?5 > 1c (1.72 ± 0.03) × 10^?5 (40°C), which is opposite to the amine basicity strength. In the case of the complexes with secondary amines, the observed trend in k_h values is in accord with amine basicity (or inductive trans effect), i.e. 2a (5.02 ± 0.22) × 10^?5 > 2b (4.18 ± 0.10) × 10^?5 > 2c (3.42 ± 0.10) × 10^?5 s^?1 (40°C). © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 387–393, 2002