Thermal stability of amino acids

Thermal decomposition of L-α-amino acids RCH₂(NH₂)COOH where R = Me₂CH, Me₂CHCH₂, MeEtCH, and C₆H₅CH₂ was studied at temperatures below the melting points of their crystals. From the effective rate constants of the first order reactions energy parameters in the Arrhenius equation were calculated. Correlations between the reaction rate constants k _R and the inductive constants σ* of substituents R and also between the rate constants of the reactions and the dipole moments of amino acids was established. Value of ρ* parameter +8.8 in the Taft equation indicates the heterolytic mechanism of transformation of the amino acids. Chromato-mass spectrometric analysis of decomposition products shows that condensation, decarboxylation, and deamination of the amino acids take place.     

Nonlinear Taft Polar Free Energy Relationship: Reactions of N‐Substituted Benzyl Amines with Benzyl Bromide in Methanol

The rates of reactions of N‐substituted benzyl amines with benzyl bromide were measured using a conductivity technique in methanol medium. The reaction followed a total second‐order path. The end product of the reaction is identified as dibenzyl alkyl amine (C₆H₅CH₂N(R)CH₂C₆H₅). The rates increased with a decrease in the electron‐donating capacity or with an increase in the Taft σ* value of electron‐donating alkyl substituents (R) such as t‐butyl (σ* = ?0.3), i‐propyl (σ* = ?0.19), n‐butyl (σ* = ?0.13), and ethyl (σ* = ?0.1) on nitrogen of the amine until the Taft σ* value becomes zero for the methyl group ( = 0.00), and then the rates decreased with an increase in the electron‐withdrawing capacity or with an increase in the Taft σ* value of electron‐withdrawing substituents (R) such H and C₆H₅ ( = 0.49 and = 0.6). The locus of the Taft polar free energy relationship has a maximum near the point for N‐methyl benzyl amine, showing that there is a sharp change in the rate‐determining step. A mechanism involving formation of an S_N2‐type transition state between the amine nucleophiles and the benzyl bromide and its subsequent decomposition is proposed. Activation parameters were calculated and are discussed.     

Structure‐Reactivity Relationships as Probes for the Inhibition Mechanism of Cholesterol Esterase by Aryl Carbamates. I. Steady‐State Kinetics

For substituted phenyl‐N‐butyl carbamates (1) and 4‐nitrophenyl‐N‐substituted carbamates (2), linear relationships between values of NH proton chemical shift (δ_NH), pKa, and logk_[OH] and Hammett substituent constant (σ) or Taft substituent constant (σ*) are observed. Carbamates 1 and 2 are pseudo‐substrate inhibitors of porcine pancreatic cholesterol esterase. Thus, the mechanism of the reaction necessitates that the inhibitor molecule and the enzyme form the enzyme‐inhibitor tetrahedral species at the K_i step of the reaction and then form the carbamyl enzyme at the k_c step of the reaction. Linear relationships between the logarithms of K_i and k_c for cholesterol esterase by carbamates 1 and σ are observed, and the reaction constants (ρs) are ?3.4 and ?0.13, respectively. Therefore, the above reaction forms the negative‐charge tetrahedral species and follows the formation of the relatively neutral carbamyl enzymes. For the inhibition of cholesterol esterase by carbamates 2 except 4‐nitrophenyl‐N‐phenyl carbamate and 4‐nitrophenyl‐N‐t‐butyl carbamate, linear relationships of ‐logK_i and logk_c with σ* are observed and the ρ* values are ?0.50 and 1.03, respectively. Since the above reaction also forms the negative‐charge tetrahedral intermediate, it is possible that the K_i step of this reaction is further divided into two steps. The first K_i step is the development of the positive‐charge at the carbamate nitrogen from the protonation of the carbamate nitrogen. The second K_i step is the formation of the tetrahedral intermediate with the negative‐charge at the carbonyl oxygen. From Arrhenius plots of a series of inhibition reactions by carbamates 1 and 2, the isokinetic and isoequilibrium temperatures are different from the reaction temperature (25°C). Therefore, the observed ρ and ρ* values only depend upon the electronic effects of the substituents. Taken together, the cholesterol esterase inhibition mechanism by carbamates 1 and 2 is proposed.     

Relative rate constants for the reactions of trichloropropenyl radical. Application of the two-parameter Taft equation

The relative rate constants for the hydrogen atom abstraction by CCl₃CH?CH· radical from CH₂Cl₂, CHCl₃, CH₃COCH₃, CH₃CN, C₆H₅CH₃, C₆H₅OCH₃, CH₃CHO, and CH₃OH in the liquid phase at 20°C have been measured. It was shown that these reaction rate constants are correlated by the two-parameter Taft equation with ρ* = 0.726 ± 0.096, r* = 1.22 ± 0.16. A relationship between r* and bond dissociation energy D(R? H) has been found for the abstraction reactions of different free radicals.     

Outer‐sphere reduction of hexacyanoferrate(III) by enolizable and nonenolizable aldehydes in alkaline medium

Outer‐sphere reduction of hexacyanoferrate(III) by some enolizable/nonenolizable aldehydes (viz., aliphatic, heterocyclic, and aromatic aldehydes) in alkaline medium has been studied spectrophotometrically at λ_max = 420 nm. The reactions are first order each in [aldehyde] and [Fe(CN)₆^3?]. The rate increases with an increase in [OH^?] in the oxidation of aliphatic and heterocyclic aldehydes, whereas it is independent of [OH^?] in the reaction with aromatic aldehydes. The intervention of free radicals in the reaction mixture was carried out using both acrylonitrile and acrylamide scavenger in two different experiments. The kinetic results indicate that the oxidation of benzaldehyde in aqueous medium proceeds at a slower rate than the aliphatic aldehydes (other than formaldehyde) and furfural. The values of third‐order rate constant (k₃) at 308 K in the oxidations of some aliphatic aldehydes and furfural follow the order (CH₃)₂CH? > CH₃CH₂? > CH₃? > C₄H₃O? > H? . The rate constants correlate with Taft's σ* value, the reaction constant being negative (–9.8). The pseudo–first‐order rate constants in the oxidations of benzaldehyde and substituted benzaldehydes follow the order ? NO₂ > ? H > ? Cl > ? OCH₃. The Hammett plot is also linear with a ρ value (0.6488) for meta‐ and para‐substituted benzaldehydes. The kinetic isotope effect for benzaldehyde (k_H/k_D = 1.93 at 303 K) was obtained. The rate‐determining step is the outer‐sphere formation of Fe(CN)₆^4? and free radicals, which is followed by the rapid oxidation of free radicals by Fe(CN)₆^3? to give products. The kinetic data and hence thermodynamic parameters have been used to distinguish enolizable and nonenolizable aldehydes. An attempt has also been made to correlate kinetic data with hydration equilibrium constants of some aliphatic aldehydes. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 494–505, 2012     

Über die Si-N-bindung: XLI. Kinetische untersuchungen zur hydrolyse von trimethylsilylurethanen des typs Me3Si(p-XC6H4)NCOOEt

Hydrolysis reactions of silylurethanes Me₃Si(p-XC₆H₄)NCOOEt (I) with X = Cl, H or Me in aqueous buffer solutions, with pH values from 1.94 to 10.00 were studied.The catalytic rate constants for the acid and base catalysed reactions and for the “non-catalysed” reaction k(H₃O⁺), k(CH₃COO^?), k(H₂PO₄^?), k(HPO₄^2?), k(NH₃), k(OH^?) and k₀ were evaluated from the pseudo first-order rate constants k_exp determined by UV spectroscopy.The Brönsted coefficients for the base-catalysed reactions were obtained from the catalytic rate constants found and the known constants of dissociation K(HB⁺).The ρ values of the reactions could be derived from the σ constants given by Jaffé.The kientical results obtained are interpreted mechanistically and are believed to also have model character for other nucleophilic substitution reactions with silicon compounds.     

Über die SiN-bindung: XLIII. Kinetische untersuchungen zur hydrolyse von silylurethanen des typs (p-XC6H4)Me2SiN(Ph)COOEt

Hydrolysis reactions of silylurethanes (p-XC₆H₄)Me₂SiN(Ph)COOEt with X = MeO, Me, H or Cl in aqueous buffer solutions, with pH values from 1.73 to 10.00 and at temperatures of 20, 30, and 40°C were studied.The catalytic rate constants for the acid- and base-catalysed reactions and for the “non-catalysed” reaction k(H₃O⁺), k(CH₃COO^?), k(NH₃), k(OH^?), and k₀ and the Brönsted coefficients for the general base-catalysed reactions were evaluated from the pseudo first-order rate constants k_exp, determined by UV spectroscopy.The ? values of the reactions can be derived from the σ constants given by Jaffé. Activation parameters can be obtained by means of the Arrhenius or Eyring equation.From the catalytic constants, Brönsted coefficients, ? values and activation parameters we derive mechanisms for the acid- and general base-catalysed reactions.Conclusions about these mechanisms are compared with the results obtained with the silylurethanes Me₃Si(p-XC₆H₄)NCOOEt.     

Prediction of ortho substituent effect in alkaline hydrolysis of phenyl esters of substituted benzoic acids in aqueous acetonitrile

The second-order rate constants k for the alkaline hydrolysis of phenyl esters of meta-, para- and ortho-substituted benzoic acids, X-C₆H₄CO₂C₆H₅, in aqueous 50.9% acetonitrile have been measured spectrophotometrically at 25°C. The log k values for meta and para derivatives correlated well with the Hammett σ_m,p substituent constants. The log k values for ortho-substituted phenyl benzoates showed good correlations with the Charton equation, containing the inductive, σ_I, resonance, σ^○ _R, and steric, E _s ^B, and Charton υ substituent constants. For ortho derivatives the predicted (log k _X)_calc values were calculated with equation (log k _ortho)_calc = (log k ^H _AN)_exp + 0.059 + 2.19σ_I + 0.304σ^○ _R + 2.79E _s ^B ? 0.0164ΔEσ_I — 0.0854ΔEσ^○ _R, where DE is the solvent electrophilicity, ΔE = E _AN — E _H20 = ?5.84 for aqueous 50.9% acetonitrile. The predicted (log k _X)_calc values for phenyl ortho-, meta- and para-substituted benzoates in aqueous 50.9% acetonitrile at 25°C precisely coincided with the experimental log k values determined in the present work. The substituent effects from the benzoyl moiety and aryl moiety were compared by correlating the log k values for the alkaline hydrolysis of phenyl esters of substituted benzoic acids, X-C₆H₄CO₂C₆H₅, in various media with the corresponding log k values for substituted phenyl benzoates, C₆H₅CO₂C₆H₄-X.     

Empirical relations between σI, σoR and taft σ* values of alkyl,acyl, benzoyl and substituted phenyl groups

The X(X) values¹ of the halogens (which resemble the Pauling electronegativities) and of some oxa substituents can be interpreted in terms of the inductive and resonance parameters σ_I and σ^o_R according to the regression equation

and η*_R=η(X)?η(R) it is found that for some substituted methyl, phenyl and benzoyl groups [σ*]^X_R=αη^X_R where α equals ?10.6 and ?10.9 for R = Me and R = Ph, CHO and PhCO respectively. Thus [σ*]^X_Rand η^x_r represent Taft σ* and [σ_I(X)?σ_oR(X)] values relative to that of the parent R group. The hydroxyl frequencies of phenol, and benzoic, acrylic, acetic and formic acids measured in dilute carbon tetrachloride solutions correlate with σ_I(X) and σ^o_R(X) according to the equations v(OH) = ?423.0 σ_I(X) + 3654.7 v(OH) = ?270.0 σ⁰_R(X) + 3586.7 where X = Ph, PhCO, CH₂=CHCO, MeCO and HCO. From these results, it is inferred that the σ* values of substituents having an α sp² hybridized carbon atom are proportional to σ⁰_R according to the equation σ*(X) = 3.97 σ⁰_R(X) + 1 New σ_I σ^o_R and σ* values of some acyi, benzoyl and substituted phenyl groups are presented.     

Rate-structure studies of the carbonylation and decarbonylation of substituted metal carbonyl complexes

The rate of CO insertion for the following complexes have been determined: RCH₂Mn(CO)₅ where R  cyclohexyl, hydrogen, phenyl, methoxy, and carboxyl; RCH₂Fe(CO)₄^? where R  cyclohexyl, n-octyl, n-heptyl, and phenyl. It appears that a Taft σ? correlation can be established for this reaction. Rate data for the decarbonylation reaction of an analogous series of substituted manganese carbonyl complexes (RCH₂COMn(CO)₅) was also obtained. As in the case of the CO insertion reaction, the rates increase as the Taft σ for RCH₂ decreases.The linear free energy relationships (LFER) generated in this study have been used to predict the relative probability of several reaction steps that have been postulated for the conversion of carbon monoxide and hydrogen into ethylene glycol.     

Conformational Analysis of Biphenyl Derivatives. I. A Novel Steric Constant from the Internal Rotation Rate of 2,2′‐Bis‐(N‐Substituted Carbamoylmethyl)Biphenyl by Means of Dynamic NMR

A novel scale of steric substituent constant E_s^D is defined from the correlation of the logarithms of the internal rotation rate (k_r) at 393 K with Hancock (E_s^c) steric constant by means of dynamic NMR. In the inhibition of Pseudomona species lipase by 2,2′‐bis‐(N‐substituted carbamoylmethyl)biphenyls (1‐8), the logarithms of bimolecular rate constants are multiply correlated with both the Taft substituent constant σ* and E_s^D.     

Rate constants for the reactions of CCl(X2II) with a series of silanes

Rate constants for the gas phase reactions of CCl generated by the flash photolysis of CHBr₂Cl with a series of silanes have been obtained by kinetic absorption spectroscopy. In general, the rate constants are very high, and range from (4.8 ± 0.5) × 10⁸ (SiH₄) to (6.4 ± 0.34) × 10⁹ for Si₂H₆. CCl does not insert into the SiC or primary CH bonds of silanes and its rate of reaction with tertiary SiH bonds is 600 times greater than with tertiary CH bonds. CCl reacts slowly with the SiSi bond. k_H/k_D varies from 1.9 to 1.0 on going from primary to tertiary SiH bonds. The electrophilic character of CCl is manifested, on a per SiH bond basis, by excellent correlations between the rate constants and the hydrilic character of the SiH bond, and between log k and the ionization potential.     

Salt-catalyzed addition reaction of monoepoxides with philosamia cynthia ricini and bombyx mori silk fibroins

The heterogeneous addition reaction of various monoepoxides with silk fibroins of Philosamia cynthia ricini and Bombyx mori was investigated at 45–75°C by use of aqueous solutions of various salts as padding catalysts. The effects of salt on the epoxide–silk fibroin reactions were attributed mainly to the nucleophilicity of the anions and also to the acidity or the electronegativity of the cations. The effect of the substituent of the epoxide on the add-ons was elucidated by the modified Taft equation, (log W ? log W₀)/σ* = ρ_p + ρ_sE_s/σ*, where W₀ and W are the add-ons for the reaction of a given compound and of its substituted derivatives, σ* and E_s are the polar and the steric substituent constants, ρ_p and ρ_s are the polar and the steric reaction constants, respectively. Histidine, lysine, arginine, tyrosine, serine, and acidic amino acids were found to react. The reactivity difference between Philosamia cynthia ricini and Bombyx mori fibroins towards the epoxide was discussed in the light of the observed phenomena.     

The two-parameter taft equation in kinetics of free radical reactions

A new method was developed for the calculation of the resonance substituent constants of the two-parameter Taft equation log k_sub/k₀=ρ*σ*+r*σ^r. It is based on a relationship between the spin density in free radicals and the rate constants of radical substitution reactions of CH₃. Possibilities and limitations of the application of this correlation equation to the investigation of substitution and addition radical reactions are discussed.     

Correlation of para substituent carbon-13 chemical shifts in aromatic compounds using polar (inductive) and resonance constants which are identical for aliphatic and aromatic compounds

A correlation of para substituted ¹³C chemical shifts in aromatic compounds with substituent polar (inductive) and resonance constants σ* and σ^r of aliphatic compounds has been studied. It has been shown that the precision of the correlations obtained corresponds to that of the Swain-Lupton and Taft two-parameter equations, but the correlation equation used in this work seems to permit a more exact separation of the substituent effects of aromatic compounds into inductive and resonance contributions. Thus, σ* and σ^r substituent constants are universal parameters which can be used in a correlation analysis of the properties of both aliphatic and aromatic compounds.     

Aminolysis of 2,4-dinitrophenyl X-substituted benzoates and Y-substituted phenyl benzoates in MeCN: effect of the reaction medium on rate and mechanism

Second‐order rate constants (k_N) have been determined spectrophotometrically for the reactions of 2,4‐dinitrophenyl X‐substituted benzoates ( 1 a – f ) and Y‐substituted phenyl benzoates ( 2 a – h ) with a series of alicyclic secondary amines in MeCN at 25.0±0.1 °C. The k_N values are only slightly larger in MeCN than in H₂O, although the amines studied are approximately 8 pK_a units more basic in the aprotic solvent than in H₂O. The Yukawa–Tsuno plot for the aminolysis of 1 a – f is linear, indicating that the electronic nature of the substituent X in the nonleaving group does not affect the rate‐determining step (RDS) or reaction mechanism. The Hammett correlation with σ^? constants also exhibits good linearity with a large slope (ρ_Y=3.54) for the reactions of 2 a – h with piperidine, implying that the leaving‐group departure occurs at the rate‐determining step. Aminolysis of 2,4‐dinitrophenyl benzoate ( 1 c ) results in a linear Brønsted‐type plot with a β_nuc value of 0.40, suggesting that bond formation between the attacking amine and the carbonyl carbon atom of 1 c is little advanced in the transition state (TS). A concerted mechanism is proposed for the aminolysis of 1 a – f in MeCN. The medium change from H₂O to MeCN appears to force the reaction to proceed concertedly by decreasing the stability of the zwitterionic tetrahedral intermediate (T^±) in aprotic solvent.     

Activation of the aromatic system by the SO2CF3 group: Kinetics study and structure–reactivity relationships

The rate constants for the reaction of 2,6‐bis(trifluoromethanesulfonyl)‐4‐nitroanisole with some substituted anilines have been measured by spectrophotometric methods in methanol at various temperatures. The data are consistent with the S_NAr mechanism. The effect of substituents on the rate of reaction has been examined. Good linear relationships were obtained from the plots of log k₁ against Hammett σ_para constants values at all temperature with negative ρ values (?1.68 to ?1.11). Activation parameters Δ^≠H varied from 41.6 to 54.3 kJ mol^?1 and Δ^≠S from ?142.7 to ?114.6 J mol^?1 K^?1. The δΔ^≠H and δΔ^≠S reaction constants were determined from the dependence of Δ^≠H and Δ^≠S activation parameters on the σ substituent constants, by analogy with the Hammett equation. A plot of Δ^≠H versus Δ^≠S for the reaction gave good straight line with 177°C isokinetic temperature. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 203–210, 2010     

QSAR in Reactions of Organophosphorus Inhibitors with Acetylcholinesterase

Abstract

Structure-activity relationships in inhibition of acetylcholinesterase by organophosphorus compounds (YO)(Z)P(O)SX have been analyzed using the equation log k_i = C + ρσ + ?π with Taft σ* for substituent elehtronegativity and Hansch Π for hydrophobicity. The obtained relationships have been used for optimizing the structures of organophosphorus inhibitors for their maximum anticholinesterase activity.     

Kinetics of Ozone-Olefin Reactions in Aqueous Solutions

The kinetics of the reactions of ozone with 12 substituted olefins in aqueous solutions at 293 K are studied by the stopped-flow method. Second-order rate constants are determined for these reactions. The dependence of the reactivity of olefins in H₂O on their structure can be described by the Taft equation.     

Theoretical studies on the alkylidene germylenoid H<Subscript>2</Subscript>C=GeLiF and its insertion reaction with R-H (R = F,OH, NH<Subscript>2</Subscript>, CH<Subscript>3</Subscript>)

The geometries and isomerization of the alkylidene germylenoid H₂C=GeLiF as well as its insertion reactions with R-H (R = F, OH, NH₂, CH₃) have been systematically investigated at the B3LYP/6-311+ G* level of theory. The potential barriers of the four insertion reactions are 110.6, 145.0, 179.4, and 250.6 kJ/mol, respectively. Here, all the mechanisms of the four reactions are identical to each other, i.e., an intermediate has been formed first during the insertion reaction. Then, the intermediate could dissociate into the substituted germylene (H₂C=GeHR) and LiF with a barrier corresponding to their respective dissociation energies. Correspondingly, the reaction energies for the four reactions are 43.6, 78.8, 113.5, and 128.0 kJ/mol, respectively. Compared with the insertion reaction of H₂C= Ge∶ and R-H, the introduction of LiF makes the insertion reaction occur more difficultly. Furthermore, the effects of halogen (F, Cl, Br) substitution and inorganic salts employed on the reaction activity have also been discussed. As a result, the relative reactivity among the four insertion reactions should be as follows: H-F > H-OH > H-NH₂ > H-CH₃.