Proton-transfer reactions between nitroalkanes and hydroxide ion under non-steady-state conditions. Apparent and real kinetic isotope effects.

The kinetics of the proton-transfer reactions between 1-nitro-1-(4-nitrophenyl)ethane (NNPE(H(D))) and hydroxide ion in water/acetonitrile (50/50 vol %) were studied at temperatures ranging from 289 to 319 K. The equilibrium constants for the reactions are large under these conditions, ensuring that the back reaction is not significant. The extent of reaction/time profiles during the first half-lives are compared with theoretical data for the simple single-step mechanism and a 2-step mechanism involving initial donor/acceptor complex formation followed by unimolecular proton transfer and dissociation of ions. In all cases, the profiles for the reactions of both NNPE(H) and NNPE(D) deviate significantly from those expected for the simple single-step mechanism. Excellent fits of experimental data with theoretical data for the complex mechanism, in the pre-steady-state time period, were observed in all cases. At all base concentrations (0.5 to 5.0 mM) and at all temperatures the apparent kinetic isotope effects (KIE(app)) were observed to increase with increasing extent of reaction. Resolution of the kinetics into microscopic rate constants at 298 K resulted in a real kinetic isotope effect (KIE(real)) for the proton-transfer step equal to 22. Significant proton tunneling was further indicated by the temperature dependence of the rate constants for proton and deuteron transfers: KIE(real) ranging from 17 to 26, E(a)(D) -- E(a)(H) equal 2.8 kcal/mol, and A(D)/A(H) equal to 4.95.     

Cotton-mouton constants and valency optical scheme for salts of pyridine,quinoline, and isoquinoline

A tensor additive scheme has been devised for the optical polarizabilities of the cations of pyridine, quinoline, and isoquinoline. The additional excess charge in these species leads to a decrease in the vertical component of the optical polarizability while the mobility of the electron cloud in the plane of the aromatic ring is retained. This effect is greater in the N-methyl derivatives than in the protonated forms.Deceased.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 615–619, March, 1991.     

Absolute rate constants for some intermolecular reactions of alpha-aminoalkylperoxyl radicals. Comparison with alkylperoxyls

Seven alpha-aminoalkylperoxyl radicals have been generated by 355 nm laser flash photolysis (LFP) of oxygen-saturated di-tert-butyl peroxide containing mono-, di-, and trialkylamines and a dialkylarylamine. All these peroxyls possess absorptions in the near-UV (strongest for the trialkylamine-derived peroxyls) which permits direct monitoring of the kinetics of their reactions with many substrates. The measured rate constants for hydrogen atom abstraction from some phenols and oxygen atom transfer to triphenylphosphine demonstrated that all seven alpha-aminoalkylperoxyls have similar reactivities toward each specific substrate. More importantly, a comparison with literature data for alkylperoxyls shows that alpha-aminoalkylperoxyls and these alkylperoxyls have essentially the same reactivities. The combination of LFP and alkylamines provides a quick, reliable method for determining absolute rate constants for alkylperoxyl radical reactions, an otherwise laborious task.     

Reaction of the pyrazinium ion having aryl and cyano groups with hydride reagents or hydroxide ion

Reduction of 3-cyano-5-(3,4-dimethoxyphenyl)-1-methylpyrazinium ion by the hydride reagents such as sodium borohydride or Hantzsch ester gave the 1,6-dihydropyrazine derivative, and the 1,4,5,6-tetra-hydropyrazine derivative on further reduction. Addition of hydroxide ion to the pyrazinium ion gave mainly a 6-hydroxy-pseudobase, accompanied by the minor formation of a 2-hydroxy-pseudobase. Photoreaction of the pseudobase mixture gave a product from the major pseudobase but thermal transformation gave another product from the minor pseudobase.     

Rate constants for some reactions of inorganic radicals with inorganic ions. Temperature and solvent dependence

Rate constants for several reactions of inorganic radicals with inorganic anions in aqueous and aqueous/acetonitrile solutions have been measured as a function of temperature by laser flash photolysis. The reactions studied were (1) Cl₂^? + N₃^?, (2) Br₂^? + N₃^?, (3) Cl₂^? + SCN^?, (4) Br₂^? + SCN^?, (5) SO₄^? + Cl^?, (6) SO₄^? + CO₃^2?, and (7) N₃? + I^?. The rate constants were corrected for ionic strength and ranged from 10⁶ to 10⁹ L mol^?1 s^?1. The Arrhenius activation energies varied from 2 to 12 kJ mol^?1 for the first 4 reactions, were higher for reaction 6, and negative for reaction 5. The pre-exponential factors also varied considerably with log A ranging from 5 to 14. The values of k₂₉₈ decreased in most cases by more than an order of magnitude upon increasing the acetonitrile (ACN) fraction from 0 to 70%. For most reactions, this decrease in k₂₉₈ was due to changes in log A with little regularity in the small changes observed in E_a. For reaction 7, k₂₉₈ was practically unchanged due to compensating effects of the changes in E_a and log A with ACN mol fraction, giving an isokinetic relationship. An isokinetic relationship was also observed in the case of reaction 6; E_a and log A change in parallel while changing ACN mol fraction. Reaction 3 (Cl₂^? + SCN^?) was also studied in water/t-butanol and water/acetic acid mixtures. Linear correlation was found between log k and the dielectric constant of the medium for water/ACN and water/t-BuOH but the lines for the two solvent mixtures had different slopes, suggesting specific solvation effects in addition to the primary solvent polarity effects. With water/acetic acid, k decreased and then increased upon addition of acetic acid. © 1993 John Wiley & Sons, Inc.     

Chemistry of thienopyridines. XXXV. Synthesis,tautomerism, and reactions of quinoline and thienopyridine systems which bear a 1-carboethoxy-1-cyanomethyl substituent in the pyridine ring,part 2

A comparison is made amongst the isosteric Systems quinoline, thieno[2,3,-b]pyridine, and thieno[3,2-b]-pyridine which bear the 1-carboethoxy-1-cyanomethyl substituent (R) alpha or gamma to the heterocyclic nitrogen atom. Treatment of thieno[3,2-b]pyridine 4-oxide with ethyl cyanoacetate and acetic anhydride at room temperature (Hamana reaction) gives the alpha R-derivative 6 (27%), formulated as an intramolecular H-bonded structure. Neither 6 nor its quinoline alpha analog reacts with refluxing acetic anhydride, while the quinoline gamma isomer 8 , existing as NH and CH tautomers, yields an N-acetyl derivative 10 (70%) under similar conditions. For each of 6 and 8 one can isolate two crystalline forms which differ considerably in color. Compound 10 and its gamma analog in the thieno[2,3-6]pyridine series (previously obtained directly from a Hamana reaction) serve as acetylating agents for aniline, 1-aminobutane, morpholine, and cholesterol. Correlations and contrasts in the three Systems are presented.     

Rate constants for some hydrogen abstraction reactions of the carbonate radical

Rate constants have been measured in aqueous solutions for the reactions of the carbonate radical, CO₃^˙?, with several saturated alcohols and one cyclic ether as a function of temperature. Arrhenius pre-exponential factors ranged from 2×10⁸ to 1×10⁹ ?? mol^?1 s^?1 and activation energies ranged from 16 to 29 kJ mol^?1. The results suggest that the reactions are not pure hydrogen abstraction, but involve an additional interaction of the radical with the ? OH or ? O? linkage. © 1993 John Wiley & Sons, Inc.     

Rate constants for some reactions of free radicals with haloacetates in aqueous solution

The kinetics of the acqueous-phase reactions of the free radicals ·OH, ·Cl, and SO· with the halogenated acetates, CH₂FCOO^?, CHF₂COO^?, CF₃COO^?, and with CH₂ClCOO^?, CHCl₂COO^?, CCl₃COO^? were investigated. Generally, the reactivity decreases with increasing halogen substitution and is in the order k(·OH) > k(SO·) > k(·Cl), but there is no general relation between the effect on reactivity of chlorine and fluorine substitution. © 1995 John Wiley & Sons, Inc.     

New spectrophotometric method for determining the stoichiometry and equilibrium constants of some redox reactions

A new spectrophotometric method is proposed for determining the stoichiometries of redox reactions; it can be used in many cases for determining the equilibrium constant of the reaction. The method is based on that of Holme and Langmyhr. Good results were obtained for several redox reactions involving iodate, bromate or periodate, and manganese(II) or some cyclohexanedione bisthiosemicarbazones.     

Test of the quantum instanton approximation for thermal rate constants for some collinear reactions

Two variants of the recently developed quantum instanton (QI) model for calculating thermal rate constants of chemical reactions are applied to several collinear atom-diatom reactions with various skew angles. The results show that the original QI version of the model is consistently more accurate than the "simplest" quantum instanton version (both being applied here with one "dividing surface") and thus to be preferred. Also, for these examples (as with other earlier applications) the QI results agree well with the correct quantum rates (to within approximately 20% or better) for all temperatures >200 K, except for situations where dynamical corrections to transition state theory (i.e., "re-crossing" dynamics) are evident. (Since re-crossing effects are substantially reduced in higher dimensionality, this is not a cause for serious concern.) A procedure is also described which facilitates use of the METROPOLIS algorithm for evaluating all quantities that appear in the QI rate expression by Monte Carlo path integral methods.     

Kinetic study and reactivity trends of hydroxide ion attack on some chromen-2-one laser dyes in binary water-methanol and water-acetone mixtures

The kinetics of base-catalyzed hydrolysis of 7-dimethylamino-4-methyl-2H-chromen-2-one (DMAC) and 7-diethylamino-4-methyl-2H-chromen-2-one (DEAC) in binary water-methanol and water-acetone mixtures were studied in the temperature range from 288 to 313 K. The activation and thermodynamic parameters of these reactions were evaluated and discussed. The change in the activation energy in going from water to aqueous methanol and aqueous acetone was estimated from the kinetic data. Base-catalyzed hydrolysis of DMAC) and DEAC in aqueous methanol and aqueous acetone follows the first-order kinetic law with respect to hydroxide ion, k _obs= k ₂[OH]. The hydrolysis rate constants of DMAC and DEAC decrease as the fraction of methanol or acetone in the binary mixture rises, which is due to destabilization of OH^? ion. The high negative entropies of activation support the proposed mechanism involving formation of an intermediate complex and reflect rigidity and stability of the latter. Opening of the pyran ring in the intermediate complex is the rate-determining step.     

Stability constants of complexes formed by some metal ions with 2-(phenyliminomethyl)pyridine and 2-(o-hydroxyphenyliminomethyl)pyridine

The stability constants of some metal complexes of these two reagents have been determined.     

Arrhenius constants for the reactions of ozone with ethylene and acetylene

The kinetics of ozonation of C₂H₄ and C₂H₂ have been studied in the gas phase from ?40 to ?95°C (C₂H₄) and +10 to ?30°C (C₂H₂). The O₃ concentrations were near 10^?4 M, and the hydrocarbons were present in 2- to 25-fold excess. A few experiments with propylene were also carried out. The reactions were followed by observing the rate of decay of O₃ absorption at 2537 Å. Reaction stoichiometries and effects of added O₂ were investigated. The second-order rate constant for C₂H₄ was log k(M^?1 sec^?1) = (6.3 ± 0.2) – (4.7 ± 0.2)/θ (θ = 2.3RT). The rate was independent of the presence of excess O₂. Rate measurements for C₃H₆ were less accurate because of aerosol interference. Combined with room temperature measurements of other workers, the C₃H₆ rate constant was log k(M^?1 sec^?1) = (6.0 ± 0.4) – (3.2 ± 0.6)/θ. The C₂H₂ rate constant was log k(M^?1 sec^?1) = (9.5 ± 0.4) – (10.8 ± 0.4)/θ. In the case of C₃H₆ the major product was propylene ozonide. Ethylene did not yield the ozonide, and the products of the O₃–C₂H₄ and O₃–C₂H₂ reactions were not identified. Pre-exponential factors for the olefin reactions are consistent with a five-membered ring transition state formed by 1,3 dipolar cycloaddition of O₃. For C₂H₂, however, the much higher observed A factor suggests a different mechanism. Possible transition states for the O₃–C₂H₂ reaction are discussed.     

Reactions on polymers with amine groups. V. Addition of pyridine and imidazole groups with acetylenecarboxylic acids

Unsaturated macromolecular carboxybetaines were obtained by reaction of poly(4-vinylpyridine) and poly(N-vinylimidazole) with propiolic acid. A kinetic model was presented for 4-methylpyridine. It consists of three coupled reactions: neutralization, addition which involves two molecules of acid and leads to a cation–anion pair structure, where the cation results from the addition of the amine nitrogen to the triple bond of acid, and an equilibrium reaction between the ion-pair structure and the betaine structure. The addition rate was found to be higher for poly(4-vinylpyridine) than for poly(N-vinylimidazole); it was also higher in water than in a water–methanol mixture. The reaction with acetylenedicarboxylic acid was carried out on poly(N-vinylimidazole), but the transformed units showed the structure that results from propiolic acid. The betaine products from 4-methylpyridine did not polymerize by radical initiation. The polymeric products show characteristics of photocrosslinking polymers. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1615–1623, 1998     

Far-i.r. spectra of charge-transfer complexes between IX (X = Cl,Br) and some pyridine and quinoline bases

Far-i.r. spectra of charge-transfer (CT) complexes of IX (X = Cl, Br) with pyridine, quinoline and their derivatives have been measured in solids. Assignments for the intermolecular NI stretching and the modified IX stretching vibrations are proposed. The force constants for these vibrations have been calculated assuming a linear triatomic model, putting a donor molecule as a point mass. The relationship between the relative decrease in the IX stretching force constants and the pK_a values of donor molecules is discussed.     

Dialyzable carbosilane dendrimers as soluble supports for the functionalization of pyridine fragments via palladium-catalyzed coupling reactions

[reaction: see text] The use of carbosilane (CS) dendrimers as soluble supports in liquid phase organic synthesis (LPOS) is described. Control of the three key steps is perfectly achieved by covalently binding a pyridine fragment to the soluble support, modifying it via coupling reactions, and releasing it at the end. Nanofiltration (dialysis) allows facile purification of the supported molecules after each step.