Hammett Relationship Research Articles

Structure and tautomerism of 3(5)-amino-5(3)-arylpyrazoles in the solid state and in solution: An X-ray and NMR study

The crystal and molecular structures of five 3(5)-amino-5(3)-arylpyrazoles differing in the nature of the substituent at the para position of the phenyl ring ( 1 : X = H; 3 .H 2 O: X = OCH 3 ; 4 : X = Cl; 5 : X = Br and 6 : X = NO 2 ) have been determined by X-ray analysis. Three situations were detected in the crystal structures: the 3-tautomer is present in 1, 3 and 4 ; the 5-tautomer is only found in 6 and both tautomers (1:1) are observed in 5 . The crystal packings are governed by NH… N O hydrogen bonds and also by OH…N interactions in the monohydrate of 3 . It is worth noting that in 1, 3, 4 and 5 there are N-H…π(arene) contacts that might play a role in stabilizing the packing. Solid state 13 C NMR results are consistent with the above crystallographic conclusions, thus allowing to determine that the only compound for which no good crystals have been obtained, the p -methyl derivative 2 should be a 3-amino tautomer. NMR solution studies ( 1 H and 13 C) allow to determine the 3-amino/5-amino tautomeric equilibrium constant, K T , which obeys a Hammett relationship with σ p . Geometry optimizations of the 3 and 5-tautomers at semi-empirical level (AM1) were performed. In all compounds, the 3-tautomer has been found to possess a relatively lower energy by approximately 2 kcal mol −1 . The potential energy surface as a function of the hybridization of the amino group and its conformation have also been analyzed. The tautomerism a b has been studied in the solid state (X-ray and CPMAS NMR) and in solution (NMR) for X = H, CH 3 , OCH 3 , Cl, Br and NO 2 . There is a relationship between the equilibrium in solution and the tautomer present in the solid state.

Toward an Understanding of the High Enantioselectivity in the Osmium-Catalyzed Asymmetric Dihydroxylation. 4. Electronic Effects in Amine-Accelerated Osmylations

Electronic effects in osmylation reactions accelerated by pyridine and quinuclidine derivatives were investigated by varying the substituents on the amine ligand as well as on the alkene substrate. Ligand substituent effects were gauged by determination of the equilibrium constants for coordination of the amines to OsO4, evaluation of structural properties and reduction potentials of the amine−OsO4 complexes, and analysis of the kinetics of osmylations in the presence of the amines. Substrate substituent effects were gauged by kinetic Hammett studies using several different amine/alkene combinations. Nonlinear Hammett relationships resulting from alkene substituent effects were observed, and the deviation from a linear free energy relationship was found to depend on the structure, binding capacity, and concentration of the amine. The results were evaluated in terms of the contending “[3 + 2]” and “[2 + 2]” mechanisms currently under consideration. A change in mechanism that depends on the structural and e...

ChemInform Abstract: Transfer of Concepts from Homogeneous to Heterogeneous Catalysis: Use of Hammett Relationships to Assess Reaction Mechanisms and Nature of Active Sites in Reactions Catalyzed by Sulfides, Metals, Clays, and Zeolites

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Transfer of concepts from homogeneous to heterogeneous catalysis: use of Hammett relationships to assess reaction mechanisms and nature of active sites in reactions catalyzed by sulfides, metals, clays and zeolites

The application of Hammett or Hammett-type relationships to a series of various reactions catalyzed by clays, zeolites, sulfides and metals is reviewed in this paper. Such relationships are, of course, of great interest for a better knowledge of the reaction mechanisms over solid catalysts and on the nature of the active species. Among the different data obtained in the laboratory over those catalytic systems, it appears that Hammett slope ϱ values for the acid-catalyzed reactions involving Brønsted species in the rate-determining step (hydration, hydrolysis, isomerization, electrophilic aromatic substitution) are of the same order of magnitude for both homogeneous and heterogeneous reactions, whereas deviations are more frequently observed for reactions involving hydrogen.

Solvent effect on preferred protonation sites in nicotinate and isonicotinate anions

Potentiometric determinations of the two successive acidities of nicotinic and isonicotinic acids (AH2+) have been carried out in aqueous dimethyl sulfoxide mixtures containing up to 95% Me2SO by volume. In both systems, the results reveal that the addition of Me2SO induces a proton transfer from the pyridinium ring to the carboxylate group, the tautomeric equilibrium between the neutral forms of the two acids being displaced toward the zwitterionic form (AH±) in aqueous solution, but toward the molecular form (AH°) in Me2SO. An analysis of the data by means of Hammett relationships previously established for benzoic acids over the whole range of H2O/Me2SO mixtures allowed the four microscopic acidity constants as well as the tautomeric equilibrium constant KTpertaining to the complete ionization scheme of the two acids to be determined. At 20 °C, there are equal populations of the tautomeric AH° and AH±species in the mixtures containing 38 and 47% Me2SO for the nicotinic and isonicotinic systems, respectively. Hammett relationships describing the ionization behaviour of a number of substituted pyridinium cations in H2O/Me2SO mixtures are also discussed. Possible reasons accounting for the peculiar effects exerted by the NH2, CONH2, and COOH substituents on the process are suggested. Key words: nicotinic and isonicotinic acids, substituted pyridines, acidities, tautomeric equilibrium, protonation sites, water – dimethyl sulfoxide mixtures.

Linear free energy relation of methanofullerene C 61-substituents with cyclic voltammetry: Strong electron withdrawal anomaly

Methanofullerenes with cyano, nitro, and carboxyethyl substituents on C 61 were prepared and characterized. The bis(cyano) methanofullerene shows a first, irreversible, reduction peak at room temperature which is ca 160 mV more positive than the corresponding step in C 60. We report for the first time a comparative study of electron-donating and electron-withdrawing substituents on C 61, using the Hammett σ m , which shows that the cyano substituent appears to be more electron-withdrawing than predicted by the Hammett relation. This anomaly could be due to kinetics of a rapid follow-up reaction, periconjugation, or “conjugation” through the cyclopropane Walsh orbitals.

Sodium Cation Transport in Synthetic Channels Obeys the Hammett Relationship in a Phospholipid Bilayer Membrane

Sodium Cation Transport in Synthetic Channels Obeys the Hammett Relationship in a Phospholipid Bilayer Membrane

Thermal decomposition of arylnitramines

The thermal decomposition of various substituted N-methyl-N-nitroanilines dissolved in indifferent solvents and piperidine has been investigated. Activation volumes and product analyses support evidence that the rate-determining step is the reversible homolysis of the nitramine bond. The activation volumes range from +18 to +36 ml mol–1. A non-linear Hammett relationship is attributed to an increase in secondary caged reactions, namely rearrangement and oxidation. Arylnitramines with electron-donating substituents yield greater amounts of the thermal rearrangement products than those with electrondeactivating groups at ambient pressures. Decomposition of arylnitramines with electron-donating substituents under high pressures (ca. 1.2 GPa) favours caged reactions over separative diffusion.

Protonation constants of some substituted 2-hydroxy-1-naphthalideneanilines in dioxan-water mixtures

The protonation constants of ten Schiff's bases synthesized by the condensation of 2-hydroxy-1-naphthaldehyde with aniline and o-, m-, and p-substituted methylanilines, ethylanilines and methoxyanilines in various dioxan-water mixtures have been determined potentiometrically using an electrode system calibrated in concentration units of the hydrogen ion. The influence of substituents in the molecular structure on the protonation constants is discussed. The log K 2 values relating to the protonation of imine nitrogen atoms are in accordance with the Hammett relationship for all solvent compositions. A reverse relationship is observed between log K 2 for all Schiff's bases and the mole fraction of dioxan. However, the log K 1 values corresponding to the protonation of naphtholate O-ions have been found to increase with the increase in concentration of dioxan. Both electrostatic and non-electrostatic contributions influencing these constants are briefly discussed.

Epoxidation of Styrenes by Hydrogen Peroxide As Catalyzed by Methylrhenium Trioxide

Methylrhenium trioxide, CH{sub 3}ReO{sub 3}, catalyzes the oxidation of styrenes by hydrogen peroxide. Kinetic studies by three methods were carried out in acidic CH{sub 3}CN/H{sub 2}O (1:1 v/v) solutions. The catalytically-active species are the mono-peroxide, CH{sub 3}Rc(O){sub 2}(O{sub 2}), A, and the bis-peroxide, CH{sub 3}Re(O)(O{sub 2}){sub 2}, B, which epoxidize a given styrene at a similar rate. The rate constants are relatively insensitive to steric hindrance, but increase with the nucleophilicity of the styrene, electron-donating groups on the olefinic carbons or on the aromatic ring enhancing the rate. The rate constants for meta- and para-substituted styrenes follow a linear Hammett relationship; correlation with {sigma}{sup +} gave {rho} = -0.93 {+-} 0.05. In CD{sub 3}CN, epoxides were observed by {sup 1}H NMR spectroscopy. cis-{beta}-Methylstyrene and trans-{beta}-methylstyrene led to the cis epoxide and the trans epoxide, respectively. In acidic CH{sub 3}CN/H{sub 2}O, the major products were 1,2-diols. In some cases C-C bond cleavage products were also observed, the extreme case being {beta}-methoxystyrene where the C-C bond was completely cleaved to yield benzaldehyde, formaldehyde, and methanol. 32 refs., 7 figs., 2 tabs.

Substituent effect in reaction of dicyclohexylcarbodiimide with substituted benzoic acids

A curved Hammett relationship was observed for the reaction of substituted benzoic acids with dicyclohexylcarbodiimide in buffered solution. The reaction is promoted by electon-withdrawing substituents because of larger concentration of the acid anion present in the reaction mixture. On the other hand, electron-donating substituents encourage nucleophilic attack of the acid anion on the protonated carbodiimide.

Vitamin B‐6 model reactions. Part VI‐a Hammett relationship for the magnitude of pseudoallylic proton‐proton spin coupling constants in substituted azomethines

AbstractFour‐bond proton‐proton spin coupling constants were measured between the azomethine proton and the glycine CH protons of some coordinated glycine Schiff bases where the parent aldehyde is aromatic. Such systems are models for vitamin B6 enzyme substrate complexes. It is demonstrated that the coupling constants are not only related to the dihedral angles between coupled systems but they are also linearly related to the Hammett σ value of the substituents on the aromatic ring. A method is suggested for the calculation of dihedral angles between proton‐carbon bonds in these pseudoallylic systems.

Kinetics and mechanism of oxidation of substituted benzaldehydes by quinolinium fluorochromate

The kinetics of oxidation of a number of meta- and para-substituted benzaldehydes by quinolinium fluorochromate, QFC has been studied in aqueous acetic acid medium in the presence of acid. The products of oxidation are the corresponding benzoic acids. The reaction is first order each in substrate, QFC and HClO4. Electron-withdrawing substituents increase the rate, while electron-releasing substituents decrease it and the rate data obey Hammett's relationship. The reaction constant ϱ for the oxidation has a value of 1.16±0.07 at 30°C. The activation enthalpies and entropies are calculated and the possible mechanism for oxidation is discussed.

Studies on the mechanism of decomposition and structural factors affecting the aqueous stability of 1,2-bis(sulfonyl)-1-alkylhydrazines.

1,2-Bis(sulfonyl)-1-alkylhydrazines are highly active experimental antineoplastic agents which decompose with first-order kinetics in neutral aqueous solutions. These agents generate approximately 2 mol of the corresponding sulfinate, 1 mol of nitrogen, and 1 mol of the appropriate alcohol, produced as a consequence of the alkylation of water. Increasing the leaving-group ability of the sulfonyl moiety on N-1 shortens the half-life, while the converse happens with N-2 substitutions. Linear Hammett relationships are found for both types of substitutions. The predictable kinetics of decomposition makes these agents potential candidates for use in regional chemotherapy, where compounds with tunable short half-lives may offer some advantage. Prodrugs of extremely short-lived derivatives of this class may also have utility as targeted alkylating agents.

Effect of Substituent Groups in Acid-catalyzed Decomposition of 1-Aryl-1-phenylethane Hydroperoxides.

1-Aryl-1-phenylethane hydroperoxides (APE-HPO) were decomposed in the presence of sulfuric acid to give phenol, substituted phenols, acetophenone, and substituted acetophenones. The effect of a variety of substituents on the ratios of cited four kinds of products was investigated. The decomposition of APE-HPO, having an electron-donating substituent, gave much more substituted phenols and acetophenone than phenol and substituted acetophenones. The decomposition rates of APE-HPO, having various substituents, followed a Hammett relationship with a negative ρ value, -1.23 and it became apparent that the product distributions and the rates are controlled by the electron-donating magnitude of a substituent in the acid-decomposition of APE-HPO.

2‐Aminothiazole derivatives as Adsorption Inhibitors for corrosion of commercial copper and brass in acid solutions

AbstractCorrosion inhibition of commercial copper and cold‐worked and annealed α‐brass (Cu/Zn : 69/31) in acid chloride solutions by some 2‐aminothiazole derivatives was studied by weight loss, polarization and capacitance measurements. The study includes the effect of substituent type at position 4, the inhibitor concentration and pH on the efficiency of inhibition. The results showed that the inhibition occured via chemisorption of the inhibitor molecules on the corroding metal following Temkin's isotherm and the adsorption process involved the electron donation from the inhibitor to the metal surface. Quantitatively the inhibition efficiency was found to run parallel to the electron density on the thiazole molecule according to Hammett's relation and the sensitivity of the process is independent of the corroding metal. The inhibition efficiency was found to decrease significantly as the solution pH deceases due to the protonation of the inhibiting molecules.

Anchimeric assistance in the specific acid-catalysed hydration of benzonitriles

2-(Acylamido)benzonitriles react to give exclusively the corresponding 2-(acylamido)benzamides under relatively mild aqueous acid conditions. The kinetics for hydration at 50 °C in aqueous HCI with ionic strength kept at 1 mol dm–3 with KCl obey the equation rate =kH×[H+][nitrile] at low acid concentrations: at higher concentrations of acid the pseudo first order rate constants show a positive deviation from linearity. The rate constants (kH) for the substituted benzamido-benzonitriles fit a Hammett relationship. log kH=–0.36 ± 0.04σ– 3.49 ± 0.01 (r= 0.963)The reactivity due to the ortho amido group is more than 25 000 that of the unsubstituted benzonitrile under the same conditions. The mechanism is considered to involve protonation of the nitrile followed by rate limiting intramolecular attack of the amido oxygen to form an intermediate which rapidly breaks down to the acylamidobenzamide.

Isomeric complexes of ruthenium(II) with neutral heterocyclic schiff base ligands. High resolution proton resonance spectra of trans- cis isomeric pairs of RuX 2L 2 (L  2-arylpyridinecarboxaldimine, X  Cl, Br) and comparison of their physical properties

The reaction of 2-arylpyridinecarboxaldimine [RH 4C 6NC(H)Py, L ( 1)] with hydrated RuX 3 (X  Cl, Br) in boiling C 2H 5OH affords dark crystals of RuX 2L 2. Two geometrical isomers of the compound have been isolated and characterized by analytical and spectroscopic data. The trans isomer of RuCl 2L 2 shows a single sharp band for ν(RuCl), whereas two bands are observed for the corresponding cis isomer. The highresolution 1H NMR spectra of the isolated complexes are reported and completely assigned. All the complexes have multiple t 2→π *(L) transitions in the visible region. Each of the complexes display a quasi-reversible oxidative response due to an Ru III/Ru II couple in the range 0.25–0.40 V vs S.C.E. at a platinum working electrode. The formal potentials of this couple obey the Hammett relationship. The ligand-based irreversible oxidations are also briefly noted.

Kinetic studies of the azo coupling reaction between 1-(4-hydroxy-6-methylpyrimidin-2-yl)-3-methylpyrazolin-5-one and substituted benzenediazonium salts

Kinetic studies have been carried out on the azo coupling reactions between 1-(4-hydroxy-6-methylpyrimidin-2-yl)-3-methylpyrazolin-5-one and seven substituted benzenediazonium salts. The effects of acidity and temperature upon the rate have been investigated and activation parameters determined, and the involvement of deprotonated substrate in the rate determining step demonstrated. Acid dissociation equilibrium of the substrate is characterized by pKaHA= 5.07 ± 0.16 at 293 K and µ= 0.2. Close activation enthalpies, between 48 and 59 kJ mol–1 and negative entropies between –22 and –53 J mol–1 K–1 have been found, depending on the electronic effects of substituents on the benzene nucleus of the diazonium salts. A Hammett relationship log10k2= 1.0 ± 0.1 +(3.3 ± 0.3)σ holds for coupling reactions. The greater the electron-withdrawing effect, the faster is the coupling rate. 1-(2-Pyrimidinyl)pyrazolin-5-one behaves similarly to 1-arylpyrazolin-5-one and phenols in reactions with arenediazonium cations.

Rates of tritium exchange in a series of N-phenyl substituted pyrroles

Rate constants kH3O+exch for the H+-catalysed protiodetritiation of a number of N-phenyl substituted 2,5-dimethyl-3,4-[3H2]pyrroles have been measured in aqueous solution. For the series of N-(para-X-phenyl substituted) derivatives studied (1a–h), the rate constants are found to obey a satisfactory linear Hammett relationship with a negative reaction constant ρ of –0.90. The low ρ value is attributed to reduced transmission of the electronic influence of the X substituent caused by steric hindrance to conjugation between the phenyl and pyrrole rings. While decreasing the thermodynamic basicity at β-C of the pyrrole ring, ortho-dimethylation of the phenyl ring increases appreciably the rate of exchange, consistent with the idea that the resulting tetramethyl-substituted pyrrolium cation but not the transition state of reaction, is subject to inhibition of solvation of positive charge which develops on the nitrogen atom. Methoxy substitution of the phenyl ring gives results which reflect a balance between the opposite resonance and inductive effects of the OCH3 group, as well as steric effects around the methoxy-bearing positions.