Hammett Equation Research Articles

Concentrated salt effects on solvolysis rates in 1,4-dioxane–H2O mixed solvent and the expansion of the use of the Hammett equation for salt effects

In the mixed solvent system 50 vol% 1,4-dioxane and H2O, the concentrated salt effects of alkali metal (M+) and alkaline earth metal (M2+) perchlorates on the solvolysis rates of aliphatic halides (RX) and related compounds were investigated. The “pseudo” first-order reaction rates (k/s−1) of typical SN1 substrates, 1-adamantyl chloride and bromide and tert-butyl chloride, increased exponentially with increasing concentration of metal salt. The effects of M2+ on the reaction rates were larger than those of M+: Na+, Li+ < Ba2+ < Mg2+. A non-metallic salt, Et4NBr, at concentrations of ≥1.0 mol dm−3 caused the log k value to decrease, although the log k was slightly increased by lower concentrations of the salt (<1.0 mol dm−3). Raman spectra of D2O containing high concentrations of Et4NBr suggested a complete distortion of the solvent structure. The increase in log k values in the presence of metal ions was explained by direct “chemical” interaction between halide ions and metal ions with the concentrated salts in the modified media: X−⋯M+ or X−⋯M2+. The solvolysis rates of typical SN2 substrates, ethyl bromide and methyl tosylate, were much decreased by the addition of concentrated LiClO4. The log k values of borderline SN1–SN2 reacting substrates, isopropyl bromide and benzyl halides, remained almost constant, however, more positive slopes in log k in the presence of LiClO4 were caused by the introduction of methyl substituents on benzyl halides; negative slopes were observed upon the introduction of strong electron-withdrawing substituents, such as NO2 and CN. We observed a correlation (of two different slopes) between the σ+ values in the Hammett equation and the Δlog k for substituted benzyl halides upon the addition of 1.0 mol dm−3 LiClO4, as well as between the log (kx/kH) values themselves. We propose here an expansion of the use of the Hammett equation for the concentrated salt effect.

Protonation of maleic and fumaric acid in aqueous sulfuric acid solutions

The protonations of maleic and fumaric acid in an acidic medium (aqueous solutions of sulfuric acid) were followed spectrophotometrically at room temperature. The acid-base equilibria were characterised qualitatively and quantitatively. The pKBH+ values were determined using the Hammett equation, employing several acid functions in order to determine which of them describes best the protonation process of the studied organic acids. The thermodynamic pKBH+ values as well as those of the solvation parameters m, m* and ? and of the thermodynamic protonation constants (or, rather, the pKa,p values) were also defermined. The method of characteristic vector analysis (CVA) was used to reconstruct the experimental spectra.

Synthesis and properties of stabilized bis(2-oxo-2H-cyclohepta[b]furan-3-yl)phenylmethyl and bis(1,2-dihydro-2-oxo-N-phenylcyclohepta[b]pyrrol-3-yl)phenylmethyl cations and their derivatives: remarkable substituent effect on the conformation and stability of the cations

The reactions of benzaldehyde and 4-substituted benzaldehyde with 2H-cyclohepta[b]furan-2-one 5 and 1,2-dihydro-N-phenylcyclohepta[b]pyrrol-2-one 6 in TFA–CH2Cl2 afford two series of bis(2-oxo-2H-cyclohepta[b]furan-3-yl)arylmethanes 7a–e and bis(1,2-dihydro-2-oxo-N-phenylcyclohepta[b]pyrrol-3-yl)arylmethanes 8a–e, respectively. Upon oxidation reaction with DDQ followed by addition of 60% HPF6, the methane derivatives 7a–e and 8a–e are converted to the corresponding methyl cations 9a–e·PF66−− and 10a–e·PF66−−, respectively. The stability of cations 9a–e and 10a–e is expressed by the pKR+ values, which are determined spectrophotometrically, as 12.4 to 7.9 and 13.9 to 11.1, respectively. The electrochemical reductions of 9a–e and 10a–e exhibit reversible waves and low reduction potentials at −0.55 to −0.23 and −0.72 to −0.44 (V vs. Ag/Ag+), respectively, upon cyclic voltammetry (CV); the values are lower than those of the related compounds, diazulen-1-ylarylmethyl cations. Good linear correlations between the pKR+ values and E1red potentials of 9a–e and 10a–e are obtained. These values (pKR+ and E1red) are dependent on the substituents on the phenyl group; however, they do not correlate with the substituent constants of the Hammett equation. This feature is ascribed to the difference in the most stable conformation of the three aromatic groups in the cations; the phenyl group having a more electron-donating substituent is prone to become more planar to the reference plane. The most stable conformation is deduced on the basis of MO calculations (MOPAC, AM1 method). The 1H NMR and UV–vis spectral studies also rationalize the conformational change of the three aromatic rings, depending on the substituent on the benzene ring. On the basis of the study, the stabilizing ability of the 4-substituted phenyl group and heteroazulenes for the cations is clarified to be in the order of 4-dimethylaminophenyl > 1,2-dihydro-2-oxo-N-phenylcyclohepta[b]pyrrol-3-yl > 2-oxo-2H-cyclohepta[b]furan-3-yl ≈ 4-methoxyphenyl.

Metathesis Reactions of Tris(adamantylimido)methylrhenium and Aldehydes and Imines

The tris(imido)methylrhenium compound CH3Re(NAd)3 (Ad = 1-adamantyl) was prepared and characterized. It reacts with aromatic aldehydes ArCHO forming the imines ArCHNAd. The reaction occurs in three stages, during which CH3Re(NAd)2O and CH3Re(NAd)O2 could be detected. In the third and slowest stage CH3ReO3 (MTO) was formed, eventually in quantitative yield. The second-order rate constant for PhCHO in C6D6 at 298 K is 1.4 × 10-4 L mol-1 s-1. Electron-donating substituents at the para-position of ArCHO cause a significant diminution in rate. Treated by the Hammett equation, the reaction constant is ρ = +0.90. The reactions between CH3Re(NAd)3 and linear aliphatic aldehydes occur much faster than do reactions of nonlinear aliphatic or aromatic aldehydes, indicating an important steric effect. Ketones do not react. The imidorhenium complex evidently undergoes a metathesis reaction with the aldehyde. Analogously, CH3Re(NAd)3 reacts with imines. Imine−imine metathesis is catalyzed by MTO homogeneously and by MTO supported on Nb2O5.

The relationship between surface derivatization and the physical properties of bisarylphosphinidenetriironnonacarbonyl clusters

A series of clusters, Fe 3(CO) 9(μ 3-P– p-C 6H 4X)(μ 3-P– p-C 6H 4X′) (X=X′=NMe 2, OCH 3, CH 3, Cl, CF 3, CN; X=CN, X′=NMe 2) has been prepared using functionalized p-XC 6H 4PCl 2 starting materials. The influence of changes in the X groups on the physical properties of the Fe 3(CO) 9 cluster core are examined. This cluster series displays two one-electron reductions to produce stable radical monoanions and diamagnetic dianions, allowing the behavior of all three species to be examined. Infrared, 31P and 1H NMR, and electron paramagnetic resonance (of the radical monoanion) spectroscopies as well as the structural characterization of two members of the series, Fe 3(CO) 9(μ 3-P– p-C 6H 4X)(μ 3-P– p-C 6H 4X′) (X=X′=NMe 2; X=CN, X′=NMe 2) by single crystal X-ray diffraction methods all indicate that the cluster structures are invariant with the nature of X. The Hammett linear free energy relationship is used to quantify the observed changes in cluster properties. Analysis of infrared, electron paramagnetic resonance and cyclic voltammetry data by the Hammett equation quantify the communication between the cluster surface and the Fe 3(CO) 9 core. Fenske–Hall molecular orbital calculations were used to assess the important orbital interactions between the substituted phenyl rings and the Fe 3(CO) 9 cluster core.

Coupling of Cyclobutenediones with Fischer Carbene Complexes: A One-Step Synthesis of Cyclopentenediones and/or 5-Alkylidenefuranones via Net Insertion of the Carbene Unit into a C−C Bond1

The reaction of Fischer carbene−chromium complexes with 3-cyclobutene-1,2-diones has been investigated. In most cases, the major product of the reaction is the C−C bond insertion product, a 2-alkoxy-4-cyclopentene-1,3-dione, accompanied by a minor amount of the partial deoxygenation product, a 4-cyclopentene-1,3-dione. In some cases, 5-alkylidenefuranones are also formed. A mechanism involving oxidative addition of the coordinatively unsaturated Fischer carbene complex followed by acyl migration and reductive elimination was proposed to account for cyclopentenedione formation. Furanone formation was thought to arise via demetalation of the acyl migration product, followed by O-acylation. An electronic dependence was noted for cyclopentenedione/alkylidene−furanone ratio, which was evaluated using the Hammett equation.

Spectral and kinetic studies of the oxidation of monosubstituted phenols and anilines by recombinant Synechocystis catalase-peroxidase compound I.

A high-level expression in Escherichia coli of a fully active recombinant form of a catalase-peroxidase (KatG) from the cyanobacterium Synechocystis PCC 6803 is reported. Since both physical and kinetic characterization revealed its identity with the wild-type protein, the large quantities of recombinant KatG allowed the first examination of second-order rate constants for the oxidation of a series of aromatic donor molecules (monosubstituted phenols and anilines) by a bifunctional catalase-peroxidase compound I using the sequential-mixing stopped-flow technique. Because of the overwhelming catalase activity, peroxoacetic acid has been used for compound I formation. A >/=50-fold excess of peroxoacetic acid is required to obtain a spectrum of relatively pure and stable compound I which is characterized by about 40% hypochromicity, a Soret maximum at 406 nm, and isosbestic points between the native enzyme and compound I at 357 and 430 nm. The apparent second-order rate constant for formation of compound I from ferric enzyme and peroxoacetic acid is (8.74 +/- 0.26) x 10(3) M(-)(1) s(-)(1) at pH 7. 0. Reduction of compound I by aromatic donor molecules is dependent upon the substituent effect on the benzene ring. The apparent second-order rate constants varied from (3.6 +/- 0.1) x 10(6) M(-)(1) s(-)(1) for p-hydroxyaniline to (5.0 +/- 0.1) x 10(2) M(-)(1) s(-)(1) for p-hydroxybenzenesulfonic acid. They are shown to correlate with the substituent constants in the Hammett equation, which suggests that in bifunctional catalase-peroxidases the aromatic donor molecule donates an electron to compound I and loses a proton simultaneously. The value of rho, the susceptibility factor in the Hammett equation, is -3.4 +/- 0.4 for the phenols and -5.1 +/- 0.8 for the anilines. The pH dependence of compound I reduction by aniline exhibits a relatively sharp maximum at pH 5. The redox intermediate formed upon reduction of compound I has spectral features which indicate that the single oxidizing equivalent in KatG compound II is contained on an amino acid which is not electronically coupled to the heme.

Preparation of aromatic fluorides: facile photo-induced fluorinative decomposition of arenediazonium salts and their related compounds using pyridine– nHF

By employing pyridine– nHF solution, the photo-induced fluorinative decomposition of arenediazonium salts (ArN 2BF 4) (fluoro-dediazoniation) and the related compounds, such as quinonediazides and triazenes, has been successfully carried out to produce the corresponding aromatic fluorides (ArF) in good yields. The rate in the fluoro-dediazoniation of para-substituted ArN 2BF 4 in pyridine– nHF solution did not obey the classical Hammett equation but conformed well to Taft's treatment [dual substituent parameter relationships (DSP)]. In the thermal fluoro-dediazoniation of ArN 2BF 4 the rate of reaction was significantly influenced by the substituents in the substrates. On the contrary, only a slight effect by the substituents was observed on the rate of the photo-induced fluoro-dediazoniation of ArN 2BF 4.

A quantitative study of substituent effects on oxidative cyclization of some 2‐aryl‐substituted aldehyde thiosemicarbazones induced by ferric chloride and cupric perchlorate

AbstractAs a development of our previous work, we performed a kinetic study of the oxidative cyclization reaction of some 2,4‐diaryl‐substituted aldehyde thiosemicarbazones 1a‐n induced by ferric chloride and by cupric perchlorate. The results of cyclization of 1a‐n were compared to those of the corresponding 2‐methyl derivatives. The kinetic data were analyzed by means of the Hammett's equation.

Voltammetry of 6,6'-dithiodinicotinic acid on a self-assembled phospholipid monolayer prive

This paper reports a voltammetric study of 6,6'-dithiodinicotinic acid (CPDS) across a biomimetic membrane system consisting of a monolayer of dioleoylphosphatidylcholine, deposited on mercury. Because of the low solubility of this compound and its potential decomposition in alkaline media, estimation of pK values for the carboxyl and amino groups of the pyridine ring of the CPDS entailed using the Hammett equation. UV spectra seem to confirm the presence of the dianionic form of CPDS above pH=3-4. Differential capacity and cyclic voltammetry measurements were made in order to characterize the voltammetric behavior directly on mercury and through a monolayer of dioleoylphosphatidylcholine. Estimation of the CPDS hydrophobicity degree from the partition coefficient in octanol/ water suggests no penetration of the dianion into the monolayer and supports the fact that the named dianion undergoes protonation at the phospholipid/solution interface to give a neutral specie which penetration into the phospholipid region, favored by its higher hydrophobicity, is followed by electrochemical reduction at the mercury surface.

Quantum Molecular Similarity. 1. BCP Space

We propose a new similarity measure operating in an abstract space spanned by properties evaluated at bond critical points defined by the theory of Atoms in Molecules. Consequently, we represent molecules compactly and reliably, extracting the relevant information from their ab initio wave function. Typical problems of continuous quantum similarity measures are thereby avoided. The practical use of this novel method is adequately illustrated via the Hammett equation for para and meta substituted benzoic acids. On the basis of our definition of distances between molecules in BCP (Bond Critical Point) space, we are able to reproduce the experimental sequence of acidities determined by the well-known σ constant of a set of substituted congeners. Moreover, our approach points out where the common reactive center of the molecules is. Due to these promising results we embark on a research program systematically addressing further issues outlined in this work. The generality and feasibility of our approach will enable predictions in medically related QSARs.

99/00859 Changes of the actual density of mineral matter of coals during their pyrolysis : Strugala, A. Karbo-EnergoChem.-Ekol., 1998, 43, (3), 105–112. (In Polish)

A method of IR analysis was used in a kinetic study of reactions of dianhydrides of 1,3-bis-(3,4-dicarboxyphenoxy)benzene and 3,3′,4,4′-tetracarboxybenzophenone with aniline and its p-methyl-, p-phenoxy-, p-bromo-, p-nitro derivatives and tetranuclear bridge-containing diamines in N,N-dimethylacetamide (DMAA) at 298 K. A PMR study was made of the kinetics of cyclization of o-carboxyamides prepared from the dianhydride of 1,3-bis(3,3-dicarboxyphenoxy)benzene and the above-mentioned diamines in DMAA solution at 433 K. The effect of a substituent in the nucleophile (amine) on rates of acylation with tetracarboxylic dianhydrides is described by a Hammett equation with p=−3·6 and −3·4 respectively. The rate of cyclization is only slightly depend on the initial amine structure. The p value for cyclization of the model o-carboxyanilides is equal to −0·2. In the case of the studied flexible-chain polymers the rates of formation and cyclization of polyamic acids depend on factors relating to the electronic structure of the reacting functional groups.

Mixed Self-Assembled Monolayers of Highly Polar Rigid Biphenyl Thiols

Mixed self-assembled monolayers (SAMs) of 4‘-nitro-4-mercaptobiphenyl (I) and 4‘-(dimethylamino)-4-mercaptobiphenyl (III) have been compared with that of 4‘-nitro-4-mercaptobiphenyl (I) and 4‘-methylmercapto-4-mercaptobiphenyl (II). The composition of the mixed SAMs was determined by external reflection Fourier transform infrared (ER−FTIR) spectroscopy and showed that both the magnitude and direction of dipole moment determined the final composition of mixed SAMs. In toluene, with a dielectric constant of 2.4, a significant enhancement dipolar interaction is observed. The composition of mixed SAMs of I and II showed a plateau at 40% surface concentration of I. These results can be explained by simple calculation based on the Hammett equation. The experimental results indicate that the equilibrium concentration of the two components in the mixed SAMI/II, in a nonpolar solvent, is driven by the formation of a two-dimensional assembly with zero net dipole moment.

A correlation between solvatochromic solvent polarity parameters and the ionization constants of various phenols in 1,4-dioxane-water mixtures

Precise thermodynamic ionization constants K for 3-nitrophenol, 3,4-dichlorophenol, and 4-cyanophenol have been obtained in 1,4-dioxane-water mixtures (0–70% volume fraction in dioxane) at 25°C using a potentiometric method. The same information for another twelve cationic, neutral, and anionic phenols were taken from the literature. Three different methods were used to study the effects of the solvents on the ionization constants: one involves a single polarity parameter, E T(30); the next involves the Kamlet–Taft multiparametric method; and the last involves the preferential solvation model. The pK values follow the preferential solvation model, but the parameters obtained are highly correlated. Using the data for the phenol molecule as reference, a linear correlation between ΔpK and E T(30) has been used to develop a new method of obtaining pK values for any binary solvent composition, with only the pK in water known. The pK(s) values correlate with the molecular parameters for the dipolarity/polarizability of the solvent π* and its hydrogen-bond donor ability α. The preferential solvation parameter, f 12/1, correlates with the parameter for the hydrogen-bond donor ability of the solvent. All the phenols follow Hammett's equation and the reaction constants have been calculated for the different water–dioxane mixtures.

Substituent Effects on Conformational Equilibria: Substituted Thiobenzanilides

Infrared absorption spectra of 38 thiobenzanilides in tetrachloromethane solutions were recorded in the ν(N-H) region. From the integrated absorption intensity of the computer-resolved N-H bands, the abundance of the Z and E conformations was calculated. The values of pertinent log KC depend on substitution in an unexpected way which cannot be described by the Hammett equation or by any other known type of modified constants σ. The only regularity observed is that the substituent effects from the two benzene rings of thioanilide are additive with a remarkable precision. The ν(N-H) frequencies are not much sensitive to substitution and the dependence is not simple: only ν(N-H) of the E conformer depend on substitution in a regular way.

Molecular basis of quantitative structure-properties relationships (QSPR): a quantum similarity approach.

Since the dawn of quantitative structure-properties relationships (QSPR), empirical parameters related to structural, electronic and hydrophobic molecular properties have been used as molecular descriptors to determine such relationships. Among all these parameters, Hammett sigma constants and the logarithm of the octanol-water partition coefficient, log P, have been massively employed in QSPR studies. In the present paper, a new molecular descriptor, based on quantum similarity measures (QSM), is proposed as a general substitute of these empirical parameters. This work continues previous analyses related to the use of QSM to QSPR, introducing molecular quantum self-similarity measures (MQS-SM) as a single working parameter in some cases. The use of MQS-SM as a molecular descriptor is first confirmed from the correlation with the aforementioned empirical parameters. The Hammett equation has been examined using MQS-SM for a series of substituted carboxylic acids. Then, for a series of aliphatic alcohols and acetic acid esters, log P values have been correlated with the self-similarity measure between density functions in water and octanol of a given molecule. And finally, some examples and applications of MQS-SM to determine QSAR are presented. In all studied cases MQS-SM appeared to be excellent molecular descriptors usable in general QSPR applications of chemical interest.

Linear free energy relationship in complex reaction: Tungsten(VI) catalyzed perborate oxidation of S-Phenylmercaptoacetic acids

Tungsten(VI) catalyzes perborate oxidation of S-phenylmercaptoacetic acid. The catalyzed oxidation is first order with respect to the oxidant, independent of [H+], and displays Michaelis–Menten dependence on [PhSCH2COOH] and [W(VI)]. The reaction requires time for equilibration between the oxidant and the catalyst. Oxodiperoxotungsten(VI) (WO(O2)2) is the probable oxidizing species, and decomposition of PhSCH2COOHWO(O2)2 complex is rate limiting. At low [PhSCH2COOH] and fixed [W(VI)], the oxidation follows a second order rate law. Operation of the linear free energy relationship in the oxidation has been tested with a few electron withdrawing and a few electron donating substituents. At high temperature, electron-withdrawing groups decelerate and electron releasing groups accelerate the oxidation, the variation in rate with the substituent is in conformity with the Hammett equation. But at low temperature, electron donating substituents fail to accelerate the oxidation. The results are rationalized. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 675–681, 1999

Protonation of Some 5-Substituted Di(2-thienyl) Ketones in Sulfuric Acid. A Comparison with Other 2-Thienyl and Phenyl Ketones

Protonation equilibria of some 5-substituted di(2-thienyl) ketones have been investigated spectrophotometrically in aqueous solutions of sulfuric acid at 298 K. The experimental pKBH+ values have been analyzed by means of the Hammett equation. The calculated ρ as well as the pKBH+ and m* values have been compared with those for substituted 2-acetyl- thiophenes and phenyl 2-thienyl ketones.

Reactions of Carbonyl Compounds in Basic Solutions. Part 34. The Mechanism of the Base-Catalysed Ring Fission of 2,3-Diphenylcycloprop-2-en-1-one

The rate coefficients for the base-catalysed ring fission of a series of 2-phenyl-3-(2-, 3- or 4-substituted phenyl)cycloprop-2-en-1-ones to give the corresponding (E)-2,3-diphenylacrylic acids have been determined in water at 30.0 °C, as well as for the unsubstituted compound at 40.0, 50.0 and 60.0 °C. The effects of meta- and para-substituents on the rates have been correlated using the Hammett equation to give a reaction constant, ρ, equal to ca 1.2 at 30 °C. For the unsubstituted compound, the activation parameters have been calculated and the kinetic solvent isotope effect has been studied. The effects of ortho-substituents on the rates appear to be mainly polar, rather than steric, in origin. The evidence indicates a mechanistic pathway which proceeds by addition of hydroxide anion to the ketone, which is rate-determining. The adduct suffers ring fission to give the final product via a carbanionic intermediate.

Role of hydrophobic effects in mechanistic QSAR

To extend the successful application of Hammett equations, previously used to predict equilibrium and rates of physico-chemical reactions with electronic and steric parameters, to the realm of biology and biochemistry, a parameter that measures hydrophobicity is required. The partition coefficient of a solute between octanol and water, expressed in log terms to put it on the same free-energy basis as the classic Hammett parameters, has been shown to be widely applicable. It is directly involved in passive transport through membranes, in binding to proteins, and in specific binding at active sites in enzymes. Methods of calculating logP(octanol) that reflect the solvation forces involved, can be useful in elucidating unusual solute conformations that may be preferred in a non-polar environment.