Hammett Equation Research Articles

Substituent and solvent effects on the UV/vis absorption spectra of 5-(3- and 4-substituted arylazo)-4,6-dimethyl-3-cyano-2-pyridones

Absorption spectra of seventeen 5-(3- and 4-substituted arylazo)-4,6-dimethyl-3-cyano-2-pyridones have been recorded in 12 protic and aprotic solvents in the range 200–600 nm. The effects of substituents on the absorption spectra of these new azo dyes are interpreted by correlation of absorption frequencies with Hammett equation. The solute–solvent interactions were clarified on the basis of linear solvation energy relationships concept proposed by Kamlet and Taft. The 2-pyridone/2-hydroxypiridine tautomeric equilibration is found to depend upon substituents as well as on solvents.

Application of ChemDraw NMR Tool: Correlation of Program-Generated 13C Chemical Shifts and pKa Values of para-Substituted Benzoic Acids

An application of ChemDraw NMR Tool was demonstrated by correlation of program-generated 13C NMR chemical shifts and pKa values of para-substituted benzoic acids. Experimental 13C NMR chemical shifts were analyzed in the same way for comparison. The project can be used as an assignment at the end of the first-year organic chemistry course to review topics or explore new techniques: Hammett equation, acid–base equilibrium theory, electronic nature of functional groups, inductive and resonance effects, structure–reactivity relationship, NMR spectroscopy, literature search, database search, and ChemDraw software.

Selection of corrosion inhibitors

Data on the inhibition of corrosion of iron by methyl pyridines in HCl, H2SO4 and H2S solutions, by para-substituted anilines in HCl solution, and by ortho-substituted benzimidazoles in HCl solution, and on the inhibition of corrosion of aluminium by methyl pyridines have been analysed in terms of the Hammett equation and in terms of a new equation relating the degree of inhibition (log I/I0) with the fraction of the electronic charge due to the substituent in the inhibitor molecule (ΔΔN). The new relationship has been found to be useful in predicting new inhibitors offering a greater degree of inhibition than the currently known inhibitor systems.

Selectivity of lipases and esterases towards phenol esters

The selectivity of 28 lipases and esterases in the hydrolysis of butanoates of o-, m- or p-substituted phenols was investigated in a microtiterplate format. The phenols released during enzyme-catalyzed hydrolysis were converted in situ with Gibbs’ reagent to form a blue indophenol complex, which was quantified spectrophotometrically at 600 nm. Substantial differences in rates were found, which exhibits that the type and position of the substituent at the alkyl group has a strong influence on the selectivity of the enzymes. For various enzymes, the p-nitro derivative was the best substrate, whereas for other enzymes the m-Cl-derivative was preferentially hydrolyzed. Analysis of the data using the Hammett equation showed that sometimes the observed changes followed a predictable trend, but in several cases the result is very unexpected.

Kinetics and mechanism of oxidation of D‐fructose and D‐glucose by sodium salts of N‐(chloro)‐mono/di‐substituted benzenesulfonamides in aqueous alkaline medium

AbstractIn an effort to introduce N‐chloroarylsulfonamides of different oxidizing strengths, nine sodium salts of mono‐ and di‐substituted N‐chloroarylsulfonamides are employed as oxidants for studying the kinetics of oxidation of D‐fructose and D‐glucose in aqueous alkaline medium. The results are analyzed along with those by the sodium salts of N‐chlorobenzenesulfonamide and N‐chloro‐4‐methylbenzenesulfonamide. The reactions show first‐order kinetics each in [oxidant], [Fru/Glu], and [OH−]. The rates slightly increase with increase in ionic strength of the medium. Further, the rate of oxidation of fructose is higher by 4 to 5 times than that of the glucose oxidation, by the same oxidant. Similarly, Ea values for glucose oxidations are higher by about 1.5 times the Ea values for fructose oxidations. The results have been explained by a plausible mechanism, and the related rate law deduced. The significant changes in the kinetics and thermodynamic data are observed with change of substituent in the benzene ring. It is because Cl+ is the effective oxidizing species in the reactions of N‐chloroarylsulfonamides. The oxidative strengths of the latter therefore depend on the ease with which Cl+ is released from them. The ease with which Cl+ is released from N‐chloroarylsulfonamides depends on the electron density of the nitrogen atom of the sulfonamide group, which in turn depends on the nature of the substituent in the benzene ring. The following Hammett equations are valid for the oxidation of fructose and glucose, log kobs = −3.13 + 0.54 σp and log kobs = −3.81 + 0.28 σp, respectively. The enthalpies and entropies of activations for oxidations by all the N‐chloroarylsulfonamides correlate well with isokinetic temperatures of 301 K and 299 K, for fructose and glucose oxidations, respectively. The effect of substitution in the oxidants on the Ea and log A for the oxidations is also considered. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 572–582, 2005

Kinetic Regularities of Reactions of O,O-Diaryl Phosphorohydrazidothioates with Phenyl Isothiocyanate in Benzene

The effect of the temperature on the rate of reactions of O,O-diaryl phosphorohydrazidothioates with phenyl isothiocyanate in benzene is studied, and the activation parameters of the reactions are calculated. It is established that the reaction series under study obeys the isokinetic relationship with an enthalpy control and an isokinetic temperature of 690±24 K. The effect of electronically diverse substituents on phosphorus on the reactivity of O,O-diaryl phosphorohydrazidothioates is additive. It is described by the Hammett equation with σP constants.

A modification of the Hammett equation for predicting ionisation constants of p-vinyl phenols

Currently there are several compounds used as drugs or studied as new chemical entities, which have an electron withdrawing group connected to a vinylic double bond in a phenolic or catecholic core structure. These compounds share a common feature – current computational methods utilizing the Hammett type equation for the prediction of ionisation constants fail to give accurate prediction of p K a's for compounds containing the vinylic moiety. The hypothesis was that the effect of electron-withdrawing substituents on the p K a of p-vinyl phenols is due to the delocalized electronic structure of these compounds. Thus, this effect should be additive for multiple substituents attached to the vinylic double bond and quantifiable by LFER-based methods. The aim of this study was to produce an improved equation with a reduced tendency to underestimate the effect of the double bond on the ionisation of the phenolic hydroxyl. To this end a set of 19 para-substituted vinyl phenols was used. The ionisation constants were measured potentiometrically, and a training set of 10 compounds was selected to build a regression model ( r 2 = 0.987 and S.E. = 0.09). The average error with an external test set of six compounds was 0.19 for our model and 1.27 for the ACD-labs 7.0. Thus, we have been able to significantly improve the existing model for prediction of the ionisation constants of substituted p-vinyl phenols.

Quantitative Structure-Property Relationship: XX. Property-Property Correlation and Nonlinear Bronsted and Hammett Equations

A property-property correlation based on the previously suggested general expression for the quantitative structure-property relationship was obtained; it allows accurate estimation of many properties of organic compounds. Its connection with the Bronsted and Hammett equations was demonstrated.

Molecular Geometry as a Source of Chemical Information. 5. Substituent Effect on Proton Transfer in Para-Substituted Phenol Complexes with Fluoridea B3LYP/6-311+G** Study

The simplified model system [p-X-PhO...H...F](-), where -X are -NO, -NO(2), -CHO, -H, -CH(3), -OCH(3), and -OH, with various O...F distance was used to simulate the wide range of the H-bond strength. Structural changes due to variation of the substituent as well as the H-bond strength are well monitored by the changes in the aromaticity index HOMA and by two empirical measures of the H-bond strength-the (1)H NMR chemical shift of proton involved and the C-O bond length. Changes in H-bonding strengths and the position of proton transfer while shortening the O...F distance are well described by the Hammett equation.

Linear Free-Energy Relationships and the Density Functional Theory: An Analog of the Hammett Equation

Density functional theory has been applied to describe electronic substituent effects, especially in the pursuit of linear relationships similar to those observed from physical organic chemistry experiments. In particular, analogues for the Hammett equation parameters (sigma, rho) have been developed. Theoretical calculations were performed on several series of organic molecules in order to validate our model and for comparison with experimental results. The trends obtained by Hammett-like relations predicted by the model were found to be in qualitative agreement with the experimental data. The results obtained in this study suggest the applicability of similar correlation analysis based on theoretical methodologies that do not make use of empirical fits to experimental data can be useful in the study of substituent effects in organic chemistry.

Linear free energy relationships of the 1H and 13C NMR chemical shifts of 3-methylene-2-substituted-1,4-pentadienes

Linear free energy relationships (LFER) were applied to the 1H and 13C NMR chemical shifts ( δ N, N= 1H and 13C, respectively) in the unsaturated backbone of cross-conjugated trienes 3-methylene-2-substituted-1,4-pentadienes. The NMR data were correlated using five different LFER models, based on the mono, the dual and the triple substituent parameter (MSP, DSP and TSP, respectively) treatment. The simple and extended Hammett equations, and the three postulated unconventional LFER models obtained by adaptation of the later, were used. The geometry data, which are needed in Karplus-type and McConnell-type analysis, were obtained using semi-empirical MNDO-PM3 calculations. In correlating the data the TSP approach was more successful than the MSP and DSP approaches. The fact that the calculated molecular geometries allow accurate prediction of the NMR data confirms the validity of unconventional LFER models used. These results suggest the s- cis conformation of the cross-conjugated triene as the preferred one. Postulated unconventional DSP and TSP equations enable the assessment of electronic substituent effects in the presence of other interfering influences.

Multiple‐substituent effects on the isomerization of unsymmetrical cis‐1,3‐diphenyltriazenes

AbstractTrans‐to‐cis photoisomerization of unsymmetrical 4,4′‐disubstituted trans‐1,3‐diphenyltriazenes renders a non‐equilibrium mixture of two isomeric cis pairs which, as in the case of symmetrical 1,3‐diphenyltriazenes, undergo thermal cis‐to‐trans isomerization by means of 1,3‐prototropic rearrangements catalyzed by general acids and general bases. A quantitative analysis, by means of a multiple‐substituent Hammett equation, of the rate constants for restricted rotation around the N‐2—N‐3 bond in cis‐1,3‐diphenyltriazenes renders ρ values of −1.93±0.08 and 0.82±0.08 for N‐1 and N‐3, respectively. Copyright © 2005 John Wiley & Sons, Ltd.

Basicity of carboxylic acids: resonance in the cation and substituent effects

Basic properties of acetic, formic and substituted benzoic acids were investigated by the density functional theory at the levels B3LYP/6-311+G(d,p) and/or B3LYP/6-311++G(2d,2p). The protonated form prefers unsymmetrical conformation E,Z on the two partially double C–O bonds; only in the case of formic acid the E,E form is still observable. Lower basicity of carboxylic acids as compared to ketones is due to the higher energy of the cation while the effect in the uncharged acid molecule is negligible. Contribution of resonance and of the inductive effect was approximately estimated by means of relatively sophisticated model compounds. The inductive effect of the hydroxy group in the cation is the deciding factor for the lowered basicity. Resonance in the cation is strong, stronger than in the acid molecule or in the carboxylate anion, but cannot overcome the inductive effect. Relative basicities of meta- and para-substituted benzoic acids are controlled by the Hammett equation with marked deviations of the para donor substituents that have been not observable with the basicities in solution.

Neighbouring group processes in the deamination of protonated phenylalanine derivatives

The gas-phase fragmentation of protonated phenylalanine and a series of its derivatives (tyrosine, 4-methylphenylalanine, 4-aminophenylalanine, 4-methoxyphenylalanine, 4-tert-butylphenylalanine, 4-fluorophenylalanine, 4-chlorophenylalanine, 4-bromophenylalanine, 4-iodophenylalanine, 4-cyanophenylalanine, 4-nitrophenylalanine, 3-fluorophenylalanine, and 3,4-dichlorophenylalanine) were examined using a combination of low energy CID in a quadrupole ion trap mass spectrometer as well as DFT calculations and RRKM modelling. In particular, the relationship between the electron-donating ability of the substituent and the competitive losses of H2O + CO and NH3 were explored through the application of the Hammett equation. It was found that electron-donating substituents promote the loss of NH3, while electron-withdrawing substituents suppress the loss of NH3 and favour the H2O + CO loss fragmentation channel instead. These observations are consistent with a neighbouring group pathway operating for the loss of NH3. Molecular orbital calculation (at the B3LYP/6-31+G(d,p) level of theory) were also performed for a range of derivatives to compare the relative transition state energy barriers for three competing mechanisms: (i) the combined loss of H2O + CO, which is triggered by an initial intramolecular proton transfer from the ammonium group to hydroxyl OH, followed by the combined loss of H2O and CO to form an immonium ion; (ii) loss of NH3 via an aryl assisted neighbouring group pathway to yield a phenonium ion; (iii) loss of NH3 via a 1,2-hydride migration process, which results in the formation of a benzyl cation. The relative energy barriers for H2O + CO loss remain nearly constant, while that for both NH3 pathways increase as the substituent moves from electron-donating to electron-withdrawing. The relative transition state energy for loss of NH3 via the aryl assisted neighbouring group pathway is always lower than that of the 1,2-hydride migration process. RRKM modelling of the DFT predicted barrier heights suggest that the rate constants for H2O + CO loss are insensitive to the substituent on the ring, while the NH3 loss channels are greatly affected by the substituent. These theoretical results are consistent with the experimental observation of the relative yields of the competing fragmentation channels. Finally, comparisons with published gas phase and condensed phase studies on related systems are made.

Measurement of a linear free energy relationship one molecule at a time

A systematic study of the dehydrogenation of substituted thiophenols by controlled charge injection from the tip of a scanning tunneling microscope (STM) reveals a pronounced dependence of the reaction yield on the position and the chemical nature of the substituent. We evaluate the dehydrogenation rate of para-halo-substituted species within a linear free energy relationship, namely the Hammett equation. The resultant rho value of 1.4 can faithfully predict the reaction rates of molecules that are meta-halo-substituted or para-methyl-substituted. The positive sign of rho suggests a negatively charged transition state at the core of the STM-induced process, and the magnitude of the rho value indicates that the presence of the substrate does not preclude substantial substituent effects. The applicability of the Hammett equation to single-molecule chemistry offers facile prediction of the rate of STM-based single-molecule chemistry in a field, which so far has been addressed by focusing on involved quantum-mechanical modeling of its underlying processes.

Kinetics of Self-Decomposition and Hydrogen Atom Transfer Reactions of Substituted Phthalimide N-Oxyl Radicals in Acetic Acid

Kinetic data have been obtained for three distinct types of reactions of phthalimide N-oxyl radicals (PINO(.)) and N-hydroxyphthalimide (NHPI) derivatives. The first is the self-decomposition of PINO(.) which was found to follow second-order kinetics. In the self-decomposition of 4-methyl-N-hydroxyphthalimide (4-Me-NHPI), H-atom abstraction competes with self-decomposition in the presence of excess 4-Me-NHPI. The second set of reactions studied is hydrogen atom transfer from NHPI to PINO(.), e.g., PINO(.) + 4-Me-NHPI <=> NHPI + 4-Me-PINO(.). The substantial KIE, k(H)/k(D) = 11 for both forward and reverse reactions, supports the assignment of H-atom transfer rather than stepwise electron-proton transfer. These data were correlated with the Marcus cross relation for hydrogen-atom transfer, and good agreement between the experimental and the calculated rate constants was obtained. The third reaction studied is hydrogen abstraction by PINO(.) from p-xylene and toluene. The reaction becomes regularly slower as the ring substituent on PINO(.) is more electron donating. Analysis by the Hammett equation gave rho = 1.1 and 1.8 for the reactions of PINO(.) with p-xylene and toluene, respectively.

Solvent and Substituent Effects on the Protonation of Anilines in Dioxane–Water Mixtures

Protonation constants of a number of mono-substituted anilines were determined potentiometrically in 0, 20, 30, 40, 50, and 60% (v/v) dioxane–water mixtures at (25.00 ± 0.0.02)°C at an ionic strength of 0.1 mol-dm−3 in sodium perchlorate. The data from the potentiometric titrations were evaluated using the BEST computer program. The trends in the values of the protonation constants of anilines were explained in terms of the nature of substituent and the solute–solvent and solvent–solvent interactions. Furthermore, the effects of the substituents on the basicity of aniline, the additivies of these effects, and the applicability of the Hammett equation to the behavior of these substituents are discussed.

Kinetics and mechanisms of the reactions of benzoyl derivatives of nucleophiles: dependence of the solvation requirement of the reaction on the structures of the nucleophile and the acyl group

AbstractThe reactions of two series of benzoyl derivatives of nucleophiles were investigated and the results were compared with those for N‐benzoylimidazole. The general structure is ArCO‐Nu, where Ar = X‐C6H4‐ and/or X2‐C6H3‐; X = 4‐Me, H, 4‐Cl, 4‐CN, 4‐NO2 and X2 = 3,5‐dinitro; Nu = iodosobenzoate, ArCO‐Iba, and phosphate dianion, ArCO‐Phos. Catalytic rate constants, kinetic solvent isotope effects, kinetic substituent effects (Hammett equation) and the dependence of ΔH≠ and ΔS≠ on σX were determined. For ArCO‐Iba, the hydrolysis occurs via two pathways, uncatalyzed (kO) and specific base‐catalyzed (kOH) water attack on the iodine atom of the iodosobenzoate ring. This conclusion is based on theoretical calculations of the partial charges on ArCO‐Iba, and the small ρ values calculated at 25°C, −0.22 and 0.92 for kO and kOH, respectively. The data for the reaction of ArCO‐Phos are consistent with a dissociative transition state, leading to elimination of the metaphosphate monoanion (PO). The dependence of the mechanistic pathway on the nucleophile is discussed. Two results are relevant to the reactions of ArCO‐Iba: (i) moderate to large substituent effects on the activation entropies suggest that solvation of the leaving benzoate anion and desolvation of the entering nucleophile contribute to the OH−‐mediated reaction; (ii) the negative and positive signs of ΔS≠ indicate large differences in solvation of the transition states of the kO and kOH pathways. For the spontaneous decomposition of ArCO‐Phos, sizeable substituent effects on both ΔH≠ and ΔS≠ were observed. This shows the contribution of solvation of the leaving benzoate and substituent‐induced shift of the structure of the transition state. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Substituent and solvent effects on the UV/Vis absorption spectra of 5-(4-substituted arylazo)-6-hydroxy-4-methyl-3-cyano-2-pyridones

Absorption spectra of ten 5-(4-substituted arylazo)-6-hydroxy-4-methyl-3-cyano-2-pyridones have been recorded in fifteen solvents in the range 200–600 nm. The substituents at the phenyl nucleus are as follows: OH, OCH 3, CH 3, C 2H 5, H, Cl, Br, I, COOH and NO 2. The effects of substituents on the absorption spectra of investigated compounds are interpreted by correlation of absorption frequencies with simple Hammett equation. The effects of solvent polarity and solvent/solute hydrogen bonding interactions are analyzed by means of linear solvation energy relationships concept proposed by Kamlet and Taft. The azo-hydrazone tuatomeric equilibration is found to depend upon substituents as well as on solvents.

Electron transfer reaction of oxo(salen)chromium(V) ion with anilines.

The kinetics of oxidation of 16 meta-, ortho-, and para-substituted anilines with nine oxo(salen)chromium(V) ions have been studied by spectrophotometric, ESIMS, and EPR techniques. During the course of the reaction, two new peaks with lambda(max) at 470 and 730 nm appear in the absorption spectrum, and these peaks are due to the formation of emeraldine forms of oligomers of aniline supported by the ESIMS peaks with m/z values 274 and 365 (for the trimer and tetramer of aniline). The rate of the reaction is highly sensitive to the change of substituents in the aryl moiety of aniline and in the salen ligand of chromium(V) complexes. Application of the Hammett equation to analyze kinetic data yields a rho value of -3.8 for the substituent variation in aniline and +2.2 for the substituent variation in the salen ligand of the metal complex. On the basis of the spectral, kinetic, and product analysis studies, a mechanism involving an electron transfer from the nitrogen of aniline to the metal complex in the rate controlling step has been proposed. The Marcus equation has been successfully applied to this system, and the calculated values are compliant with the measured values.