Hammett Equation Research Articles

Synergic Effects of Ordered Mesoporous Bifunctional Ionic Liquid: A Recyclable Catalyst to Access Chemoselective N-Protected Indoline-2,3-dione Analogous

SBA-15 and organic ionic liquid were incorporated in a post-grafting technique for generating a bifunctional ionic liquid embedded mesoporous SBA-15. The prepared heterogeneous catalyst was employed for the first time to synthesize N-alkylated indoline-2,3-dione at mild conditions to afford excellent yields in a short reaction time. The synthesized DABCOIL@SBA-15 catalyst was meticulously characterized by various techniques, such as FT-IR, solid-state 13C NMR, solid-state 29Si NMR, small-angle X-ray diffraction (XRD), and N2 adsorption–desorption. Further, the morphological behavior of the catalyst was studied by SEM and TEM. The thermal stability and number of active sites were determined by thermogravimetric analysis (TGA). The Hammett equation was used to analyze the synergetic effect of the catalyst and substituent effects on the N-alkylated products of 5-substituted isatin derivatives, which resulted in a negative slope. This negative slope indicates a positive charge in the transition state. Notably, the DABCOIL@SBA-15 catalyst demonstrated its practicality by being reused for seven cycles with consistently high catalytic activity.

Acidity Constants of Boronic Acids as Simply as Possible: Experimental, Correlations, and Prediction.

The wide use of boronic compounds, especially boronic acids and benzoxaboroles, in virtually all fields of chemistry is related to their specific properties. The most important of them are the ability to form cyclic esters with diols and the complexation of anions. In both cases, the equilibrium of the reaction depends mainly on the acidity of the compounds, although other factors must also be taken into account. Quantification of the acidity (pKa value) is a fundamental factor considered when designing new compounds of practical importance. The aim of the current work was to collect available values of the acidity constants of monosubstituted phenylboronic acids, critically evaluate these data, and supplement the database with data for missing compounds. Measurements were made using various methods, as a result of which a fast and reliable method for determining the pKa of boronic compounds was selected. For an extensive database of monosubstituted phenylboronic acids, their correlation with their Brønsted analogues-namely carboxylic acids-was examined. Compounds with ortho substituents do not show any correlation, which is due to the different natures of both types of acids. Nonetheless, both meta- and para-substituted compounds show excellent correlation. From a practical point of view, acidity constants are best determined from the Hammett equation. Computational approaches for determining acidity constants were also analyzed. In general, the reported calculated values are not compatible with experimental ones, providing comparable results only for selected groups of compounds.

Changes in Mechanism and Transition State Structure for Solvolysis Reactions of Ring Substituted Benzyl Chlorides in Aqueous Solution.

Rate and product data are reported for the solvolysis reactions of twenty-seven mono, di (3,4) and tri (3,4,5) ring-substituted benzyl chlorides. The first order rate constant for solvolysis in 20% acetonitrile in water decrease from k solv = 2.2 s-1 for 4-methoxybenzyl chloride to 1.1 x 10-8 s-1 for 3,4-dinitrobenzyl chloride. The product rate constant ratios k MeOH/k TFE for solvolysis in 70/27/3 (v/v/v) HOH/TFE/MeOH range from a minimum of k MeOH/k TFE = 8 to a maximum of 110. The rate data were fit to a four-parameter Hammett equation that separates the resonance and polar effects of the aromatic ring substituents on the reaction rate. Increases in the values of the Hammett reaction constants and are observed as the substituent constants or are increased. A sharp decrease in the product selectivity k MeOH/k TFE = 26 for stepwise solvolysis of 4-methoxybenzyl chloride is observed as electron-withdrawing meta-substituents are added to 4-methoxybenzyl ring due to a Hammond-effect on the position of the transition state for solvent addition to the substituted 4-methoxybenzyl carbocation reaction intermediates. Sharp increases in the selectivity k MeOH/k TFE are observed with decreasing reactivity of other 3,4,5-subsituted benzyl chlorides due to anti-Hammond shifts on a two-dimensional More-O'Ferrall reaction coordinate diagram in the position of the transition state for a concerted solvolysis reaction.

Application of Hammett and Taft Equations Together on the Deprotonation Equilibriums of Isoxazolium Cations: A One Hour Graduate Classroom Teaching

Application of Hammett and Taft Equations Together on the Deprotonation Equilibriums of Isoxazolium Cations: A One Hour Graduate Classroom Teaching

Aromatic aldehyde oxidation by hexacyanoferrate(III) catalyzed by Ru(VI) in alkaline medium

In this study, the selective oxidation of aromatic aldehydes has been investigated using Ru(VI) as a catalyst and hexacyanoferrate (III) as a cooxidant in an alkaline medium. The reaction order with respect to the oxidant and the aromatic aldehyde is complex while the reaction order with respect to Ru(VI) is one. The proposed mechanism presents two catalytic cycles for the two active species of the catalyst. In each cycle, Ru(VI) species forms a complex with the anion of hydrated benzaldehyde which decomposes to form benzoic acid. The function of hexacyanoferrate(III) is the regeneration of catalyst species. A quantitative structure–activity relationship was analyzed using the Hammett equation for decomposition complex step. This analysis showed that deactivating groups in para- position enhances this process.

Tuning the acidity of halloysite by polyionic liquid to develop an efficient catalyst for the conversion of fructose to 5-hydroxymethylfurfural

In an attempt to prepare a low-cost and efficient acidic heterogeneous catalyst for the conversion of fructose to 5-hydroxymethylfurfural under mild reaction conditions, the acidity of halloysite was improved by covalent grafting of an acidic polyionic liquid. More precisely, halloysite was first vinyl functionalized and then polymerized with vinyl imidazole and 2-acrylamido-2-methylpropanesulfonic acid. The tangling imidazole rings were further converted to acidic ionic liquids by treating them with chlorosulfuric acid. UV–Vis spectroscopy and Hammett equation confirmed that conjugation of acid polyionic liquid resulted in the increase of the acidity of halloysite. Investigation of the efficiency of the catalyst for the synthesis of 5-hydroxymethylfurfural and optimization of reaction variables showed that 5-hydroxymethylfurfural was yielded in 97.8% after 30 min under the optimum conditions, i.e. catalyst loading of 20 wt% at 70 °C. Notably, the catalyst was highly reusable and it could be reused for at least seven reaction runs with insignificant loss of its activity. Furthermore, this catalyst could also promote the conversion of sucrose and maltose to give moderate yields of 5-hydroxymethylfurfural.

Remote substituent effects on catalytic activity of metal-organic frameworks: a linker orbital energy model

The Hammett equation is commonly used to theoretically depict the remote electronic effects of substituents on catalytic activities of metal nodes of metal-organic frameworks (MOFs). However, the application of the theory to MOF catalysts usually encounters problems because it relies heavily on empirical parameters with unknown transferability. To develop an alternative prediction theory, the linker orbital energy model has been proposed by density functional theory calculations. The model provides a simple method to approximately depict the remote electronic substituent effects on catalytic activities of metal nodes of MOFs, and its general applicability to MOFs is supported by extensively revisiting the structure-activity relationships reported in the literatures. The model can be used to design catalytic activity of metal nodes of MOFs by engineering the electronic properties of linkers and substituents.

Quantum Alchemy Based Bonding Trends and Their Link to Hammett's Equation and Pauling's Electronegativity Model.

We present an intuitive and general analytical approximation estimating the energy of covalent single and double bonds between participating atoms in terms of their respective nuclear charges with just three parameters, [EAB ≈ a - bZAZB + c(ZA7/3 + ZB7/3) ]. The functional form of our expression models an alchemical atomic energy decomposition between participating atoms A and B. After calibration, reasonably accurate bond dissociation energy estimates are obtained for hydrogen-saturated diatomics composed of p-block elements coming from the same row 2 ≤ n ≤ 4 in the periodic table. Corresponding changes in bond dissociation energies due to substitution of atom B by C can be obtained via simple formulas. While being of different functional form and origin, our model is as simple and accurate as Pauling's well-known electronegativity model. Analysis indicates that the model's response in covalent bonding to variation in nuclear charge is near-linear, which is consistent with Hammett's equation.

Reduction Potential Predictions for Thirty-Seven 1,4-di-N-Oxide Quinoxaline-2-Carboxamide Derivatives with Anti-Tuberculosis Activity

The ability for density functional theory with the B3LYP functional with the lanl2dz basis set to predict the 1st (Wave 1) and 2nd (Wave 2) reductions of the diazine ring in a series of thirty-seven (37) 1,4-di-N-oxide quinoxaline-2-carboxamide derivatives in dimethylformamide was examined. The B3LYP/lanl2dz method had a strong correlation and low correlation to the experimental potentials for Wave 1 and Wave 2, respectively. There are nine identifiable analogs based on similarities of structure. The predicted reduction potentials for the derivatives of each analog generally fit the modified Hammett equation. The B3LYP/lanl2dz method is shown to be useful in accurately predicting the Wave 1 potentials for quinoxaline-di-N-oxide derivatives. For derivatives with assessable anti-tuberculosis activity, the predicted Wave 1 potentials have a similar correlation with the bioactivity when compared to the experimental wave 1 potentials.

Novel Carbene Hydroxymethylene Derivatives: Gibbs free energy, NBO, AIM, and Hammett approaches via DFT and MP2 methods

Density functional theory (DFT) and the Møller-Plesset expansion (MP2) calculations were benchmarked to characterize the structure of novel derivatives of carbene hydroxymethylene. In this regard, eleven numbers of primary aldehydes including formaldehyde, acetaldehyde, formyl chloride, 2-chloroacetaldehyde, formyl fluoride, 2-fluoroacetaldehyde, formic acid, propionaldehyde, glyoxal, methyl formate, and formyl bromide (1H-11Br) were scrutinized, at the B3LYP/6-311++G(3d,3p) level of theory. Thereafter, the process of keto-enol tautomerism via a [1,2]-H-shift of 1H-11Br was monitored to reach their corresponding enol forms (1P-11P). For all species (except for one structure), the TS involves a triangle structure of C-O-H and yields cis or trans isomers of a singlet ground-state (S) carbene that contrasts the multiplicity of ordinary carbenes. The bond dissociation energy (BDE) calculations were carried out for the homolytic cleavage of C−H bonds of 1H-11Br. The AIM, as a promising analysis that definitely reveals the nature of the bond, was conducted to characterize the eigenvalues at the bond critical points (BCPs). In addition, the relation between the Hammett equation and thermodynamic parameters was obtained. Our investigation suggests new stable S derivatives of carbene hydroxymethylene that can be isolated.

Bis(thienyl)ethenes with α-methoxymethyl groups. Syntheses, spectroscopic Hammett plots, and stabilities in PMMA films.

A series of bis(thienyl)ethenes (BTEs) possessing perfluorocyclopentene backbones and methoxymethyl groups (MOM) in the 2/2'-positions of the thiophenes was prepared and examined. The substitution pattern of the 5/5'-positions was varied, covering the range from electron-donating to electron-withdrawing. The substituent effects of the absorption wavelengths of the ring-opened and the ring-closed isomers, which are interconverted by reversible 6π-electrocyclizations and cycloreversions, are studied by means of the spectroscopic Hammett equation and the Hammett-Brown equation. Excellent correlations of these linear free energy relationships were found, when the σp values of the Hammett equation, which summarize inductive, mesomeric and field effects, were replaced to the Hammett-Brown σp+ and σp- values which also take direct conjugation into account. We studied solvent effects on the spectroscopic properties and embedded the BTEs into polymethylmethacrylate (PMMA) coatings to examine their fatigue resistance. By our studies, the spectroscopic properties of BTEs can be adjusted by variation of the substitution pattern to a desired excitation wavelength for switching processes.

An Experimental Teaching Design for Learning Hammett Equation by Suzuki Coupling Reaction

An Experimental Teaching Design for Learning Hammett Equation by Suzuki Coupling Reaction

Supplementary data: Quantum Alchemy Based Bonding Trends and Their Link to Hammett's Equation and Pauling's Electronegativity Model

Supplementary data: Quantum Alchemy Based Bonding Trends and Their Link to Hammett's Equation and Pauling's Electronegativity Model

Supplementary data: From quantum alchemy to Hammett's equation: Covalent bonding from atomic energy partitioning

Supplementary data: From quantum alchemy to Hammett's equation: Covalent bonding from atomic energy partitioning

Correlation between Absorption and Substitution of Photochromic 1,2‐Bis(thienyl)ethenes (BTEs) Using Modified Spectroscopic Hammett Equations

AbstractSeries of photochromic 1,2‐bis(thienyl)ethenes possessing perfluorocyclopentene backbones, either hydrogen or methyl groups at the β‐positions of the thiophenes, and a variety of substituents in their α'‐positions were prepared, which cover the range from electron‐donating to electron‐withdrawing (Me, −CH2OH, −OTBS, −TMS, −Br, 1,3‐dioxan‐2‐yl, pyridin‐4‐yl, −CH2OH, −COOH). As a linear free energy relationship the spectroscopic Hammett equation gives fair to excellent fits to the excitation energy of the absorption maxima of the ring‐opened as well as the ring‐closed forms of the BTEs, when Hammett substituent constants σp were replaced by Brown's modified substituent constants σp+ and σp−. Vice versa, hitherto unknown Hammett‐Brown substituent constants can be estimated from the UV spectra. Furthermore, we compared the experimentally measured absorption maxima with values which we calculated by three different methods (DFT STEOM‐DLPNO‐CCSD/def2‐TZVPP, TD‐DFT ωB97X‐D3/6‐31G*, TD‐DFT ωB97X‐D3/6‐311++G**).

Is Hammett Correlation of Dipole Moments of <i>mono</i>-Substituted Benzenes Helpful to Distinguish <i>o,p</i>-Directing Groups from <i>m</i>-Directing Groups in an Aromatic Electrophilic Substitution? A Chemical Education Perspective

In chemistry literature it is so far achieved to distinguish o,p-directing groups from m-directing groups purely on the basis of only relative yields of ortho, para and meta electrophilic substituted products in the benzene ring and it is also based on their ability to disturb the pi-electron density at the carbons. It is for the first time in chemistry literature using dipole moment data a linear free energy relationship (LFER), Hammett equation is used to distinguish o,p-directing groups from m-directing groups. This could be achieved by constructing Hammett plots with dipole moments versus Hammett para substituent constants for electron donating groups and Hammett meta substituent constants for electron withdrawing groups. Good straight lines are obtained with correlation coefficients close to 0.9 with certainly an unmistakable trend.

Surface Tuning of Wood via Covalent Modification of Its Lignocellulosic Biopolymers with Substituted Benzoates-A Study on Reactivity, Efficiency, and Durability.

Chemical modification of wood applying benzotriazolyl-activated carboxylic acids has proven to be a versatile method for the durable functionalization of its lignocellulosic biopolymers. Through this process, the material properties of wood can be influenced and specifically optimized. To check the scope and limitations of this modification method, various benzamide derivatives with electron-withdrawing (EWG) or electron-donating (EDG) functional groups in different positions of the aromatic ring were synthesized and applied for covalent modification of Scots pine (Pinus sylvestris L.) sapwood in this study. The bonded amounts of substances (up to 2.20 mmol) were compared with the reactivity constants of the Hammett equation, revealing a significant correlation between the modification efficiency and the theoretical reactivity constants of the corresponding aromatic substitution pattern. The successful covalent attachment of the respective substituted benzamides was proven by attenuated total reflection infrared (ATR-IR) spectroscopy, while the stability of the newly formed ester bond was proven in a standardized leaching test.

Cerium Ammonium Nitrate-Mediated Access to Biaryl Lactones: Substrate Scopes and Mechanism Studies.

Herein we described an access to biaryl lactones from ortho-aryl benzoic acids via intramolecular O-H/C-H oxidative coupling with the commonly used cerium ammonium nitrate (CAN) as the one-electron oxidant under a thermal condition. The radical interrupting experiment suggested a radical process, while the kinetic isotope effect (KIE) showed that the C-H cleavage likely was not involved in the rate-determining step. Competitive reactions, especially the strikingly different ρ values of Hammett equations, indicated that the reaction rate was more sensitive to the electronic properties on the aryl moiety rather than the carboxylic moiety, which corresponded to the first single electron transfer (SET) step. In addition, the quite negative ρ values (-4.7) of the aryl moiety unveiled the remarkable electrophilic nature of the second intramolecular radical addition process, which was also consistent with product yields and regioselectivity. Moreover, control experiments disclosed that the single electron in the third step was also transferred to CeIV instead of molecular oxygen. Besides, the possible role of co-solvents trifluoroethanol (TFE) and its influences on the CeIV species were discussed. This work elucidated the possible mechanism by proposing the step that had more effects on the total reaction rate and the species that was responsible for the last single electron transfer.

The concept of substituent-induced force in the rationale of substituent effect.

Controlling the thermochemistry and kinetics of chemical reactions is a central problem in chemistry. Among factors permitting this control, the substituent effect constitutes a remarkable example. Here, we develop a model accounting for the effect of a substituent on the potential energy surface of the substrate (i.e., substituted molecule). We show that substituents affect the substrate by exerting forces on the nuclei. These substituent-induced forces are able to develop a work when the molecule follows a given reaction path. By applying a simple mechanical model, it becomes possible to quantify this work, which corresponds to the energy variation due to the effect of the substituent along a specific pathway. Our model accounts for the Hammett equation as a particular case, providing the first non-empirical scale for the σ and ρ constants, which, in the developed model, are related to the forces exerted by the substituents (σ) and the reaction path length (ρ), giving their product (σ · ρ) the well-known variation on the reaction energy due to the substituent.

Exploring Unusual Effects of the Ring Substituents in the Type II Reaction with TD-DFT and DFT.

The type II photoelimination reactions are important in biological systems. The different roles played by ring substituents are often not obvious from quantum yields for product formation and the rate constant for hydrogen abstraction is quite low (<107 s-1 ), but Hautala (Tetrahedron Lett. 1977, 18, 2499-2502) reported unusual effects of the nitro substituent in aryl ketones. They reported the first examples of such and discuss for another role played by the nitro group. They determined an upper limit of less than 2×10-5 to the quantum yield for formation of nitro substituent. They hypothesized the carbonyl localized state may be populated with high quantum efficiency, rapid deactivation occurs to populate the lower lying nitro localized state. We focus on the case where the theoretical studies are of great importance and indispensable, such as resolving the debate on the stepwise and concerted mechanism of hydrogen abstraction. Our proposed new mechanisms for 2-ethoxy-1-phenylethanone are based on the energy, HOMO-LUMO, dipole moments, etc. We provide modifications to experimentally proposed mechanisms or completely new mechanism with Hammett, synchronicity, singlet fission and relative rate equations to explain successfully the previous result. At the end, we show why an unusual effect of the nitro substituent exists in it.