Hammett Equation Research Articles

Kinetics of chlorination of phenol and monosubstituted phenols by t-butyl hypochlorite in aqueous alkaline medium

The kinetics of chlorination of the parent and sixteen monosubstituted phenols (2-chloro, 2-methyl, 2-carboxy, 2-nitro, 3-chloro, 3-methyl, 3-carboxy, 4-fluoro, 4-chloro, 4-bromo, 4-methyl, 4-ethyl, 4-methoxy, 4-carboxy, 4-acetyl and 4-nitro) by t-BuOCI have been studied in aqueous alkaline medium. The rates of reactions show first order kinetics each in [t-BuOCl] and [XC 6 H 4 OH] and inverse first order in [OH - ]. Variation in either ionic strength or addition of reaction product has no significant effect on the rates of reactions, while lowering of the dielectric constant of the medium increases the rate. The rates are measured at different temperatures and the activation parameters for all the phenols computed. A mechanism involving the electrophilic attack of phenoxide ions by HOCl in the rate determining step is suggested. The rates decrease in the order : 3-CH 3 > 2-CH 3 > 4-OCH 3 > 4-CH 3 > 4-C 2 H 5 > H > 3-Cl > 3-COO - > 4-F > 2-COO - > 4-Br > 2-Cl > 4-Cl > 4-COO - > 4-COCH 3 > 2-NO 2 > 4-NO 2 . Hammett equation of the type, log k = -3.44 - 2.35 p is found to be valid for substituent effects. The enthalpy and entropy of activation are correlated.

The mode of transmission of electrical effects: evidence from infrared stretching frequencies

The transmission of the universal electrical effect in infrared stretching frequencies is well fit by the modified field effect model with m ≈ −1.5 ± 0.2. The considerable variation in rm/p, the ratio of transmission of the electrical effect through meta-substituted benzene derivatives to that through para-substituted benzene derivatives and rc/t, the ratio of the transmission of the electrical effect through the s-cis conformer to that through the s-trans conformer and the failure of these ratios to show a Gaussian distribution provides further evidence against the existence of the inductive effect. The values of rm/p obtained show that, in the general case, the Hammett equation is inferior to either di- or tri-parametric equations, as it assumes the equality of ρm and ρp. This is also true of the Yukawa–Tsuno equation. It has been shown that the inductive effect does not fit the experimental data on the transmission of electrical effects in a range of chemical reactions and in infrared stretching frequencies. Recent quantum chemical calculations support the field effect as the mode of electrical effect transmission. The inductive effect model of electrical effect transmission should be abandoned. Copyright © 2001 John Wiley & Sons, Ltd.

Hydroxamic Acids as Weak Base Indicators: Protonation in Strong Acid Media

The protonation equilibria of N-phenylbenzohydroxamic, benzohydroxamic, salicylhydroxamic, and N-p-tolylcinnamohydroxamic acids have been studied at 25 degrees C in concentrated sulfuric, hydrochloric, and perchloric acid media; the UV-vis spectral measurements were analyzed using the Hammett equation and the Bunnett-Olsen and excess acidity methods. The medium effects observed in the UV spectral curves were corrected with the Cox-Yates and vector analysis methods. The H(A) acidity function based on benzamides provided the best results. The range of variation of the solvation coefficient m is similar to that of amides, this indicating similar solvation requirements for amides and hydroxamic acids. For the same substrate, the observed variations of pK(BH)(+) with the mineral acid used was justified by formation of solvent-separated ion pairs; for the same mineral acid, the observed changes in pK(BH)(+) can be explained by the solvation of BH(+). The change of the pK(BH)(+) values was in reasonably good agreement with the sequence of the catalytic efficiency of the mineral acids used, HCl > H(2)SO(4) > HClO(4).

The mode of transmission of electrical effects: evidence from infrared stretching frequencies

AbstractThe transmission of the universal electrical effect in infrared stretching frequencies is well fit by the modified field effect model with m ≈ −1.5 ± 0.2. The considerable variation in rm/p, the ratio of transmission of the electrical effect through meta‐substituted benzene derivatives to that through para‐substituted benzene derivatives and rc/t, the ratio of the transmission of the electrical effect through the s‐cis conformer to that through the s‐trans conformer and the failure of these ratios to show a Gaussian distribution provides further evidence against the existence of the inductive effect. The values of rm/p obtained show that, in the general case, the Hammett equation is inferior to either di‐ or tri‐parametric equations, as it assumes the equality of ρm and ρp. This is also true of the Yukawa–Tsuno equation. It has been shown that the inductive effect does not fit the experimental data on the transmission of electrical effects in a range of chemical reactions and in infrared stretching frequencies. Recent quantum chemical calculations support the field effect as the mode of electrical effect transmission. The inductive effect model of electrical effect transmission should be abandoned. Copyright © 2001 John Wiley & Sons, Ltd.

MECHANISTIC INVESTIGATIONS OF THE OXIDATION OF SUBSTITUTED PHENETHYL ALCOHOLS BY MANGANESE(III) SULFATE CATALYZED BY RUTHENIUM(III) IN ACID SOLUTION

The kinetics of the Ru(III) catalysed oxidation of six derivatives of phenethyl alcohol (X-C6H4-CH2CH2OH where X═H, Cl, Br, CH3, OCH3 or NO2) by manganese(III) sulfate, in sulfuric acid solution at 30°C, were followed spectrophotometrically at λmax =500 nm. The rate shows a first-order dependence on [Mn(III)] and Michaelis-Menten kinetics was observed with the substrate. The rate was of fractional-order in [Ru(III)]. Hammett acidity function (─H0) has been invoked to explain the retardation of the rate by acid. Bunnett plots indicated the absence of water molecule participation in the rate-limiting step. Variation of the ionic strength and the dielectric constant and the addition of the reaction product Mn(II) and anions such as C1−, SO2+ 4, and F− have no effect on the rate. The solvent isotope effect k(H2O)/k(D2O) =1.38 was observed. Proton inventory studies were made in H2O-D2O mixtures. A linear free energy relationship was observed when the results were analyzed by Hammett and Okamoto-Brown equations. Activation parameters were computed. A mechanism consistent with the observed kinetics has been proposed.

Linear free energy study of ring-substituted aniline ozonation for developing treatment of aniline-based pesticide wastes.

The relative rate constants for the reaction of ozone were determined for several substituted anilines in aqueous solutions at pH 6.5 and 1.5. At pH 6.5, with the exception of m- and p-nitroaniline, the rate constants obey Hammett's equation: log(k(X)/k(H)) = rho sigma. The departure of m- and p-nitroaniline may be explained by direct conjugation of the reaction center. The commonly used sigma(p)(-) value of 1.27, which extends the range of applicability of the Hammett equation, was insufficient to account for the conjugation effects on ozonation of p-nitroaniline; rho = -1.48 (R = 0.973). Use of amine group atomic charge determinations significantly improved correlations: (k(X)/k(H)) = 48.7 delta - 18.2 (R = 0.996). A linear plot of Hammett constants versus relative rate data at pH 1.5 showed poor correlation: rho = 0.72 (R = 0.572). Poor correlation was similarly observed for amine group atomic charge determinations, suggesting varied reaction mechanisms.

Electrochemistry of Nickel and Copper β-Octahalogeno-meso-tetraarylporphyrins. Evidence for Important Role Played by Saddling-Induced Metal(dx2-y2)−Porphyrin(“a2u”) Orbital Interactions

This is an electrochemical study of the Ni(II) and Cu(II) complexes of a series of β-octahalogeno-meso-tetrakis(p-X-phenyl)porphyrins, M[Y8T(p-X-P)P] (M = Ni, Cu; Y = Cl, Br; X = CH3, H, F, Br, COOMe, CF3, NO2). We have analyzed the oxidation and reduction potentials by means of the Hammett equation. The Ni and Cu series exhibit significant differences in electrochemical behavior in terms of the absolute values of the redox potentials and substituent effects. DFT calculations suggest that these differences may be ascribed to differences in saddling-induced metal(dx2-y2)−porphyrin(a2u) orbital interaction between the Ni and Cu porphyrins.

Physical Organic Chemistry of Transition Metal Carbene Complexes. 22.1 Substituent Effects on the Reversible Cyclization of Fischer Carbene Complexes (CO)5WC(SCH2CH2OH)C6H4Z in Aqueous Acetonitrile

A kinetic study of the reversible cyclization of (((2-hydroxyethyl)thio)arylcarbene)pentacarbonyltungsten(0) complexes, 2-SR(Z), in 50% MeCN−50% water (v/v) at 25 °C is reported. The results are consistent with Scheme 1, which shows a pathway in basic solution that involves rapid deprotonation of the OH group followed by rate-limiting cyclization and a pathway in acidic solution that involves direct nucleophilic attack by the OH group. Rate and equilibrium constants for the various elementary steps were determined as a function of the phenyl substituent. They were found to correlate well with the Hammett equation. The ρ values provide insights into (a) the reasons that the equilibrium constants are highly sensitive to substituent effects despite the fact that the negative charge of the adducts is strongly dispersed, (b) the mechanistic details of the cyclization pathway in acidic solution, and (c) the extra stabilizing effect of soft−soft interactions in previously studied adducts between a soft thiolate ion and the extra-soft carbene complex (2-SR(Z)).

Class Projects in Physical Organic Chemistry: The Hammett Equation

This paper provides a brief introduction to the Hammett equation. A laboratory experiment is described that requires students to determine the pKa of three para-substituted phenols through the use of UV-spectroscopy. A student's individual data are combined with other students' data to provide a class set. The students analyze the class set of data to determine whether σp or σp- is more appropriate for the reaction studied, and they also determine ρ for the reaction.

The Hammett equation applied to the nucleophilic displacement of ions and ion pairs on substituted benzenesulphonates

AbstractNucleophilic substitution on meta‐ and para‐substituted methyl benzenesulphonates was studied with two chloride salts with different structures: NBu4Cl or KCl‐Kryptofix 2,2,2. Treating the results with the Acree equation shows that the reaction proceeds by two reaction paths, one involving the chloride ion and the other, slower one, involving the ion pairs. Treating the results with the Hammett equation gives consistent data, and shows that ρ is positive and nearly the same for the two reaction paths (ρ ≈ +2). The reactivity of methyl p‐nitrobenzenesulphonate was compared with that of the corresponding ethyl derivative, and it is shown that the methyl derivative reacts faster than the ethyl derivative in both paths. The results are interpreted based on the assumption that in both paths a negative charge is developed on the leaving group in the transition state, and that the activated complex is linear. Copyright © 2001 John Wiley and Sons, Ltd.

Synthesis, Acidity Constants and Tautomeric Structure of 7‐Arylhydrazono[1,2,4]Triazolo[3,4‐b][1,3,4]thiadiazines in Ground and Excited States

Abstract7‐Arylhydrazono[1,2,4]triazolo[3,4‐b][1,3,4]thiadiazines 4 were synthesized from the reactions of 4‐amino‐5‐phenyl‐4H‐[1,2,4]triazole‐3‐thiol 2 and 2‐(2‐naphthyl)‐2‐oxoethanehydrazonoyl bromides 1 and their acid dissociation constants pK and pK*, in the ground and excited states, respectively, were determined. Both pK and pK* constants were correlated by Hammett equation. The pK and the spectral data presented indicate that the title compounds exist predominantly in the hydrazone tautomeric form.

Classical Coset Hamiltonian for the Electronic Motion and itsApplication to Anderson Localization and Hammett Equation

A classical coset Hamiltonian is introduced for the system of oneelectron in multi-sites. By this Hamiltonian, the dynamical behaviour ofthe electronic motion can be readily simulated. The simulationreproduces the retardation of the electron density decay in a latticewith site energies randomly distributed - an analogy with Andersonlocalization. This algorithm is also applied to reproduce the Hammettequation which relates the reaction rate with the property of thesubstitutions in the organic chemical reactions. The advantages andshortcomings of this algorithm, as contrasted with traditionalquantum methods such as the molecular orbital theory, are also discussed.

Effects of Solvent, Structure, and Temperature on the Ionization Equilibrium Constants of Phenols and Pyridinium Ions in Mixtures of Water and 1,4-Dioxane

An extensive collection of data has been used to study the effects of solvent, structure, and temperature on the ionization equilibrium constants of some substituted phenols and pyridinium ions in water–1,4-dioxane mixtures (0–70% weight fraction in dioxane) and temperatures ranging from 10 to 50°C. The effects of structure and solvent are explained using Hammett's equation and the Marshall–Quist model at all temperatures. An equation allowing an analysis of the three effects together on the pK values has been developed. The pK data under all experimental conditions fit this equation well, with standard errors of less than 0.3 pH units. Hammett's reaction constant for the ionization of phenols and pyridinium ions has been obtained for all the experimental conditions. The pK and Hammett's reaction constants for the different ionizations in water–1,4-dioxane mixtures correlate well with Kamlet and Taft's solvatochromic parameters π* and α, which measure the dipolarity/polarizability and hydrogen-bonding capacity of the solvent, respectively. These correlations explain more thoroughly the different contributions and origin of the effects of the solvent on the pK.

The Hammett Equation and Micellar Effects on SN2 Reactions of Methyl Benzenesulfonates ?ms The Role of Micellar Polarity

Substituent effects on the reaction of H2O, OH−, and Br− with p-substituted methyl benzenesulfonates in cationic micelles of cetyl trialkylammonium ion surfactants (n-C16H33NR3X, X = OH, Br, R = Me, Et, nPr, nBu) and in water were analyzed by using the Hammett equation. Values of ρ in the various media confirm that micellar interfacial regions are less polar than water and polarities decrease with increasing bulk of the surfactant head-group.

Charge Variations in Substituted Alkanes: Evidence for a Through-Space Effect

Ab initio quantum mechanical calculations at the 6-31+G* basis set are reported on over 75 substituted ethanes and higher alkanes. The charges on the carbon and hydrogen atoms of these species are determined by topological- and orbital-based methods. The relative change in charge is used to determine how a substituent group affects atoms remote from it. Arguments are presented to account for the charge on the hydrogen atoms in substituted ethanes and higher alkanes. Fluorine atoms within about 2.7 Å of hydrogen atoms cause an increase in positive charge on those hydrogen atoms. Charge is moved to the periphery of the molecule when a charged substituent is present. A model that involves through-space interactions between the charged group and the alkane framework accounts for these data. The charge calculations indicate a substituted carbon atom has a charge that is more dependent on the nature of the atom to which it is attached than on that atom's charge or on other atoms in the substituent. No evidence of systematic charge variation on nonsubstituted carbon atoms was found. Energy changes for some isodesmic reactions are explained on the basis of the model of charge distribution. The results are applicable to studies of the Hammett equation.

Formation of Peracetic Acid upon Aging of Perborate in Acetic Acid. Kinetics of the Oxidation of S-Phenylmercaptoacetic Acids

Upon aging, perborate in glacial acetic acid generates peracetic acid and thus oxidizes S-phenylmercaptoacetic acid rapidly. Perborate dissolved in ethylene glycol, however, does not show the aging effect, and the corresponding oxidation proceeds smoothly. The oxidation is of second order and not acid catalyzed. Boric acid and borate do not influence the oxidation. In the smooth oxidation, is the reactive species. The oxidation of some para-substituted S-phenylmercaptoacetic acids conforms to the Exner relationship, indicating operation of a common mechanism. Also, the oxidation obeys the Hammett equation with a negative reaction constant. However, the oxidation of p-nitro-S-phenylmercaptoacetic acid follows a different kinetic pathway.

Multiple substituent effects in the CC reduction of phenyl styryl ketones: cyclic voltammetry as a tool

Figure options Download full-size image Download as PowerPoint slide Cyclic voltammetric studies on 16 mono- and disubstituted phenyl styryl ketones (PSKs) or chalcones have been performed. Multiple substituent effects on the first reduction potential (E) were analysed with the help of the Hammett equation and the ρ values were evaluated for various substituents in one ring for each substituent in the other ring and vice versa. An interactive Gibbs energy relationship was applicable to this system. The reduction of alkenyl instead of carbonyl is expected to give similar values for cross-interaction constants qx and qy and the same were obtained (qx=−0.6720 and qy=−0.6710).

Micellar Charge Effects upon Hydrolyses of Substituted Benzoyl Chlorides. Their Relation to Mechanism

Micellar rate effects on hydrolyses of substituted benzoyl chlorides, 1, depend on headgroup charge and electron donation or withdrawal by substituents. Micellized sodium dodecyl sulfate, SDS, inhibits hydrolyses, and first-order rate constants in the micellar pseudophase, k‘M, decrease, relative to those in water, k‘W, over a range of ca. 10, but in cetyl trimethylammonium chloride, CTACl, k‘M/k‘W > 1 for hydrolyses of 1,3,5-(NO2)2 and 1, 4-NO2, and decreases steeply with electron-donating substituents in the following sequence: 1,4-Cl ≈ 4-Br > 4-H > 4-Me > 4-OMe, over a range of >103. Cetyl trimethylammonium bromide and mesylate behave like CTACl. Fits to the Hammett equation give ρ ≈ 1 in SDS and ρ ≈ 4 in CTACl. Anionic micelles have higher interfacial polarities than cationic micelles, but micellar and solvent effects do not correspond because over a range of solvents, H2O to H2O−MeCN, 1:1 w/w plots of log k‘W against σ go through minima with positive ρ for 1, 3,5-(NO2)2, and 4-NO2 and negative for t...

A new method for measuring carbonate radical reactivity toward pesticides

AbstractThe carbonate radical is produced from the scavenging of the hydroxyl radical by carbonate/bicarbonate and is strongly electrophilic toward electron‐rich compounds such as aromatic amine and sulfur‐containing compounds. Carbonate radical reactivity (CO3⋅−) was measured by setting up a competition between a chemical of known reactivity and a compound with unknown reactivity toward CO3⋅−. The carbonate radical was produced using a novel method whereby KOONO (potassium peroxonitrite) was dissolved into 0.01 NaHCO3 solution initially producing a HO⋅, which was rapidly scavenged by HCO3⋅−, yielding CO3⋅− CO3⋅− production was monitored using a stopped‐flow spectrometer at the absorption band for CO3⋅− of 600 nm. Results using this competition kinetic method indicated the second‐order rate constants for substituted anilines corresponded to reference values with a relative error less than 10%. Using σpara+ values, the rate constants of substituted anilines at pH 9.0 correlated well (R2 = 0.98) with the Hammett equation yielding ρ = −0.89. To measure the reactivity of pesticides toward the carbonate radical, p‐nitroaniline (PNA) was selected as the standard competitor. Of the nine pesticides tested, fenthion was the most reactive, followed by phorate, with rate constants above 107/M/s. Fluometuron and atrazine were of intermediate reactivity, and methyl parathion was one of the least reactive.

Photocyclization Reaction of some 2-Methyl-4-phenyl- Substituted Aldehyde Thiosemicarbazones. Mechanistic Aspects

Irradiation of 2-methyl-4-phenyl- substituted benzaldehyde thiosemicarbazones led with good yields to the corresponding Δ 2-1,2,4-triazoline-5-thione derivatives through the formation of stable 1,2,4-triazolidine-5-thione intermediates. The relative quantum yields of photocyclization at 313 nm were analysed by means of the Hammett equation and ρ values determined. We interpreted the results in terms of the mechanism of photocyclization of 2-methyl-4-phenyl-substituted benzaldehyde thiosemicarbazones.