Taft Relationship Research Articles

Non-Linear Taft Relationship applied to surface tensions of aliphatic acids: Inter-molecular hydrogen bonding versus intra-molecular hydrogen bonding

Hammett and Taft equations in their most basic forms are linear free energy relationships between standard free energies of two reaction series. Therefore any chemical or physical property inherently associated with any of the thermodynamic property like enthalpy H, free energy G, internal energy E or entropy S are subjected to substituent effects. One such physical property is the surface tension. Surface tension is a kind of surface free energy (G) of any liquid and is also supposed to be effected by the structure of liquids. Hence the present study has been selected to see the effect of substituents on surface tension of some aliphatic acids. The quick glance at the present study ended up with a non-linearity of Taft equation to the surface tension data of aliphatic acids. Hence Non-Linear Taft Relationship (NLTR) was applied to surface tensions (γ) of some aliphatic acids. The non-linearity of log γ versus Taft σ* correlation was explained in terms of inter-molecular hydrogen bonding versus intra-molecular hydrogen bonding and in terms of steric effects.

Tertiary Butyl Nitrite Triggered Nitration of Phenols: Solvent‐ and Structure‐Dependent Kinetic Study

ABSTRACTNitration of phenols with tertiary butyl nitrite (TBN) obeyed second‐order kinetics with a first‐order dependence on [TBN] and [phenol] under acid‐free conditions. Reaction rates were significantly altered by a change in the dielectric constant and other physical properties of solvent. The rate of nitration increased with an increase in temperature (303–323 K) in different solvent media (acetonitrile, dichloroethane, CCl4, dimethyl formamide (DMF), and toluene). The rates of nitration (log k) could not fit into either Amis or Kirkwood plots [log k’ vs. (1/D) or [(D – 1)/(2D + 1)], but the trends were better explained by the basic form of multivariate linear solvent energy relationships (MLSER) suggested by the Koppel and Palm approach on the one hand and the Kamlet and Taft approach on the other hand. These observations probably substantiate that cumulative contributions of basic solvent parameters (equilibrium as well as frictional solvent effects) and solvent–solute interactions for solvation of transition state during nitration of phenols. Reaction rates accelerated with the introduction of electron‐donating groups and retarded with electron‐withdrawing groups. Accordingly, the reactivity of structurally different phenols was found to follow the following sequence: p‐OH > p‐MeO > p‐Me > H > m‐Me > p‐Cl > p‐Br > m‐Cl > p‐NO2 > m‐OH. The results are interpreted by Hammett's theory of linear free energy relationship. The reaction constant (Hammett's ρ) is a measure of the sensitivity of the reaction toward the electronic effects of the substituent. The rho (ρ) values obtained from the present experiments are fairly large negative values (ρ < 0), indicating attack of an electrophile on the aromatic ring. An increase in temperature decreases the reaction constant (ρ) values. According to Exner, ρ values for a given reaction are influenced by the temperature according to the following relation: ρ = A [1 – β/T]. Obtained “isokinetic temperature (β)” values are in the range of 225–290. These values are far below the experimental temperature range (303–323 K), indicating that the entropy factors are probably more important in controlling the reaction. This point can be seen from the negative entropy values and linearity of multiple linear regression analysis (MLRA). Furthermore, in the present study, rate constants for TBN nitration of ortho‐substituted phenols could not fit into Taft's plots of log(k/kCH3) versus σ* or, Es or combined Taft's relationship. However, Charton's MLRA of the log k with polar, resonance, steric, hydrophobicity, and molar refractivity showing a very good linear relationship was obtained. It is of interest to note that when log kexp values are correlated with log kcal a perfect linearity is obtained with a correlation coefficient of unity, indicating the consonance between experimental and calculated rate constants in the present work.

Effect of solvent and encapsulation in β-cyclodextrin on the photophysical properties of 4-[5-(thiophen-2-yl)furan-2-yl]benzamidine

Abstract Photophysical characteristics of the ground and excited states of the bichalcophene derivative 4-[5-(thiophen-2-yl)furan-2-yl]benzamidine (TFB) were investigated in different solvents and in β-cyclodextrin (β-CD). The photophysical properties of TFB were correlated with the two commonly used solvent polarity parameters viz., ETN and reaction field factor Δf. In addition, the photophysical properties of TFB were also found to be well correlated with Kamlet–Taft relationship. Multiple linear regression analysis of solvent dependent photophysical parameters indicated that both non-specific dipolar interaction and specific hydrogen bonding interactions play important roles in determining the photophysical properties of TFB. It has been found that the non-specific dipolar interaction is the dominating parameter. Fluorescence decay curves are well fitted to double exponential decay function, with shorter component having the higher relative integrated intensity. Molecular encapsulation of TFB by β-CD in aqueous solution has been studied by steady state and time resolved fluorescence techniques. Steady state measurements show that the fluorescence quantum yield is enhanced strongly upon the addition of β-CD and increases from 0.03 ± 0.01 in aqueous solution to 0.11 ± 0.02 at 7.5 mM concentration of β-CD. Steady state fluorescence measurements show 1:1 inclusion of TFB in the β-cyclodextrin cavity with an association constant of 2.3 ± 0.2 × 103 M−1, while data derived from time resolved measurements gave very close value for the association constant of 2.6 ± 0.3 × 103 M−1. Time resolved measurements and global analysis of the results indicated that the excited state lifetime is strongly affected by the presence of β-CD. The changes observed for the chemical shifts of TFB and β-CD 1H NMR spectra and corresponding 1H NMR spectra of their mixture confirmed the formation of the inclusion complex and showed that TFB is partially encapsulated in β-CD cavity.

Chemically Triggered C–ON Bond Homolysis of Alkoxyamines. 5. Cybotactic Effect

In a recent work (Org. Lett. 2012, 14, 358), we showed that the rate constant k(d) of the chemically activated C-ON bond homolysis of alkoxyamines was subject to solvent effects. However, we showed that solvent effects were weak for the nonactivated alkoxyamine 1 (diethyl (1-(tert-butyl(1-(pyridin-4-yl)ethoxy)amino)-2,2-dimethylpropyl)phosphonate) and its N(+)-O(-) oxide activated version 3. On the other hand, the activated N-methylated version 2 of 1 experienced a strong solvent effect for a radical reaction, i.e., a 24-fold increase in k(d) from tert-butylbenzene (tBuPh) to 2,2,2-trifluoroethanol (TFE). Good correlations were observed with the normalized Reichardt solvent polarity constant E(T)(N) and the nitrogen hyperfine coupling constant of the released nitroxide a(N,SG1), meaning that the stabilization of the nitroxide played an important role in C-ON bond homolysis in alkoxyamines. The Kalmet-Abboud--Taft relationships described successfully the solvent effect for each diastereoismer of 1 and 2, as for example with the minor diastereoisomer of 1 log(k(d)'/s(-1)) = -4.84 + 0.37π* + 0.21α and that of 2 log(k(d)'/s(-1)) = -3.11 + 0.37π* + 0.47α, with π* being the polarity/polarizability and α the hydrogen bond donor (HBD) ability of the solvent. Surprisingly, the HBD effect is larger for 2 than for 1, whereas no extra lone pair is available in 2. This amazing effect was ascribed to the solvation of the counteranion, which is expected to be better solvated in a HBD solvent.

Chemically triggered C–ON bond homolysis of alkoxyamines. Part 4: solvent effect

In a recent work (Org. Lett., 2012, 14, 358), we showed that the rate constants kd for the C–ON bond homolysis of chemically activated alkoxyamines were subject to solvent effects. We then investigated solvent effects on the non-activated alkoxyamine 1 ((diethyl(1-(tert-butyl(1-(pyridin-4-yl)ethoxy)amino)-2,2-dimethylpropyl) phosphonate) and its N+–O− oxide activated version 2, using 14 solvents exhibiting different solvent parameters – dipolar moments μ, dielectrical constants e, cohesive pressures c, Reichardt solvent polarity constants ET, viscosity η, hydrogen bond donor and hydrogen bond acceptor constants α and β, respectively, and nitrogen hyperfine coupling constants aN. Weak solvent effects were observed both for 1 (4–5-fold from n-octane to 2,2,2-trifluoroethanol TFE) and for 2 (2-fold from n-octane to water) although kd increased 27-fold in n-octane and 19-fold in TFE from 1 to 2. It was shown that the C–ON bond homolysis rate constant kd increased with the aN values, meaning that the stabilization of the nitroxide was the main factor involved in the solvent effect. Approaches relying on the Koppel–Palm and Kalmet–Abboud–Taft relationships failed to describe the solvent effect for all diastereoisomers of 1 and 2. Nevertheless, the solvent polarity/polarizability (π*) and hydrogen bond donor (α) properties are the main effects involved in the solvent effects at TS and on products.

Hyperaromatic Stabilization of Arenium Ions: Acid-Catalyzed Dehydration of 2-Substituted 1,2-Dihydro-1-naphthols

Rate constants for acid-catalyzed dehydration of cis-2-substituted 1,2-dihydro-naphthols are well correlated by the Taft relationship log k = -0.49 - 8.8σ(I), with minor negative deviations for OH and OMe. By contrast the trans substituents show a poor correlation with σ(I) and in most cases react more slowly than their cis isomers. The behavior is consistent with rate-determining formation of a 2-substituted carbocation (naphthalenium ion) intermediate that for cis reactants possesses a 2-C-H bond suitably oriented for hyperconjugation with the charge center. For the trans isomers the 2-substituent itself is oriented for hyperconjugation in the initially formed conformation of the cation. It is argued that k(cis)/k(trans) rate ratios for substituents (Me, 8.4; Bu(t), 12.7; Ph, 3.8; NH(3)(+), 160; OH, 440) reflect their hyperconjugating ability relative to hydrogen. Faster reactions of trans isomers are observed for substitutents known (RS, N(3)) or suspected (EtSO, EtSO(2)) of stabilizing the cation by a π or σ neighboring group effect. The good Taft correlation is taken to indicate that cis substuents are reacting normally, differentiated only by their inductive effects. The slower reactions of the trans isomers are the judged to be "abnormal". This is confirmed by comparing effects of cis and trans β-OH substituents on the reactivities of dihydro phenols, naphthols, and phenanthrols. Whereas k(H)/k(OH) for cis substituents varies by less than 8-fold and is consistent with the influence of an inductive effect of the OH group (k(H)/k(OH) ≈ 2000), k(H)/k(OH) for the trans substituents varies by 3 orders of magnitude, reflecting the additional influence of the lesser hyperconjugating ability of a C-OH bond compared to a C-H bond. The magnitude and variation of this difference is consistent with C-H hyperconjugation conferring aromatic character on the arenium ions.

Reactivity of mixed organozinc and mixed organocopper reagents. Part 4: a kinetic study of group transfer selectivity in CC coupling of mixed diorganocuprates

AbstractThe competitive rate data and Taft relationships for the coupling of bromomagnesium n‐butyl (substituted phenyl) cuprates with alkyl bromides show that selective n‐butyl transfer can be explained by an oxidative addition mechanism. Taft reaction constants also show that the residual group FG‐C6H4 in the mixed cuprate n‐Bu(FG‐C6H4)CuMgBr changes the ability of the copper nucleophile to react with the electrophile RBr. These results provide support for the commonly accepted hypothesis regarding the dependence of the R1 group transfer ability on the strength of R2Cu bond in reactions of R1R2CuMgBr reagents. Copyright © 2009 John Wiley & Sons, Ltd.

Solvent Effects on the Redox Properties of Cu Complexes Used as Mediators in Atom Transfer Radical Polymerization

Solvent effects on the redox properties of six Cu(I) complexes used as mediators in atom transfer radical polymerization (ATRP) have been studied using cyclic voltammetry. The six ligands used were tris[2-(dimethylamino)ethyl]amine, N-(n-propyl)-2-pyridylmethanimine, N,N,N',N',N'-pentamethyldiethylenetriamine, 1,1,4,7,10,10-hexamethyl-triethylenetetramine, 2,2'-bipyridine, and 1,4,8,11-tetraaza-1,4,8,11-tetramethylcyclotetradecan. The solvents used were DMSO, DMF, MeCN, MeOH, IP, and BuOH. Significant solvent effects were observed and quantitatively analyzed in terms of Kamlet-Taft relationships. The resulting Kamlet-Taft equations were found to successfully describe the solvent effects and could thus be used as tools for the design of ATRP in new solvents. The solvent sensitivity of the different ligands and the nature of the solvent effects are also discussed to some extent.

Solvent Effects on the Redox Properties of Thioethers

The one-electron reduction potential of the radical cations of thioanisole (1), benzyl methyl sulfide (2) and 2-hydroxyethyl benzyl sulfide (3) in water, formamide, acetonitrile, acetone, 1,1,1,3,3,3-hexafluoropropan-2-ol, methanol and 2-propanol was investigated by cyclic voltammetry. For comparison the one-electron reduction potentials in water were also measured using pulse radiolysis. The redox potential is strongly influenced by the nature of the solvent and the solvent sensitivity increases with charge localization. The present results have been used to evaluate solvent effects in view of the Kamlet-Taft relationship. The Kamlet-Taft expression quantitatively describes the solvent effects on the redox properties of 1-3 and gives the relative importance of the different solvent properties. The dominating contribution to the solvent effects is the solvent dipolarity/polarizability pi*, whereas alpha appears to be of minor importance. Furthermore, the relationship between the pi* and reduction potential of radical cations of 1-3 appear to be linear. It was also possible to find the same trend between the solvent dipole moment and peak potential of 1-3. These facts indicate that the nature of solvation is mainly nonspecific.

Inclusion complex of 2-naphthylamine-6-sulfonate with β-cyclodextrin: Intramolecular charge transfer versus hydrogen bonding effects

The photophysical characteristics of the ground and excited states of 2-naphthylamine-6-sulfonate (2-NA-6-S) were investigated in different solvents and in β-cyclodextrin (β-CD). The spectral shifts are well correlated with Kamlet–Taft relationship. Multiple linear regression analysis indicated that both non-specific dipolar interaction and specific hydrogen bonding interactions play competitive roles in determining the position of the absorption maximum, while the dipolar interaction is the dominating parameter in determining the emission maximum. For the Stokes shift, both the nonspecific interaction and the hydrogen donation property of the solvent are participating equally. The molecular encapsulation of 2-NA-6-S by β-CD in aqueous solution has been studied by different spectroscopic techniques. Fluorescence measurements show that the dielectric constant of β-CD experienced by the included 2-NA-6-S is intermediate between water and methanol. The changes observed in the absorption and fluorescence spectra of 2-NA-6-S upon inclusion in β-CD allowed the association constant to be calculated and found to be 465 ± 100 and 495 ± 100 M −1, respectively. The changes observed for the chemical shifts of 2-NA-6-S and β-CD 1H NMR spectra and the corresponding 1H NMR spectra of their mixture confirmed the formation of the inclusion complex and showed that 2-NA-6-S is encapsulated in β-CD cavity in a tilted equatorial approach.

On the Nature of Solvent Effects on Redox Properties

The one-electron reduction potentials of six radical cations, four cations, and four neutral radicals in tetrahydrofuran, dichloromethane, dimethyl sulfoxide, N-methyl-2-pyrrolidinone, N,N-dimethylformamide, acetonitrile, methanol, ethanol, 2-propanol, acetone, formamide, and 1,1,1,3,3,3-hexafluoropropan-2-ol have been measured by cyclic voltammetry. For 10 of the redox couples, the redox process was reversible in all solvents. These results have been used to evaluate solvent effects by means of the Kamlet−Taft relationship. The relative importance of the solvent parameters α, β, π*, and δH is 54.9, 9.6, 15.5 and 20.0%, respectively, for the radical cation displaying the strongest solvent dependence. In addition, we have studied the entropy contribution to some of the observed solvent effects by measuring the redox potentials as a function of temperature. The absolute value of the entropy appears to increase with increasing hydrogen bond donor ability of the solvent. The variation in entropy indicates tha...

The ionization constants of 2‐substituted 4,6‐diamino‐s‐triazines: The applicability to the hammett and taft equations

AbstractThe ionization (or basicity) constants (pKb) were determined for many 2‐substituted 4,6‐diamino‐s‐tri‐azines (I) by means of the electrometric titration. I includes 2‐alkoxy or aryloxy‐(Ia), 2‐alkyl‐ or 2‐aryl‐(Ib), and 2‐alkylamino‐ or 2‐arylamino‐4,6‐diamino‐s‐triazines (Ic). For the series with the same alkyl or aryl group, the order of the basicity was found to be Ic < Ib < Ia.A study was made of relationships between the pKb, values of I, and the substituent constants, σp, σm, σp+, σm+, σpO, σmo, σI, σn, and σ*. The Hammett relationships were observed between the pKa values of I, and the substituent constants σm, (or the combination ones, [0.97σm + 0.03σp] as well as another [0.77σI + 0.23σR]). The Taft relationships were also found between the pKa values of Ia, Ib, and Ic and the constants σ*, respectively. Furthermore, in the case of Ic a linear relationship was observed between the pKa values and Σσ8.

Esterification of different acids over heterogeneous and homogeneous catalysts and correlation with the Taft equation

Esterification of acetic, propanoic and pentanoic acid with methanol, ethanol, 1-propanol, 2-propanol, butanol and 2-butanol was studied in the presence of a fibrous polymer-supported sulphonic acid catalyst, Smopex-101. The reaction temperature in the experiments was 60 °C. Comparative experiments were carried out on an ion-exchange resin, Amberlyst 15 and with a homogeneous catalyst, liquid HCl. The effect of different molar ratios between the reactants (acid and alcohol) on the reaction rate was investigated. The rate constants were related to the substituent effects of the reacting molecules according to the Taft equation. The substituent effects of alcohols were found to follow the Taft relationship, which was not the case for the acids. The experimental results were modelled according to a simple second-order model and a more advanced adsorption-based model. The adsorption-based model, which includes the adsorption of carboxylic acid and water and is consistent with structure–activity relationship, was superior to the second-order model.

Solvent effects on redox properties of radical ions 1

The one-electron reduction potentials of the radical cations of 1,4-diazabicyclo[2.2.2]octane (DABCO) and N,N,N′,N′-tetramethylphenylene-1,4-diamine (TMPD) in propan-2-ol, ethanol, methanol, acetone, acetonitrile and dimethyl sulfoxide have been measured by cyclic voltammetry. Furthermore, the one-electron reduction potentials of 1,4-benzoquinone, 1,4-benzoquinone radical anion, methyl viologen dication and methyl viologen radical cation also have been measured in a number of solvents. The present results, together with previously published data on radical anions, have been used to evaluate solvent effects in view of the Kamlet–Taft relationship. The main factors affecting the magnitude of the solvent effects are the gas-phase redox properties of the corresponding neutral molecule (ionization potential and electron affinity) and the charge. In general, the magnitude of the solvent effects on solution redox properties of radical ions decreases with increasing redox stability of the radical ion, reflected by low ionization potential of the corresponding neutral molecule for radical cations and by high electron affinity of the corresponding molecule for radical anions.

Photoelectrochemistry using quinone radical anions

The photoelectrochemistry of quinone radical anions has been demonstrated qualitatively by the photoassisted reduction of methyl viologen with benzoquinone and of neutral red with chloranil. Data were then collected for the estimation of quenching rate constants using Marcus–Weller theory. Reduction potentials of seven quinones were obtained in four solvents (and two aqueous mixtures) by cyclic voltammetry. The solvent effects on these potentials were studied by fitting them to the Taft relationship. The effects of proton donors were also noted. Absorption spectra of the radical anions were measured and the solvent effects noted and commented upon. From the molar absorption coefficients of the radical anions, the mean lifetimes of the excited states were estimated. Fluorescence spectra were obtained for anthraquinone and naphthaquinone radical anions and excitation energies were calculated. These values were estimated for the other quinones. Values of redox potentials for the excited radical anions were thence obtained. The Gibbs energies of the electron transfers between the excited quinone radical anions and the various substrates were obtained and hence the Gibbs energies of activation were calculated using the Marcus equation. The quenching rate constants were calculated using the Rehm–Weller equation and plotted vs. ΔG giving a characteristic Marcus plot including some data in the inverted region. The significance of the inverted region is discussed.

Substituenteneffekte in den 13C‐NMR‐Spektren von diastereomeren Chalkondihalogeniden. VIII. Untersuchungen an α‐ und β‐Halogendihydrochalkonen und Berechnung von 13C‐chemischen Verschiebungen bei diastereomeren Chalkondihalogeniden und Chalkonhalogenhydrinen

Substituent Effects in 13C‐N.M.R. Spectra of Diastereomeric Chalcone Dihalogenides. VIII. Investigations of α‐ and β‐Halogene Dihydrochalcones and Calculation of 13C‐chemical Shifts of Diastereomeric Chalcone Dihalogenides and Chalcone HalogenohydrinsThe chemical shifts of C‐α and C‐β in the 13C‐n.m.r. spectra are determined and compared with calculated values from an incremental system and Taft relationship. The correlations are extended for erythro and threo isomers of halohydrins and diastereometric dihalogenides 6–18.The chemical shifts of C‐α and C‐β atoms of 6–18 are described in the dependence of the shifts calculated by an incremental system. Excellent correlations allowed the computation of chemical shifts of C‐α and C‐β for diastereomeric α,β‐disubstituted dihydrochalcone derivatives likely 6–18 in advance.

The three-parameter Nieuwdorp equation is the optimal linear free energy relationship for the prediction of missing data

It has been shown that the linear free energy relationship proposed by Nieuwdorp (1979) gives a better fit to available data on substituent effects than the relationships proposed by Taft (1958), Swain (1983), and Yukawa–Tsuno (1980). One reason is that the Nieuwdorp relationship contains three parameters, while the Taft relationship has only two. It is shown here that the Nieuwdorp relationship gives the best prediction of missing data on substituent effects, and that nothing can be gained by adding a fourth p parameter. This is demonstrated with three sets of data: (1) the set of 76 series of data on 17 substituents from which Nieuwdorp derived the values of his σI, σR, and σE variables; (2) a set of 164 series of data on 14 substituents for which the fit by the Nieuwdorp relationship was not satisfactory; and (3) a set of 28 series of data on 14 substituents, selected with the view to provide a large variance that cannot be explained by the Nieuwdorp relationship.

A carbon-13 nuclear magnetic resonance study of a series of 5-substituted benzimidazoles

Carbon-13 nmr chemical shift assignments are reported for a series of 5-substituted benzimidazoles, 2-methylbenzimidazole, and 5,6-dimethylbenzimidazole. By employing the three-parameter equation of Smith and Proulx, the chemical shifts of all the ring carbons of the 5-substituted compounds are correlated with F and R of Swain and Lupton and Q of Schaefer; only the ipso carbon, C-5, has chemical shifts with a correlation coefficient less than 0.953. These correlations are compared with those obtained from the Swain–Lupton and Taft relationships; in general those from the latter two approaches are found to be less satisfactory. Correlations of the same chemical shifts with excess charge density values, calculated at the INDO level of approximation, are also presented.

Methyl radical addition to nitroalkane aci-anions in aqueous solution: rate constants and optical absorption spectra

Absolute rate constants have been measured for the ˙CH3 radical addition to the aci-anions of nitromethane, nitro-ethane, 1-nitropropane, and 2-nitropropane. The respective values of (1.35 ± 0.2)× 108, (1.60 ± 0.3)× 107, (1.35 ± 0.3)× 107, and (2.35 ± 0.5)× 106 dm3 mol–1 s–1 indicate a nucleophilic character of the methyl radical. From Taft's relationship on the inductive substituent effect, ρ*+ 8.9 is derived. The optical absorption spectra of the methyl radical adducts, R1R2CH3NO2·-, are characterized by λmax 270 nm and extinction coefficients of (2.8–3.0)× 103 dm3 mol–1 cm–1.

Slolvent effects on rates and equilibria of chelate ring-closure ? ring-opening nucleophilic substitution reactions of octahedral rhodium(III) complexes

The solvent effects on rates and equilibria of reversible reactions (i) have been studied [Xn–= SCN–, mer-[RhL(L′)Cl2X]1–n⇌trans-[RhL2Cl2]++ Xn–(i) SeCN–, N3–, NO2–, or pyridine; L and L′=(o-dimethylaminophenyl)dimethylarsine-NAs and -As respectively]. When Xn– is pyridine the reactions are both kinetically and thermodynamically independent of the nature of the solvent. When Xn– is anionic the equilibria and the rates of ring closure [forward reactions (i)] are strongly affected by the solvent as well as by the leaving group. Linear free-energy relationships of type (ii) exist for the activation and standard free energies of ΔG(m→t)‡=αΔG(m→t)⊖+β(ii) either reactions of different mer complexes carried out in a given solvent or reactions of a given mer complex carried out in different solvents. An analysis of the solvent effect through Grunwald–Winstein or Kamlet–Taft relationships enables the observed results to be interpreted in terms of a dissociative mechanism for forward reactions (i).