Steric Substituent Constants Research Articles

Ring–chain tautomerism. Part V. 8-Aroyl- and 8-acyl-1-naphthoic acids

The equilibrium constants for ring–chain tautomerism in a series of 8-aroyl-1-naphthoic acids in dioxan have been determined by an i.r. spectroscopic method and have been correlated with the substituent constants σ1 and σR0 using a modified Hammett equation. The observed pKa values, which have been measured in 80%(w/w) 2-methoxyethanol–water, have been corrected for these tautomeric equilibria to give the true pKaT value. The effect of substitution on the latter values have been assessed using the Hammett equation. The equilibrium constants for ring–chain tautomerism in a series of 8-acyl-1-naphthoic acids have been determined by using the observed pKa and estimated true pKaT values in 80%(w/w) 2-methoxyethanol–water. These equilibria constants have been correlated with steric substituent constants Es. The ‘cis’ and ‘trans’ conformers of methyl 8-aroyl- and 8-acyl-1-naphthoates has been detected by i.r. spectroscopy and the carbonyl stretching frequencies of the ester carbonyl groups have been assigned. The results are related to the polar, resonance, and steric effects of the substituents.

The influence of steric and electronic parameters on the substrate behavior of -oxo acids to yeast pyruvate decarboxylase.

The Michaelis constants of yeast pyruvate decarboxylase at full substrate activation and the maximum decarboxylation rates were determined from a total of 21 aliphatic and aromatic α‐oxo acids in the absence of buffer substances. By means of a pyruvamide‐activated enzyme it was possible to confirm experimentally the Km‐values obtained by extrapolation. The substrate binding rate of pyruvate decarboxylase increases linearly with increasing H+ concentration, which indicates that only the undissociated pyruvic acid acts as the substrate of the enzyme. Whilst the substrate binding rate can be correlated linearly with the steric substituent constants of aliphatic α‐oxo acids, it was not possible to find a corresponding relation between the aromatic α‐oxo acids and the Hammett σ‐constants, which indicates steric peculiarities in the active centre of pyruvate decarboxylase. On the other hand, a linear relation between the maximum velocities and the σ+‐values for direct conjugation was established, leading to some conclusions concerning the mechanism of the splitting‐off of aldehyde.

Polar and steric substituent constants for an alkylperoxy group and related ether groups

Polar and steric substituent constants for an alkylperoxy group and related ether groups

Comparison of the Electronic Spectra of Geometric Isomers. III. β-Substituted Styrenes

Abstract The ultraviolet absorption spectra of cis and trans isomers of various β-substituted styrenes have been measured both in the gaseous state and in n-hexane solution. The excitation energies observed for cis isomers are either smaller or greater than those of the trans, depending on the type of β-substituents. The isomeric difference in the energy of the lowest allowed transition was found to be linearly correlated with Taft’s steric substituent constants. The electronic states of the various isomeric pairs were calculated by the LCAO–SCF–CI method of the Pariser-Parr-Pople type. The results were found to be entirely in line with the observed spectral features.

Kinetics and mechanism of urethane formation in DMF: The reaction of 4, 4′‐di‐isocyanatodiphenylmethane with alcohols

AbstractThe rate constants of reactions between 4, 4′‐di‐isocyanato‐diphenylmethane (MDI) and alcohols in N, N‐dimethylformamide (DMF) have been determined by an UV‐spectroscopic method. On the basis of the kinetics observed and the values of the activation parameters ΔS ≠ and ΔH ≠, a proposal is made for a mechanism with a four‐centre transition state. The rate constants for the reaction of several alcohols with MDI in DMF are correlated with the extended Taft equation: equation image (Ecs = corrected steric substituent constant according to Hancock).

Reactivity of mono‐epoxides toward silk fibroin

AbstractThe addition reaction of various mono‐epoxides such as epichlorohydrin, glycidol, allylglycidyl ether, butyl glycidyl ether, phenyl glycidyl ether, propylene oxide, styrene oxide, hexadecene oxide, and vinyl cyclohexene monoepoxide with silk fibroin was investigated in the presence of some salts in different solvents at 30–80°C. Histidine, lysine, arginine, aspartic acid and glutamic acid, serine and tyrosine reacted. The weight gains decreased with increasing solubility parameter of the solvents, reached a minimum value at about 10 or at the solubility parameter of the epoxide, and then increased with the parameter. The salt effects on the epoxide‐silk reaction were described partly by the hydration ability or by the acidity of the cations, and also by the basicity or the nucleophilicity of the anions. The substituent effects of the epoxide on the add‐ons were elucidated by the modified TAFT equation, (log k − log k0)/σ* = ρp + ρsEs/σ*, where k0 and k are the rate constants for the reaction of a given compound and for the same reaction of one of its derivatives; σ* is the polar substituent constant characteristic of the substituent, Es is the steric substituent constant, and ρp and ρs are the reaction constants as measure of the susceptibility of the reaction to the polar and steric effects of substituents.

Effect of unsaturated substituents on the hydrolysis of esters

A study of the acid and alkaline hydrolysis of esters (R1CO2R2) containing unsaturated substituents in aqueous acetone confirms that where the unsaturation in R1 is conjugated with CO2R2 in the initial state, the value of the Taft steric substituent constant, Es, is considerably more negative than might be expected from normal steric interactions. When there is no conjugation, Es has values consistent with steric interactions by R1 and R2. On the other hand, the Taft polar substituent constant, σ*, has fairly large positive values regardless of the presence or absence of conjugation. This may be attributed to the electron-withdrawing power of the unsaturated substituents but the property falls off as the seat of unsaturation becomes further removed from the ester group.

Copolymerization of styrene. IV. Copolymerization with esters of benzylidenecyanoacetic acid

AbstractStyrene was copolymerized in bulk with a number of esters of benzylidenecyanoacetic acid. The kinetic scheme of all pairs fitted the improved scheme of copolymerization, taking into account the effect of the penultimate unit. The Alfrey‐Price Q and e values were calculated. Using the modified Taft equation, log (1/r1) = ρ*σ* + δEs, it was found that the relative reactivities of the ester monomers toward the polystyryl radical were correlated by the polar substituent constants σ* of the ester alkyl groups (ρ* = 0.14) and not by their steric substituent constants Es (δ = 0.008).

Effect of structure of vinyl esters on radical polymerization

AbstractA study has been made to clarify the correlation between the structures of vinyl esters and their radical reactivities by applying the following TAFT's equation: From the results of the copolymerizations of seven vinyl esters (M2) with vinyl chloride (M1) it was found that the relative reactivities (1/r1) of vinyl esters towards the poly(vinyl chloride) radical are dependent on the polar substituent constants (σ*) of the acyl groups, but not on their steric substituent constants (Es). The ρ* and δ values found were 0.094 ± 0.009 and 0, respectively. It was also observed that the Q and e values for vinyl esters tend to increase with their σ* values.Moreover, from the determination of 1.2‐glycol units in the poly(vinyl alcohols) derived from the poly(vinyl esters), it was shown that the proportion of a head‐to‐head addition in homopolymerizations of vinyl esters decreases with increasing σ* values, except for the case of vinyl benzoate.

Argentation Constants of Styrene Derivatives

Abstract Equilibrium constants for the formation of silver ion complexes (argentation constants) of styrene derivatives in water were measured at 0 and 25°C by Andrews and Keefer’s solubility method. The argentation constants for the 1 : 1 complex (K1) decreased in this order: cis-β-alkylstyrene>α-alkylstyrene>trans-β-alkylstyrene, for a given alkyl group. This order was related to differences in the resonance stabilization of the double bond. The K1 values of the α- and β-alkylstyrene series did not seem to correlate with Taft’s steric substituent constant, Es, within the respective series. However, when the hyper conjugation effect of alkyl substituents was considered, a good linearity between logK1 and Hancock’s parameter, Esc, was obtained. Thus, the steric effect was separated from the hyperconjugation effect, and it was found that the steric effects for the three series are nearly identical. Thermodynamic data indicated that an isokinetic relationship holds between ΔH and ΔS and that the argentation equilibrium is controlled mainly by the entropy factor. The degree of methyl substitution at the double bond of styrene slightly influenced the K1 values, although 1,2-dihydronaphthalene had an abnormally large K1 value.

Transmission of polar effects. Part XI. Polar and steric effects in the reactions of arylaliphatic carboxylic acids

The rate coefficients for the reaction with diazodiphenylmethane in ethanol at 30° and the acid-catalyzed esterification with methanol at 60° of a number of arylaliphatic carboxylic acids and their pKa values in 80% 2-methoxyethanol–water have been measured. Rate coefficients have also been measured for the alkaline hydrolysis in 70% dioxan–water of their 2-diethylaminoethyl esters at 45°. The pKa values and diazodiphenylmethane esterification rate coefficients were also measured for a series of 2-substituted fluorene-9-carboxylic acids. The latter results have been correlated by Hammett equations and the transmission of polar effects in fluorene is discussed. Polar and steric substituent constants are evaluated for the arylaliphatic systems. These values are used to successfully correlate the hydrolysis and ionization reactions using the Taft–Ingold equation.

Effect of ester alkyl groups on the copolymerizations of alkyl thiolacrylates with styrene

AbstractA study of the effect of ester alkyl groups In alkyl thiolacrylates (RTA) on the copolymerization reactivities of these monomers (M2) having five kinds of ester alkyl groups with styrene (M1) has been made at 60°C. From the results obtained the monomer reactivity ratios and the Q2 and e2 values were determined. It was found that the relative reactivities (1/r1) of RTA to the polystyryl radical were correlated by the polar substituent constants (σ*) of the ester alkyl groups in RTA, not by their steric substituent constants (Es) in the modified TAFT equation: From the results obtained, the ρ* and δ values were obtained as 0.28 and 0, respectively. It was also observed that the Q2 and e2 values for RTA were correlated by the TAFT σ* constants of the alkyl groups in RTA.

Determination of propagation and termination rate constants for some methacrylates in their radical polymerizations

AbstractRotating sector determinations of kp and 2kt for ten methacrylates undergoing radical polymerization were carried out at 30°C. Ester groups in the monomers were: isopropyl, ethyl, β‐cyclohexylethyl, methyl, γ‐phenylpropyl, β‐phenylethyl, β‐methoxyethyl, benzyl, β‐chloroethyl, and phenyl. Values of kp obtained were 121, 126, 1190, 141, 149, 228, 249, 1250, 254, and 411 l./mole‐sec., respectively; values of 2kt × 10−6 were 4.52, 7.35, 32.8, 11.6, 0.813, 1.88, 9.30, 41.9, 6.71, and 11.9 l./mole‐sec., respectively.Omitting the data for the β‐cyclohexylethyl and benzyl esters, a Taft correlation, log kp = (0.70 ± 0.18)σ* + 2.2, was established, where σ* denotes Taft's polar substitutent constants for the above‐mentioned ester groups. The steric substituent constants Es were found to have no influence on kp. Combination of kp with r2 data from copolymerization studies with styrene or methyl methacrylate as M1 comonomer revealed that the more reactive monomer gave rise to the more reactive polymer radical. Monomer viscosities and molar volumes of the ester groups were found to correlate with 2kt.

The separation of polar and steric effects. Part VI. The kinetics of the acid-catalysed esterification of substituted benzoic acids by methanol

Rate coefficients have been determined for the acid-catalysed esterification of benzoic acid and of o-XC6H4·CO2H (X = F, Cl, Br, I, Me, Et, or Pri) by methanol, with toluene-p-sulphonic acid as catalyst, at several temperatures. The acids m-XC6H4·CO2H (X = Cl, Me, or NO2) have been similarly studied at 60°, and approximate rate coefficients have been determined for the esterification of o-t-butylbenzoic acid at 100 and 120°.Arrhenius parameters and the related quantities of transition-state theory have been evaluated for the reactions of the ortho-substituted acids. The effect of ortho-substitution on the entropy of activation (ΔS‡) in esterification is discussed in terms of a ‘bulk effect’ which tends to decrease ΔS‡, and steric inhibition of solvation of the transition state which tends to increase ΔS‡. All the ortho-substituents studied (except t-butyl) depress the enthalpy of activation (ΔH‡) relative to that for the reaction of the parent acid and two effects are tentatively suggested to account for this, namely, the secondary steric effect of ortho-substituents, and a steric inhibition of ‘ring solvation’ of the initial state, which may be brought about by substituents in any position in the ring.Taft steric substituent constants (Es) have been evaluated for the ortho-substituted acids, and these are compared with values obtained from other systems. The differences found are discussed in terms of the Hammett reaction constant (ρ) and of the nature of the solvent.It is also concluded that results from the kinetics of esterification cannot replace those from acid-catalysed ester hydrolysis for the purpose of accurately satisfying the conditions of the Taft relation.

The Acid Catalysed Formation of the Peracids from ω-Phenyl-and ω-Phenoxy Fatty Acids with Hydrogen Peroxide

The acid catalysed formation of peracids from ω-phenyl and ω-phenoxy fatty acids (benzoic (I), phenylacetic (II), β-phenylpropionic (III), γ-phenylbutyric (IV), δ-phenylvaleric (V), phenoxyacetic (VI) and β-phenoxypropionic (VII) acids) has been studied kinetically in dioxane. The comparison of the second order rate constants indicated the rapid formation of the peracids from II, III, IV and VII. Plots of logarithms of the rate constants vs Taft's steric substituent constants show that the reaction is controlled by steric rather than polar effects. The activation parameters suggest the effect of benzene ring on the carboxyl group of II, which increases freedom of the transition state for the peracid formation.

Kinetic Studies of Bimolecular Nucleophilic Substitution. II. Structural Effects of Alkyl Halides on the Rate of SN2 Reactions —A Reinvestigation of the Linear Free-Energy Relationships for the Structural Variation of the Alkyl Groups

Abstract The linear free-energy relationships have been examined for 35 SN2 reaction series, including 4 series of the SN2 reactions of alkyl bromides which rates were measured in these laboratories. These results have then been treated using the equation, log(k⁄k0)=rα, where α is a structural constant for methyl, ethyl, n-propyl, n-butyl, i-propyl, i-butyl, and neopentyl groups and where r is a reaction constant. The α values have been estimated as the arithmetic means of the logkSN2 values of the reaction series examined for each alkyl. The correlation coefficients for r’s are greater than 0.960 for all of the reaction series except two. The inapplicability of Taft’s polar and steric substituent constants (σ* and Es) to the SN2 reactivity of these alkyl groups has been discussed.

Functional-group and substituent effects, both steric and electronic, upon the ultraviolet absorption spectra of some conjugated heteroenoid compounds

The positions of the long-wavelength maxima in the spectra of some 300 compounds of generalized formula I may be accommodated by assuming that the observed maximum is equal to that of styrene + Δλ (the conjugative effect of A) + Δλ′ (the inductive effect of B) + ΣΔλ″ (the sum of the steric effects of the A, B, and phenyl groups) + D (an empirical parameter related to X and Y). These arguments have been extended to the spectra of II. Δλ′ is related to the σ value for meta-substituted benzene derivatives or to σ*. D for meta- and [Formula: see text]para-substituted derivatives roughly correlates with the appropriate σ+ values, whereas D for ortho-substituted derivatives is related to a simple function involving the D value for para substitution and the steric substituent constant, ES. For compounds of generalized formula III, the electronic effects of the A and B groups cancel but the steric effects are additive.Multiple bands occur in the long-wavelength region of the spectra for compounds containing halogen, alkoxy, or amino groups. These groups are highly polarizable and are considered to function both as electron donors and electron sinks.

Some considerations about solvent effect on infra-red absorption intensities

Measurements have been made on the infra-red absorption intensities of the CN bands of acetonitrile, propionitrile, n-butyronitrile and the CO bands of acetone and methyl ethyl ketone in fifteen solvents. With the object of relating the intensities in solution to the gas-phase values, solvent effects on the band intensities were studied with a view to the specific molecular interaction effect between solute and solvent. An attempt was made to interpret the results using a new equation formed by application of the electronic theory, log A s / A g = aσ* + bE s + c, where σ* and E s are the Taft's inductive factor and the steric substituent constant.

Reactivities of alkyl methacrylates in their radical polymerization

AbstractThe effect of the alkyl group on radical polymerizations of alkyl methacrylates (RMA) has been investigated. From the results on copolymerizations of RMA(M2) with styrene or B‐chloroethyl methacrylate (M1), the relative reactivities (lr1) of RMA toward attack of polymer radicals were plotted by the Taft's equation: log (1/r1) = ρ* σ*+δE8. The plot could correlate with polar substituent constants (σ*), but not steric substituent constants (E8) of the alkyl substituents in RMA. These results might strongly indicate that the radical reactivities of RMA depended on the polar character of the alkyl groups, but not on their steric character. This conclusion is compared with other experimental results and discussed.

The Relative Reactivities of Alkyl Acrylates in Their Radical Copolymerizations

Abstract In order to clarify the effect of the alkyl groups in alkyl acrylates (RA) on their reactivities to the attack of a radical, the copolymerization of RA (M2) with styrene (M1) has been carried out at 60°C. From the results obtained, the monomer reactivity ratios and the Q2 and e2 values were determined. It was found that the relative reactivities (1/r1) of RA to a polystyryl radical were correlated only by the polar substituent constant (σ*) of the alkyl group in RA, not by their steric substituent constant (Es) in Taft’s equation: log(1⁄r1)=ρ*σ*+δEs According to the above equation, the ρ* and δ values were obtained as 0.56 and 0, respectively. It was also observed that the Q2 and e2 values for RA were correlated by Taft’s σ* substituent constant.