Effects Of Meta Substituents Research Articles

Steric effects of meta substituents in substituted tetraphenylporphyrin complexes of ruthenium, indium, titanium, and gallium

Steric effects of meta substituents in substituted tetraphenylporphyrin complexes of ruthenium, indium, titanium, and gallium

Rate parameters for transfer reactions of some polymer radicals: in terms of Swain and Lupton's f and r and the unique positional weighting factors of Williams and Norrington

Rate parameters of the reaction of phenols, deuterated phenols and nitrobenzenes with polyvinylacetate radicals and of aromatic thiols with polymethylmethacrylate radicals have been correlated with Swain and Lupton's substituent constants ( F k and R k ) and Williams and Norrington's unique positional weighting factors ( f j and r j ) by the following equation: P i = α iƒ jF k + β ir jR k + e i + P 0 i where P i 's are the rate parameters, P 0 i being that for a standard reference state. The correlations were found to be quite satisfactory. The sign and magnitude, and the ratio of the reaction dependent parameters α i and β i throw light on the nature of the transition state and the relative contributions of the mesomeric and inductive effects. The present studies also show that the mesomeric effect of meta-substituents is significantly greater than reported by earlier workers.

Carbon‐13 NMR spectra of monosubstituted pyrazines

AbstractCarbon‐13 NMR chemical shifts and one‐bond carbon–hydrogen coupling constants have been obtained at 15·09 MHz. The trends in the carbon chemical shifts obtained for the pyrazines parallel those of monosubstituted benzenes and 2‐substituted pyridines, except for the direct effect of substitution where the pyrazines resemble pyridines not benzenes. The substituent effects on the 13C NMR spectra are generally quite similar to those in the 1H NMR spectra. The 13C NMR spectrum of the tautomeric hydroxypyrazine has been compared with the 13C NMR spectra of 2‐, 3‐ and 4‐hydroxypyridines. Hydroxy compounds that can exist as a cyclic amide show a large meta substituent effect on the chemical carbon shift.

The kinetics of the reactions of picryl chloride with some substituted anilines. Part II.

Arrhenius parameters have been determined for the reactions of picryl chloride with 3-R, 4-R, and 3,5-R2-anilines (where R = NO2, CF3, or SO2Me) in methanol and acetonitrile. In acetonitrile for all three groups, and for the trifluoromethyl and methylsulphonyl groups in methanol, the rate sequence of the 3-substituted anilines is governed entirely by the entropy of activation. The results are discussed in terms of the electronic effects of the substituents and linear free energy correlations are made. The effects of meta-substituents are additive in acetonitrile.

Substituent effects on the formation constants of substituted acetophenone–iodine complexes

The formation constants of complexes of iodine with 11 meta- and para-substituted acetophenones are reported for heptane solutions at 25° and correlated by various linear free energy relationships. Neither σ, σ+, or a combination of σ0(or σn) and σ+ satisfactorily correlate the data. Dual substituent parameter equations furnish better results and show that the effects of meta-substituents are not of the σ0 type whilst those of para-substituents are of the σR+ type. The Thirot equation gives the best empirical fit to the data.

Electrophilic aromatic reactivities via pyrolysis of 1-arylethyl acetates. Part IX. The effect of the trimethylsilyl substituent

The three isomeric 1-(trimethylsilylphenyl)ethyl acetates have been prepared and their rates of pyrolysis together with that of 1-phenylethyl acetate measured between 340·6 and 392·9°. The trimethylsilyl substituent activates in all positions according to the sequence o > m > p as theoretically predicted and found in protodesilylation, but contrary to the activation order observed in nitration, and the deactivation order found in solvolysis of 1-aryl-1-methylethyl chlorides. The variability of the substituent effect in these and some other reactions is attributed mainly to varying demands for resonance stabilisation of the transition states together with some solvent–substituent interactions. The variation in the meta-substituent effect coupled with the high activation by the ortho-substituent indicates that the trimethylsilyl group supplies electrons inductomerically. In the light of the present results the reactivity pattern in nitration of phenyltrimethylsilane is seen to be affected by steric hindrance. Values of σ+ are –0·16 (m-SiMe3) and –0·09 (p-SiMe3).

Restricted Rotation in Aromatic Ketones. I. Substituent Effects on the Barrier to Rotation about the Benzene-to-Carbonyl Bond

Abstract Several aryl isopropyl ketones and aryl benzyl ketones were synthesized and their NMR spectra were recorded at various temperatures. Their temperature dependence was accounted for in terms of the restricted rotation about the benzene-to-carbonyl bond, whose free energies of activation at the coalescence temperatures varied from 9 to 15 kcal/mol according to the steric effect of ortho substituents, the buttressing effect of meta substituents, and the steric effect of the side-chain groups.

Relative Reactivities in the Addition of Free Trichloromethyl Radicals to Substituted Styrenes. An Attempt to Separate Polar and Resonance Effects

Abstract Relative reactivities in dibenzoyl peroxide-catalyzed addition of bromotrichloromethane to nine nuclearsubstituted styrenes have been investigated with unsubstituted styrene. The effect of meta substituents upon the relative rates was correlated with the simple Hammett equation using either σ or σ+ constants. The relative rates of para-substituted styrenes, however, did not obey the simple Hammett relation, with the results best explained on the basis of polar (ρσ+) and resonance (ED) terms. All para substituents examined increase the reactivity. log(k⁄kH)=ρσ++ED The ED term shows a striking correlation with logQ, of the corresponding para styrene in Alfrey-Price′s Q-e scheme of free-radical copolymerization. That the polar term originates in charge-transfer interactions between the olefin and the electrophilic trichloromethyl radical and can be expressed by ρσ+ is further supported by the evidence that charge-transfer energies of substituted styrenes showed good correlation with σ+. Factors influencing the reactivity have been discussed.

The separation of polar and steric effects. Part IX. The kinetics of the reactions of ortho-substituted benzoic acids with diazodiphenylmethane in several solvents

Rate coefficients are reported for the reactions at 30° of diazodiphenylmethane with benzoic acid and its ortho-substituted derivatives (substituents: Me, Et, Pri, But, F, Cl, Br, and I) in dioxan, ethyl acetate, 2-methoxyethanol, ethanol, dimethylformamide, and dimethyl sulphoxide.In dioxan, ethyl acetate, 2-methoxyethanol, and ethanol the rate coefficients for o-alkylbenzoic acids lie in the order H > Me < Et < Pri < But. The retarding influence of an ortho-methyl group, which varies with the solvent, is probably due to a polar effect, combined with a steric effect on solvation. The increase in rate coefficient through the series of acids is due to a secondary steric effect, which does not depend greatly on the solvent. In dimethylformamide or dimethyl sulphoxide, an ortho-methyl group also retards the reaction, particularly in dimethylformamide, but increase in the size of the alkyl group produces little change. This is attributed to powerful solvation of the carboxylic proton, which greatly reduces the conjugation of the carboxy-group with the ring. o-Halogeno-substituents markedly increase the reactivity of benzoic acids towards diazodiphenylmethane in the order H ≪ F < Cl < Br ≈ I, with o-Cl ≫p-Cl. With dimethylformamide as solvent, the accelerating influence of ortho-halogens is not so marked as with the other solvents. The influence of ortho-halogens is believed to be due mainly to a large polar effect. The pKa values for o-halogeno- and o-methyl-benzoic acids are taken to define a rough scale of polar effects, and approximate polar susceptibility constants, ρo, are evaluated for the reaction with diazodiphenylmethane. These are less solvent-dependent than the Hammett ρ-values based on the effect of meta-substituents.Polar substituent constants, σo, for ortho-halogen and -methyl based on the pKa values of benzoic acids, are compared with values calculated on the basis of simple models of the polar effect.

Molecular structure and photochemical reactivity. IX. Photolytic and radiolytic Type II elimination reaction of n-butyrophenones. Effects of meta substituents

Molecular structure and photochemical reactivity. IX. Photolytic and radiolytic Type II elimination reaction of n-butyrophenones. Effects of meta substituents

Aromatic reactivity. Part XXXV. Alkali cleavage of aryltrimethylstannanes: an unusual electrophilic aromatic substitution

We have measured spectrophotometrically the rates of cleavage by aqueous methanolic alkali of the aryl–tin bonds in a series of X·C6H4·SnMe3 compounds. In this reaction, the rate-determining step of which probably involves separation of the carbanion X·C6H4–, the effects of meta substituents are in accord with the Hammett σ constants (σ= 2·18), but the effects of para substituents show an abnormal pattern. For example, there is rate-acceleration not only by electron-withdrawing groups such as p-CF3 and p-Cl, but also by normally electron-releasing groups such as p-OMe and p-NMe2. For the para substituents, the effects are roughly in line with their σ1 constants.The analogy with base-catalysed aromatic hydrogen-exchange is discussed, and substituent effects in the two reactions are compared. An explanation previously advanced for the abnormalities in the hydrogen exchange is examined in the light of the results for the cleavage.

Meta-substituent effects on the kinetics of methoxydechlorination of some 2- and 4-chloroquinolines

The kinetic effects of some meta-substituents on the methoxydechlorination of 2-chloro and 4-chloroquinolines in methanol have been investigated. For all cases where the data for each substituent were available in both series a linear correlation is observed at 75°. The reaction at the 2-position is appreciably less selective than that at the 4-position. The results are consistent with those obtained for the substituent effects from the heteronuclear positions. It is shown that with electron-releasing substituents, including OR, SCH 3, Cl, the reactions are slower than predicted by Hammett's σ m constants, at least partly because of mesomeric interaction with an ortho or para aza-group. The methyl group is affected but slightly by such an interaction. The observed effect of the CN group seems to indicate that in the case of electron-withdrawing groups σ m constants may still predict reactivities correctly in aromatic nucleophilic substitutions. The activation entropies were found to vary at least as widely as the activation enthalpies.