Use of two-parameter correlations for studying the radical reactivity of aromatic compounds

Two-parameter equation correlations are reported for radical reactions of aromatic compounds. In these correlations polar and resonance substituent constants identical with the substituent constants of aliphatic compounds were used. The equations correlate the rate constants for H abstraction reactions and for the addition of a variety of free radicals to the ortho-, meta-, and para-substituted aromatic compounds. Besides, they correlate parameters of the spectra for substituted aromatic radicals. The correlations show that the effects of para substituents on the reactions studied are nearly entirely resonance effects, whereas for the meta- and ortho-substituted compounds polar (inductive) effects become essential. Application of the two-parameter correlations permits also to determine the structure of transition states (σ or π-complex) in free-radical reactions.     

Electrophilic bromination of gaseous aromatic compounds: Mechanism and linear free energy effects on reaction rates

Electrophilic bromination of monosubstituted aromatic compounds is effected in a pentaquadrupole mass spectrometer using BrCO⁺ and CH₃NH₂Br⁺ as mass-selected reagent ions. Reaction normally occurs at the ring and the brominated product can be mass selected in turn and caused to dissociate by Br˙ loss upon collision-induced dissociation. Linear free energy correlations with Brown substituent σ⁺ constants describe the extent of gas-phase bromine cation addition under the non-equilibrium, low-pressure and solvent-free conditions which pertain in quadruple collision cells. The electrophilic addition reaction proceeds via a σ-complex to the ring as suggested by MS³ spectra, except in the case of nitrobenzene, where substituent bromination is suggested by the occurrence of a competitive process in which the nitrosubstituent is displaced by bromine. The reactivity parameters ρ are ?0.23 and ?0.56 for the gaseous reagents, BrCO⁺ and CH₃NH₂Br⁺, respectively. Both values are much less negative than corresponding values for bromination in solution. The greater reactivity of BrCO⁺ is evident by the fact that it reacts even with the strongly deactivated substrates and this is consistent with a weak Br? CO bond. Competitive protonation occurs in the case of CH₃NH₂Br⁺ and, unlike bromination, the rate of this reaction does not correlate with σ⁺ values. This is suggested to be a consequence of protonation at the ring in some cases and at the substituent in others, including acetophenone and benzonitrile. Evidence for this is that, in contrast to its lack of correlation with substituent constants, the rate of protonation correlates linearly with proton affinity.     

Rate constants for reactions of perfluorobutylperoxyl radical with alkenes

Perfluorobutylperoxyl radicals were produced by radiolytic reduction of perfluorobutyl iodide in aerated methanol solutions. Rate constants for the reactions of this peroxyl radical with various organic compounds were determined by kinetic spectrophotometric pulse radiolysis. The rate constants for alkanes and alkenes were determined by competition kinetics using chlorpromazine as a reference. The results indicate that hydrogen abstraction from aliphatic compounds takes place with a rate constant that is too slow to measure in our system (<10⁵ M^?1 s^?1), and that abstraction of allylic and doubly allylic hydrogens is slow compared with addition. Addition to alkenes takes place with rate constants of the order of k = 10⁶ ? 10⁸ M^?1 s^?1. Good correlation was obtained between log k and the Taft substituent constants σ* for the various substituents on the double bond. Perfluorobutylperoxyl radical is found to be more reactive than trichloromethylperoxyl and other peroxyl radicals.     

The Derivation of Inductive Substituent Constants from pKa Values of 4-Substituted Quinuclidines. Polar effects. Part I

Thermodynamic pK_a-values have been determined for 38 4-substituted quinuclidinium perchlorates. They are remarkably sensitive to the polar effect of the substituent and cover a range of 3.63 pK_a units. Furthermore, they vary linearly and almost equally with temperature since the contribution of the TΔS° term to the free energy of ionization is relatively small and constant. The magnitude of the polar effect of the 4-cyano group varies with the solvent and appears to depend on its ability to form hydrogen bonds to the substituent rather than its dielectric constant. New inductive substituent constants σ_I^q are derived from the pK_a values. Their correlation with known inductive constants is only fair or unsatisfactory, especially as regards the relative order of hydrogen and the alkyl groups. The discrepancies can be ascribed mainly to the different models used to derive the substituent constants.     

A 19F NMR study of the transmission of electronic effects in triarylantimony and triarylbismuth compounds. A comparison with the nitrogen- and carbon-containing analogues

A ¹⁹F NMR study of the transmission of electronic effects has been made for the systems Ar₂EC₆H₄F-4 (E = Sb, Bi, CH, N). The fluorine chemical shifts obtained are correlated with the polar constants (Σσ^o and Σσ) of the substituents, suggesting that electronic effects are transmitted through the SbC_ar, BiC_ar and CC_ar bonds predominantly by an inductive mechanism, whereas the transmission through the NC_ar bonds is contributed to significantly by classical resonance effects due to competitive conjugation of the lone pair with the aromatic rings, and the substituents therein. A dual parameter correlation of the fluorine chemical shifts with the inductive (σ_I) and resonance (σ^o_R and σ_R) parameters of the substituents in the aromatic rings has led to similar conclusions. The inductive transmission through the bridging Sb and Bi atoms has been assigned to the absence of conjugation of lone pair and vacant d-orbitals of the metals with π-electron systems of the aromatic rings. On the basis of the values of the ? coefficients for the correlation equations obtained it has been established that the transmitting ability of the BiC_ar bonds is close to that of the C_alC_ar bonds and considerably lower than the transmitting ability of the NC_ar bonds.     

Scf-perturbational analysis of substituent effects in the dediazoniation of aryldiazonium ions

A M.O. rationalization of the substituent effects on the rate constants for the dediazoniation of substituted aryldiazonium ions has been performed using SCF-perturbation theory. The total perturbation energies arising from a distortion of the Ar-N bonds reflect the substituent effects on the rate constants. The substituent effects are analysed by means of energy components characterising the role of the unperturbed electron density distribution, its relaxation via mixing of orbitals and the nuclear repulsion energy. The influences of the substituents are separated into σ and π contributions using energy increments for localised orbitals. These energy components of the perturbation energy correlate reasonably with the σ_l and σ_R⁺ constants for the inductive and resonance effects, respectively, of the substituents.     

The two-parameter taft equation in kinetics of free radical reactions

A new method was developed for the calculation of the resonance substituent constants of the two-parameter Taft equation log k_sub/k₀=ρ*σ*+r*σ^r. It is based on a relationship between the spin density in free radicals and the rate constants of radical substitution reactions of CH₃. Possibilities and limitations of the application of this correlation equation to the investigation of substitution and addition radical reactions are discussed.     

Inductive substituent constants σji from olefinic geminal 1h, 1h nmr coupling constants

Theoretical and experimental arguments are presented supporting the postulate that olefinic NMR coupling constants ²J_HH are insensitive to β-substituent influences on the olefinic π orbital. A new set of substituent constants, σ^J_I, is proposed to measure directly the inductive substituent effect transmitted by σ-bonds. The previously available range of inductive substituent constants can be appreciably extended in this way. Comparisons of σ^J_I with other observables and parameters for selected substituents are made as a test of consistency.     

Conformational Analysis of Biphenyl Derivatives. I. A Novel Steric Constant from the Internal Rotation Rate of 2,2′‐Bis‐(N‐Substituted Carbamoylmethyl)Biphenyl by Means of Dynamic NMR

A novel scale of steric substituent constant E_s^D is defined from the correlation of the logarithms of the internal rotation rate (k_r) at 393 K with Hancock (E_s^c) steric constant by means of dynamic NMR. In the inhibition of Pseudomona species lipase by 2,2′‐bis‐(N‐substituted carbamoylmethyl)biphenyls (1‐8), the logarithms of bimolecular rate constants are multiply correlated with both the Taft substituent constant σ* and E_s^D.     

Correlation for prediction of interfacial tensions of pure alkane,naphthenic and aromatic compounds between their freezing and critical points

A simple equation has been developed for predicting the interfacial tensions of pure alkanes, aliphatic and aromatic hydrocarbons between their freezing and critical points. The equation has the form

and represents the interfacial tension using a generalized correlation developed by Sivaraman et al. (1984) for predicting the latent heats of vaporization of normal fluids and coal-liquid model compounds. Here σ^* is the reduced dimensionless interfacial tension, and L^*₍₀₎ and L^*₍₁₎ are the reduced dimensionless latent heats of vaporization; A and N are system-independent constants. Subsequent tests of this correlation in predicting interfacial tension over a broad domain of reduced temperatures for a large number of different types of compounds have confirmed the validity of our approach. The percentage deviations in interfacial tension are in the range? 3.87 to 5.99 in the range of reduced temperatures 0.03 < ? = (T_c ? T)/T_c < 0.55.     

14N nuclear quadrupole resonance in N-acetyl amino acids

¹⁴N Nuclear quadrupole resonance (NQR) spectra of several N-acetyl amino acids and related compounds are reported and analyzed within the framework of the Townes and Dailey theory. The inductive effect of the chloroacetyl group on the nitrogen is discussed. A positive correlation between the π-σ_NC electron density at the nitrogen and the Taft inductive parameter σ* is observed, suggesting that the nitrogen π-charge density in the N-acetyl amino acids does not vary appreciably. NQR data for hippuric acid are compared to the ¹⁴N quadrupole parameters extracted from ENDOR data reported for X-irradiated hippuric acid.     

Selective nucleophilic substitution reactions on poly(epichlorohydrin) using aromatic and aliphatic thiol compounds

Poly(epichlorohydrin) has been modified chemically using aromatic and aliphatic thiol compounds. The reactivity and kinetics of these modifiers with respect to substitution and elimination was studied. Therefore, the chemical structure of the reaction products was analysed using ¹³C NMR, ¹H NMR and ¹³C-DEPT spectroscopies. It is shown that both, aromatic as well as aliphatic thiols, are highly selective with respect to nucleophilic substitution as reaction conditions can be found which allow one to achieve degrees of modification of up to 90% without any elimination side-reaction. As a consequence no degradative chain-scission takes place what has been confirmed by GPC analysis.A comparison between both types of thiol modifiers shows that aromatic ones react faster and that higher degrees of modification are reached than with their aliphatic homologues.     

Natural abundance 13C NMR investigation of substituted bromobenzene using noise decoupling of proton resonances

The natural abundance ¹³C NMR spectra of bromobenzene and ten derivatives have been obtained using the technique of noise modulated decoupling of the proton resonances. Assignments have been made for all ¹³C resonance signals using an additivity rule. The chemical shifts of the aromatic carbon nuclei in the para-substituted compounds are discussed in terms of the Taft parameters (σ_R, σ_I) and the electronegativity of the substituent.     

Bond dissociation energies and the Hammett correlation,Part 1: Remotely substituted aromatic compounds and vitamin E

Four hundred and seventy nine experimental values of X? Z bond dissociation energies (BDEs) of para‐, ortho‐, meta‐, and multisubstituted aromatic compounds were reproduced by the following simple equations: Here X represents a wide variety of univalent atoms or groups; Y is one or several various remote substituents; and Z represents an oxygen, nitrogen, carbon, or sulfur atom. DH⁰(X? ZC₅H₆) is a known reference/anchor point for a series of X? Z bonds. E_sr is the remote conjugation energy between the substituents and the reaction center (broken bond), and can be expressed as a Hammett‐type correlation. σ⁺(Y) is the Brown–Okamoto constant of a substituent Y or the sum of several substituent constants. The Hammett slope or reaction constant ρ is a linear function of the difference between the covalent potentials V_x(Z) and V_x(X). It has been found that and The set of equations are very powerful for the prediction of BDEs, chemical reactivity, and reaction center (or active site) in antioxidants, nutrients, pharmaceuticals, toxicants, carcinogens, and explosives. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 453–466, 2002     

Carbon-13 and proton magnetic resonance studies of some carbonyl-bis-(amino acid esters)

A number of carbonyl-bis-(amino acid esters) have been examined by proton and carbon-13 nuclear magnetic resonance techniques. All but one of the compounds were synthesized with two chiral centers of like-configuration. In one series, the diastereotopic nonequivalence of isopropyl methyl groups attached to the asymmetric centers is apparent in both the proton and the carbon spectra, and the relative magnitude of the observed nonequivalence increases slightly with increasing ‘bulk’ of the neighboring ester groups. Carbon-13 chemical shifts are reported, and a linear correlation of Taft σ* inductive constants with ester carbonyl carbon chemical shifts and with amide proton chemical shifts (for a series in which only variation of the ester substituent occurs) is presented. In addition, the effect in terms of chemical shift differences of keeping the same ester group at the asymmetric centers while varying the other substituent group, is examined.     

Quantum-chemical study on nitrosonium complexes of Bi- and polycyclic aromatic compounds

The affinities of bi- and polycyclic aromatic compounds for nitrosonium ion (A_NO⁺ A_{NO^ + } ) were calculated at the RI-MP2/L1 and DFT/PBE/3z levels of theory. Both methods gave generally similar geometric parameters of nitrosonium complexes. The formation of pincer complexes in which the NO⁺ ion is linked to two aromatic rings is more energetically favorable than the formation of analogous complexes through external binding of NO⁺ to one aromatic ring. Linear correlations were found between the calculated A_NO⁺ A_{NO^ + } values of aromatic compounds and experimental equilibrium constants for the reactions of these compounds with nitrosonium ion in solution. Structural peculiarities of the examined nitrosonium complexes are discussed.     

Polarization effect in the electronically excited state of benzene derivatives and related compounds

The energies of intramolecular charge-transfer bands and excited singlet and triplet states, as well as the molar absorption coefficients of mono- and para-disubstituted benzenes and monosubstituted naphthalenes and pyridines depend not only on the inductive and resonance substituent effects, but also on their polarization effect which can be characterized by σ_α constants. The polarization effect determines the lifetime of the triplet state and phosphorescence quantum yield of monosubstituted naphthalenes.     

Correlation of ground and excited state dissociation constants of trans -para- and ortho-substituted cinnamic acids

The ground and excited state dissociation constants oftrans- para- and ortho-substituted cinnamic acids have been determined in 50% (v/v) dioxan-water mixture at 30°C using the Forster cycle. The measured dissociation constants are analysed in the light of single and dual substituent parameter (DSP) equations. Excited state dissociation constants ofp-substituted cinnamic acids correlate well with the exalted substituent constants. The DSP method of analysis shows that resonance effect is predominant relative to inductive effect in the excited state than in the ground state for the para-substituted acids. The single parameter equation gives poor correlation for the ortho-substituted acids. However, the DSP analysis shows fairly good correlation. The inductive effect is predominant relative to the resonance effect in the excited state than in the ground state     

A Reexamination of Inductive Substituent Constants Derived from pKa Values of 4-Substituted Quinuclidines. Polar effects. Part VIII

pK_a Values of 42 quinuclidinium perchlorates I have been measured in 0.1 M aqueous KCl-solution. In a few cases small corrections of earlier thermodynamic pK_a values are indicated. These measurements, in conjunction with recent X-ray structure determinations, confirm the reliability of the inductive substituent constants σ_I^q derived from them.     

13C, 13C coupling constants and 13C chemical shifts of aromatic carbonyl compounds. Effects of ortho- and peri-interactions involving the carbonyl substituent

Carbon-13 n.m.r. data have been determined for a series of 26 aromatic carbonyl compounds including benzoyl, naphthoyl and pyrenoyl derivatives ¹³C labelled in the carbonyl group. Doubly labelled anthraquinone has also been included. The compounds investigated comprise non-hindered molecules and molecules in which the carbonyl substituent is subject to ortho- or peri-interactions affecting conjugation of the carbonyl group with the aromatic ring. The dependence of long range ¹³C,¹³C coupling constants involving the carbonyl carbon, in particular ²J and ³J, on steric conditions is discussed, as is the possibility of deciding on the orientation of the carbonyl bond. The following results have emerged. ²J(s-t)>²J(s-c) for ketones and aldehydes, and the reverse is valid for acids and acid derivatives. (s-t and s-c refer to the orientation of the C?O group relative to the aromatic bond in question with respect to the connecting single bond). For ketones ³J(t,s-c)<³J(t,s-t), and both of these ³J(t) values decrease with increasing angle of twist, θ, about the single bond, whereas ³J(c,s-c) increases with θ. For acids and acid derivatives no similar regularity was found. (The initial t and c refer to the geometry of the three-bond coupling path). Generally it is found that ³J(t)>³J(c) and ³J(t)>²J, confirming earlier results. Theoretical calculations on a few model compounds are qualitatively in accordance with the experimental results. Some sign determinations for coupling constants are presented. A short discussion is given of substituent effects on chemical shifts. Observed trends are consistent with earlier results.