Range of validity of the Hammett equation: acidity of substituted ethynylbenzenes

Abstract

Acidities of 19 meta-and para-substituted ethynylbenzenes were calculated at the B3LYP/6-311þG(d,p) level and correlated within the framework of the Hammett equation with the calculated acidities of equally substituted benzoic acids. The substituent effects were decomposed in terms of isodesmic reactions into those operating in the anions and in the uncharged molecules. Characteristic deviations from the Hammett equation were found for para-substituents, both for acceptors and donors; the former can be interpreted by the resonance formula only with an electron sextet. With reference to the series of ionization reactions investigated previously, it was possible to reinvestigate the validity of the Hammett equation on the basis of calculated reaction energies using a more homogeneous data set than had been ever accessible from the experimental reactivities. The equation was fulfilled for all meta-substituents with a higher accuracy than commonly attainable with the experimental data. When para-substituents were included, deviations occurred according to the character of the functional group: When this group was an acceptor, the donor substituents showed deviations and vice versa. Another series of reactions proceeding between uncharged groups bonded directly on the benzene ring was investigated in the same way: The Hammett equation held with a similar precision, although its original range of validity was surpassed. The properties of a set of common substituents were investigated by principal component analysis and cluster analysis. There is a fundamental difference between uniform acceptors and more discriminated donors but clustering is not so strong to depreciate common statistical analysis.The Hammett equation has become popular, in spite of its restricted applicability, as the first attempt to predict reactivity by means of an empirical formula. For many years, it has remained the most general and simplest structure-property relationship.1,2 In energy terms, it is expressed by eqn (1).The symbol E(X) may mean the reaction energy or enthalpy but in most cases it stands for the Gibbs energy, or activation Gibbs energy, in a series of reactions of meta-or para-substituted benzene derivatives. The empirical parameter s m or s p characterizes the substituent X, the parameter r is pertinent to the given reaction, E(H) relates to the unsubstituted compound (X = H). In simple terms of organic chemistry, eqn (1) means that the variable substituents X raise changes of reactivity always in the same succession: for instance the substituent effect of 4-CO 2 CH 3 is always twice the effect of 4-Cl. Eqn (1) has the main importance in interpreting the values of the parameters 2 s and r; less in predicting the unknown values of E. Nevertheless, it serves still as reference when investigating reactivities or activities of benzene derivatives. 3The exact range of validity is of central importance for any empirical relationship 2 and numerous attempts were made to delimit it for eqn (1) with more precision. 2,4 In kinetics, the...

exo-Substituent effects in halogenated icosahedral (B12H122–) and octahedral (B6H62–) closo-borane skeletons: chemical reactivity studied by experimental and quantum chemical methods

“…The spectra were measured with B(OMe) 3 in a glass capillary as the internal reference (18.1 ppm). The coupling constants 1 J( 11 B-1 H) are given in Hz and are taken from the resolutionenhanced 11 B spectra with a digital resolution of 2 Hz. The NMR data presented in the text below are in the following format: 11 B NMR: 11 B chemical shifts, δ( 11 B) in ppm, multiplicity, coupling constants J( 11 B-1 H) in Hz.…”

“…Two basis sets, namely the def2-SVP and def2-TZVP (for details, see above) were combined with either of the two above-mentioned methods. 11 B chemical shifts -δ( 11 B) -were calculated relative to diborane (B 2 H 6 ) and converted to the standard BF 3 ·OEt 2 scale by using an experimental δ( 11 B) value of diborane of 16.6 ppm (Eq. (2)) 33 .…”

“…A comparison will be made with the aromatic organic systems, such as the benzene molecule, as other representatives of structures with highly delocalized electronic distribution. For these, the substituent effects are well-known and have also been studied theoretically in great depth by Exner and Böhm [10][11][12][13][14] . Furthermore, the geometrical consequences of benzene ring exo-substitutions have been carefully examined by Domenicano 15 , and this approach has been applied to exo-substitutions in the closo-1,12-C 2 B 10 H 12 cluster, i.e.…”

“…In the first part, we will present the experimental data on the halogenation reactions of B12 along with the results from 11 B NMR spectroscopy. Subsequently, we will investigate the electronic structure and equilibrium geometries, 11 B NMR chemical shifts, the stability, and halogenation reactions of the B12 and B6 skeletons using quantum chemical (QM) calculations. In the end, we will draw, based on both the experimental and theoretical data, conclusions concerning the directive effects of the halogen substituents on the B12 and B6 cages.…”

Substituent Effects of the Alkyl Groups: Polarity vs. Polarizability

“…[45] The main shortcoming of the traditional treatment was that different phenomena were treated under one heading. The results of this paper belong into this category.…”