Analysis of the ortho effect: acidity of 2-substituted benzoic acids

Abstract

The classical term ortho effect was quantitatively analyzed and decomposed into its real or supposed components, using the acidity of eleven 2-substituted benzoic acids as a sample. The substituent effects were evaluated by calculations at the B3LYP/6-311þG(d,p) level by means of isodesmic reactions, separately for the acid molecules and for their anions. An intramolecular hydrogen bond, affecting the acidity moderately, was found only in the case of 2-methoxy-and 2-dimethylaminobenzoic acids. For the other acids, the oftenemployed interpretation invoking a hydrogen bond is wrong: the pertinent form was not revealed by calculations or by infrared spectroscopy. Another widespread explanation relying on the steric inhibition of resonance of the COOH group is also not correct in most cases: only some acid molecules are nonplanar (with substituents tert-Bu, CH 2 Cl, CF 3 , NO 2 , Cl) and the steric inhibition exists also in the anions, thus partly cancelling this effect.The main part of the ortho effect consists of polar effects (inductive and resonance) transmitted through the ring and of purely steric effects. Their tentative separation was based on comparison with the 4-substituted isomers. Polar effects are most important for the acidity and are not very different in the ortho and para positions (rough estimate 0.8:1). Steric effects are strong but mostly of similar magnitude in the acid as in the anion; hence they are of restricted importance for the acidity. They may be better described as van der Waals interactions rather than as short-range field effects. The whole analysis leads to the conclusion that the acidity of ortho-substituted benzoic acids cannot be interpreted by a simple universal theory.Benzene ortho derivatives have received less attention compared to their meta and para isomers. While in the latter the dependence of the reactivity and properties on structure is simple and can be approximated by the Hammett equation, effects of ortho substituents are much more complex. 1,2 Mostly they have been assumed to arise from two contributions: one similar as in the meta and para isomers, called inductive effect and resonance, 1,3 the other commonly called steric. The latter was further divided into several tentative components, 2,4,5 which were not always exactly identified: primary steric effect, steric inhibition to resonance (SIR), short-range field effect, hydrogen bonds (HB), steric inhibition to solvation and others. In numerous treatments of this subject, certain of these effects were more or less stressed and sometimes they were expressed quantitatively by an empirical equation with limited success. 4 Note that some of these effects are only simplified models, sometimes poorly defined: for instance, one can hardly distinguish a short-range field effect from a primary steric effect. As an extreme opinion, reactivities of ortho derivatives were explained without referring to any steric effects at all, 3 either by a blend of inductive and resonance effects 3a,b or most simply, by correlation with the...

Structures, luminescence and vibrational spectroscopy of europium and terbium nitro- and dinitro-substituted benzoates. Nitro groups as quenchers of Ln3+ luminescence

“…It is known, that polar substituent effects transmitted through the benzene ring are similar in the cases of substituents in the ortho and para positions, and are different from effects for substituents in the meta position [35][36][37][38]. However, in the case of substituents in the ortho position a steric factor may be significant that will counteract to the polar substituent effect.…”

On the Harmonic Oscillator Model of Electron Delocalization (HOMED) Index and its Application to Heteroatomic π-Electron Systems

“…Differences in the calculated bond lengths are not larger than 0.0002 Å. Since the DFT(B3LYP)/6-311+G(d,p) level has been very often used in the literature to study the geometry of π-electron systems [10,11,[36][37][38][39], we chose this level of theory for the HOMED estimation of all heteroatomic π-electron systems considered in this paper. Table 7 summarizes the HOMED indices for the DFT structures of the simplest triad molecules.…”

“…hyperconjugation, conjugation, aromaticity, tautomerism, proton-transfer, and/or hydrogen bonding, may take place [10,11,[36][37][38][39][40]. We analyzed π-electron delocalization for simple σ-π hyperconjugated, n-π conjugated, π -π conjugated, and aromatic compounds including those displaying prototropic tautomerism.…”

Analysis of the Ortho Effect: Basicity of 2-Substituted Benzonitriles

“…Some of these effects could be quantitatively estimated by means of suitable model compounds 2 ; in other cases this was not possible 11 . Our results 2,11 and most of the literature data 3-7 were obtained on 2-substituted benzoic acids, mainly on their dissociation constants in solution. In terms of isodesmic 12 (and homodesmotic 13 ) reactions the dissociation reaction is formulated as Eq.…”

“…steric component 1,[6][7][8] , but this treatment was criticized 4 . The classical problem is of standing interest [7][8][9][10] .…”