Steric inhibition of resonance and intramolecular hydrogen bonding compete in determining molecular conformation and acidity.
the verdict
CONTESTED
contested - evenly split
refutedsupported
the weight of evidence
1 source for · 1 against
Chemical literature evaluates steric inhibition of resonance and intramolecular hydrogen bonding as factors influencing molecular conformation and acidity, but disputes their relative prevalence and importance across different systems.
The structures of ortho-substituted benzoic acids with substituents bearing hydrogen atoms (OH, NH2, COOH and SO2NH2) were investigated by means of IR spectroscopy and of density functional theory at the B3LYP/6-311 + G(d,p) level. All possible conformations, hydrogen bonds, tautomeric forms and zwitterions were taken into consideration and particular attention was given to intramolecular H-bonds and their effect on acidity. Strong H-bonds in the anions of all four acids, were revealed by calculations. In three cases they were confirmed by the IR spectra of the tetrabutylammonium salts in tetrachloromethane solution, while the salt of 1,2-benzenedicarboxylic acid was not sufficiently soluble. The H-bonds are of different strengths but in all cases they are the main cause of the strengthened acidity of these acids in the gas phase and also in solution, although their effect is opposed by weaker H-bonds present in the undissociated acid molecules. The substituent effect on the acidity was evaluated in terms of isodesmic reactions, separately in the acid molecules and in the anions. While the acidity of the 2-OH and 2-NH2 acids is determined essentially by the H-bonds, that of the 2-COOH and 2-SO2NH2 acids is strengthened by the polar effect operating in the undissociated molecule in addition to the H-bond in the anion. The steric inhibition of resonance (SIR), estimated from model conformations with fixed torsional angles, is of little importance. This analysis goes significant
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/6-311+G(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 often-employed 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, CH2Cl, CF3, NO2, 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