Infrared spectra and conformation of methyl-substituted benzoic acids

Abstract

Infrared spectra of all isomers of polymethyl-substituted benzoic acids were recorded in the carbonyl and hydroxyl regions in tetrachloromethane at various concentrations and interpreted in terms of conformation. According to a plot of #(C=O) of the monomeric form vs Hammett substituent constants ', these compounds may be classified into two classes. Derivatives with none or only one methyl group in the ortho position are concluded to exist in an equilibrium of two planar conformations, unless the equilibrium is degenerate. Derivatives with two orthosituated methyl groups are concluded to take up one non-planar conformation. These findings are supported by the shape of the hydroxyl and carbonyl bands, which are unsymmetrical in the former class, although they could not be reliably separated into bands pertinent to the individual conformers. This conclusion is at variance with the common interpretation which has invariably ascribed to these and similar ortho derivatives non-planar conformations with a variable torsional angle.

Structure of Methyl 2-Methylbenzoate: Steric Effect of a Smallortho-Substituent

“…The ultimate conclusion can be formulated in two ways: if one should decide between a one-conformer or a two-conformer model, both methods agree in favor of the latter, and our previous analyses [14,16,17] were confirmed. When more detailed information is required and the question is whether the conformation is planar or slightly nonplanar, the two methods need to give identical answers.…”

“…[5,12] The variable angle φ served as a generally accepted explanation of stronger or weaker effects in individual compounds. However, this explanation may be valid only in the case of more strongly hindered compounds, [13,14] and was challenged [15,16] for spectively. On the other hand, the corresponding absolute values found by GED differed by 29°from the values of 0°and 180°, that is, the average absolute torsional angles were found to be 29°and 151°.…”

“…Hence, the theory of hindered resonance had to be revised and restricted in its application. [14,16] The angle φ does not vary much in individual benzoic acids and is often either zero or near to 60°. The effect on the acid strength, rather different according to the number and size of the substituents, is attributed mainly to an electrostatic interaction stabilizing the anion.…”

“…[18,19] The actual experimental proofs of the planar conformation of 1B and 1C (and of further polymethylbenzoic acids with only one ortho methyl group) are mostly indirect. They include the X-ray analysis of 2-methyl-5-nitrobenzoic acid in the crystalline state, [15] but are mainly based on detailed comparison of all polymethyl-substituted benzoic acids; for this, their IR spectra in solution, [14] the gas-phase acidities, or the gas-phase enthalpies of formation was used. [16] On the other hand, the conformation of methyl 2-methyl-5nitrobenzoate in crystal is not perfectly planar, and its dipole moment in solution could be explained by both a non- Figure 1.…”

Intramolecular hydrogen bonding in o‐hydroxyaryl Schiff bases

“…Steric effect research has seen major advances in the last decade, with important contributions from Exner's group. [1][2][3][4][5][6][7][8][9][10][11][12] This paper focuses on several aspects of the o-hydroxyaryl Schiff bases research. The paper focuses on the intramolecular hydrogen bond of the O-HÁ Á ÁN type which forms between the hydroxyl group of the phenol ring (the proton donor) and the nitrogen atom of the imine (the proton acceptor) (Scheme 1).…”