Concentration and solvent effects on the conformational equilibrium of cis-3-ethoxycyclohexanol by 1H NMR and IR spectroscopy

Abstract

NMR data, in CCl 4, show that an increase in the concentration of cis-3-ethoxycyclohexanol ( cis-3-ECH) shifts the conformational equilibrium from the ax–ax conformer (X ax–ax=51% at 0.01 mol L −1), stabilized by an intramolecular hydrogen bond (IAHB), to the eq–eq conformer (X eq–eq=67% at 0.40 mol L −1), which forms intermolecular hydrogen bonds (IEHB), as confirmed by IR data. The Δν values, obtained by IR spectra, indicated that cis-3-ECH presents a stronger IAHB than the cis-3-methoxycyclohexanol, due to the increase in substituent steric and group electronegativity effects. The percentage of eq–eq conformer also increases with the solvent basicity, from 51% (ΔG eqeq– axax=−0.03 kcal mol −1 in CCl 4 to 97% (ΔG eqeq– axax=−2.05 kcal mol −1) in DMSO. Values of 4.58 and 5.39 kcal mol −1, for the IAHB strength in cis-3-ECH, were obtained from the theoretical data at CBS-4M and B3LYP/6-311+G** levels, respectively.