Síntese e estudo experimental e teórico dos equilíbrios conformacionais de 2-halocicloeptanonas e 2-halociclooctanonas

Abstract

In this work we synthesized the 2-fluor, 2-chloro, 2-bromo and 2-iodine derivatives of the cicloheptanone and cyclooctanone. The procedures used are all known from the literature for similar compounds and have been adapted according to the needs. The compounds were purified by distillation, with reasonable incomes, except in the case of 2-fluoro-derivatives, which were purified by HPLC. The study of the conformational equilibrium of 2-halocicloheptanonas and 2- halociclooctanonas was developed by means of infrared spectroscopy and theoretical calculations. The infrared analysis was performed with the bands observed in the fundamental carbonyl stretching and its first overtone in different solvents in order to observe the influence of polarity of the medium in the conformational equilibrium of these compounds. The bands were properly deconvoluted using the GRAMS program in order to establish the contribution of each conformer in equilibrium. Theoretical calculations were performed with density functional theory and ab initio methods, considering both the molecules in vacuum and in the presence of solvents, using a continuous solvation models for this. The conformational preference observed by both infrared spectroscopy and by theoretical calculations is strongly dependent with the polarity of the medium. The conformers with higher dipole moment is favored by increasing the dielectric constant of the solvent. Moreover, stereoelectronic effects, like hiperconjugative and steric effects, also influence the conformational preference. The analysis with the natural bond orbital theory (NBO) shows that the eletronic delocalization for the π*C=O orbital is directly involved in the observed stability of certain conformations. The effect of substituted halogen type can also be noticed, with changes in the conformational preference and energy involved in the interactions of NBO according the period of the halogen.