Claudimar Junker Duarte

Este trabalho contempla um relato de experiência do evento “Alquimagia Circus Show” organizado pelo núcleo de Química do IFMG- Campus Bambuí e que envolveu a associação entre a disciplina de química e os números apresentados em espetáculos circenses. A metodologia adotada consistiu em dividir os discentes que cursavam as disciplinas de Química e que pertenciam aos cursos técnicos integrados e superiores, em oito equipes. Cada uma destas ficou responsável pela elaboração de uma apresentação circense com duração entre 10 e 15 minutos, relacionando a química em seus números. As apresentações aconteceram no anfiteatro aberto do Campus, contando com a presença da comunidade acadêmica e externa. As equipes foram avaliadas por uma banca composta por servidores internos e externos ao Campus. Os resultados revelaram que o evento desenvolvido logrou êxito na medida em que além de contemplar a química no circo, foi capaz de integrar os participantes, os coordenadores, avaliadores e público visitante em um verdadeiro espetáculo. Os resultados indicaram que o evento pode atuar como um recurso didático-pedagógico favorecedor do processo de ensino e aprendizagem.

2-Fluorocyclohexanone undergoes chair inversion, giving rise to axial and equatorial conformers, with the equatorial form being highly preferred in solution, for example, 87% in chloroform and 93% in methylene chloride. Modifications in the conformational preferences can modify macroscopic properties of 2-fluoro ketones. The introduction of an endocyclic oxygen in 2-fluorocyclohexanone to give 3-fluorodihydro-2H-pyran-4(3H)-one would be expected to create a gauche effect in the axial conformer along with the O–C–C–F moiety, inducing an increase of its population. However, small changes were verified in the conformational populations both in the gas phase and solution because the carbonyl group plays an important role for the hyperconjugation in the equatorial conformer, despite experiencing strong dipolar repulsion with the fluorine atom. These data were obtained theoretically and by NMR spectroscopy, while the nature of the interactions governing these conformational shifts were investigated on the basis of natural bond orbital analysis.

The gauche effect in fluorinated alkylammonium salts is well known and attributed either to an intramolecular hydrogen bond or to an electrostatic attraction between the positively charged nitrogen and the vicinal electronegative fluorine atom. This work reports the effect of adding a fluorine atom in 2-fluoroethylamine hydrochloride on the conformational isomerism of the resulting 2,2-difluoroethylamine chloride (2). The analysis was carried out using NMR coupling constants in D₂O solution, in order to mimic the equilibrium conditions in a physiological medium, in the gas phase and in implicit water through theoretical calculations. Despite the presence of σ_CH→σ*_CF and σ_CH→σ*_CN interactions, which usually rule the hyperconjugative gauche effect in 1,2-disubstituted ethanes, the most important forces leading to the double gauche effect (⁺NH₃ in the gauche relationship with both fluorine atoms) in 2 are the Lewis-type ones. Particularly, electrostatic interactions are operative even in water solution, where they should be significantly attenuated, whereas hyperconjugation and hydrogen bond have secondary importance.

Salicylic acid is the precursor of an important analgesic drug, and fluorination may modify its properties, including pK_a in solution, viscosity, polarity and molecular conformation. The intramolecular hydrogen bond is an interaction that modifies the conformational equilibrium of molecules and, therefore, it can modify their macroscopic properties, such as bioactivity. Competition between F and N as proton acceptors for the acidic COOH hydrogen in 6-fluoroanthranilic acid could not be experimentally probed earlier because of, among other factors, its tendency to form a zwitterionic species due to the higher basicity of the nitrogen. In 6-fluorosalicylic acid (1), the fluorine atom competes with oxygen for the COOH proton, thus affecting the conformational equilibrium when compared to salicylic acid itself. Indeed, 1 shows two major conformers in the gas phase and solution, while the isomer 4-fluorosalicylic acid is expected to exhibit only one among three possible conformers. Theoretical calculations indicate that the second most stable conformer of 1 exhibits intramolecular F⋯H(OOC) hydrogen bonds, which is corroborated by the small, but detectable correlation between ¹⁹F and ¹H(OOC) in the HETCOR NMR spectrum in benzene-d₆ solution.

A competition between the terminal fluorine and hydroxyl groups by the central hydroxyl group as hydrogen bond donor in 3-fluoro-1,2-propanediol would be expected to dictate the conformational isomerism of this compound, but also the repulsion between the electronegative and bulky vicinal substituents. Indeed, an intramolecular hydrogen bond has been verified only for a local minimum using QTAIM calculations, while the most stable conformer exhibits an all-gauche conformation with a small stabilizing contribution from the n_F→σ^* _OH interaction. The preferred orientation of the OH and F substituents was confirmed from the chemical shifts and coupling constants of the diastereotopic hydrogens. This conformational preference, which is calculated to exist both in the gas phase and solution (using implicit CHCl₃ and CH₃CN solvents), is better described by predominant hyperconjugative interactions over Lewis-type interactions. The strong contribution from antiperiplanar interactions involving σ_CH and σ_CC as electron donors and σ^* _CF and σ^* _CO as electron acceptors dictates the gauche effect in 3-fluoro-1,2-propanediol rather than a hydrogen bond. The absence of J_F,H(O) and J_H(O),H(O) coupling constants confirms that any influence from a hydrogen bond to the conformational isomerism of 3-fluoro-1,2-propanediol is secondary.

Suitable ³J_H,H coupling constants and theoretical calculations were used to define the conformational preferences of trans-2-bromoalkoxycyclohexanes (alkoxy = OMe, OEt, OⁱPr, and O^tBu) for the isolated molecule and as a function of the medium. The diaxial conformer was preponderant, or at least similarly populated to the diequatorial form, for the tert-butoxy derivative only, while the diequatorial conformer was prevalent for the remaining alkoxy derivatives (except for the OMe derivative in CCl₄ solution). The conformational behavior of these compounds was analyzed on the basis of classical steric effects and attractive electron delocalizations, by means of natural bond orbital analysis.