Theoretical Assessment of Carbon Dioxide Reactivity in Methylpiperidines: A Conformational Investigation

Abstract

In this work, the possible mechanisms for the reactions of CO 2 with various positional isomers of methylpiperidines (MPs) (N-MP, 2-MP, 3-MP, and 4-MP) including the effect of aqueous solvation have been explored using quantum chemical methods. The major pathways investigated for CO 2 capture in aqueous amines are carbamate formation, its hydrolysis, and the bicarbonate formation (CO 2 + H 2 O + MP) reaction. The calculations indicate that an axial orientation for the methyl group and an equatorial for the COO − group could be energetically ideal in the carbamate product of MPs. The proton abstraction step in the carbamate pathway is almost barrierless for the zwitterionamine route, while a much higher energy barrier is observed for the zwitterion-H 2 O route. During carbamate hydrolysis, the addition of even two explicit water molecules does not exhibit any notable effect on the already high energy barrier associated with this reaction. This indicates that bicarbonate formation is less likely to occur via carbamate hydrolysis. The calculations suggest that, although the carbamate pathway is kinetically favored, the MP carbamate could still be a minor product, especially for sterically hindered conformations, and the bicarbonate pathway should be predominant in aqueous MPs.

Phase Equilibrium Conditions and Crystal Structure of Binary Hydrates Formed with 1-Methylpiperidine

“…A reason for this may be the chemical reaction between CO 2 , 1-methylpiperidine, and water occurring in these systems to prevent the structure H hydrate formation. Equation shows the chemical reaction of a tertiary amine, such as 1-methylpiperidine, with CO 2 in the aqueous phase to form bicarbonate in the system. , The result of pH measurements for the CO 2 + 1-methylpepiridine + water system was 7.27, while the pH values of carbonated water and 1-methylpiperidine aqueous solutions used in this study were 3.83 and 12.62, respectively. The alkaline 1-methylpiperidine aqueous solution changed to neutral by supplying CO 2 .…”

“…Equation 4shows the chemical reaction of a tertiary amine, such as 1-methylpiperidine, with CO 2 in the aqueous phase to form bicarbonate in the system. 37,38 R N CO H O R NH HCO…”