The gas phase aldose‐ketone isomerization mechanism: Direct interconversion of the model hydroxycarbonyls 2‐hydroxypropanal and hydroxyacetone

Abstract

We report a novel mechanism for the interconversion of 2-hydroxypropanal with its more-stable ketone isomer hydroxyacetone. Reaction proceeds via concerted transfer of two H atoms, requires a barrier of only 40 kcal mol 21 , bypasses the enediol intermediate, and is general for a-hydroxy carbonyls. A similar isomerization mechanism is shown to persist for b, g, and d-hydroxy carbonyls; these compounds are skeletal forms of the monosaccharides and this work, therefore, discloses the gas-phase mechanism for aldose-ketose isomerization. As an example, the isomerization of glyceraldehyde to dihydroxyacetone is shown to proceed via this mechanism with a barrier of 31 kcal mol 21 . Rate coefficients and thermochemical properties are reported for the isomerization of 2-hydroxypropanal and hydroxyacetone for use in detailed kinetic models. Additionally, RRKM theory k(E) values for this reaction suggest that it may transpire in the troposphere following solar excitation. K E Y W O R D S computational chemistry, gas-phase reactions, pyrolysis, reaction mechanisms 1 | I N T R O D U C T I O N Hydroxycarbonyls are common oxygenated intermediates formed in the photochemical oxidation of volatile organic compounds and in the thermal processing of lignocellulosic material. Hydroxyacetone (HOCH 2 C(O)CH 3 ), the simplest a-hydroxy ketone, is a widely-detected trace gas in the troposphere, [1] and its photochemical oxidation mechanisms have been studied extensively. [2-5] 2-Hydroxypropanal (CH 3 CHOH-CHO), an isomer of hydroxyacetone, is an intermediate in the pyrolysis and combustion chemistry of glyceral. [6] More broadly, the open-chain forms of the monosaccharides are hydroxycarbonyls, with an aldehyde (aldose) or ketone (ketose) moiety and a hydroxyl group at each remaining carbon. Hydroyxacetone and 2-hydroxypropanal thus serve as model compounds for the a-hydroxy carbonyl functionality in these simple sugars.Recently, we reported on the isomerization mechanism of glycolaldehyde (HOCH 2 CHO), [7] the archetypal a-hydroxy carbonyl and an important tropospheric compound in its own right. [8] This work revealed that glycolaldehyde could undergo keto-enol tautomerization via intramolecular transfer of two H atoms, mediated by the hydroxyl group. This process is reminiscent of a bimolecular mechanism previously shown to catalyze keto-enol tautomerizations. [9] In the course of our investigation into glycolaldehyde it became apparent that this compound could undergo degenerate isomerization reactions in which the unique O atoms (i.e., the alcohol and aldehyde functional groups) were effectively scrambled. The present work extends this analysis to hydroxycarbonyl compounds in which the alcohol and aldehyde/ketone groups are not interchangeable, starting with the hydroxyacetone/2-hydroxypropanal pairing, through the application of ab initio calculations and reaction rate theory modelling. This study expands our understanding of rearrangement reactions available to hydroxycarbonyl compounds in thermal environments and in the troposph...

Intramolecular proton transfer in the isomerization of hydroxyacetone: Characterization based on reaction force analysis and the bond fragility spectrum

“…In summary, our results add two new distinctions for this mechanism: (1) The synchronicity of the proton transfers have been characterized fully within the framework of the reaction force constant and the bond fragility spectrum detailing the concerted mechanism as mildly asynchronous and (2) The reaction electronic flux along the intrinsic reaction coordinate accounts for the favorable electronic rearrangement associated with the charge transfer of the concerted mechanism. Hence, the previously reported thermodynamics/kinetics calculations [41] on this isomerization should be considered together with our results to gain a more complete understanding of this mechanism. The results on this system could serve as a model for similar processes of other α-hydroxycarbonyls including open-chain monosaccharides like glucose that undergo similar intramolecular proton transfers.…”

“…[34][35][36][37][38][39][40] A general scheme for the isomerization of hydroxyacetone to 2-hydroxypropanal is shown in Figure 1 with the DPT mechanism occurring via three unique pathways, the first two pathways are stepwise processes that proceed through stable isomers (Z and E) of their shared enediol intermediate. The third pathway is a concerted mechanism that was recently discovered by Gabriel da Silva and coworkers, [41] it bypasses the enediol intermediate and was found to be energetically favorable by approximately 20 kcal mol −1 suggesting this may be a general mechanism of importance for all hydroxycarbonyls.…”

In‐situ IR Spectroscopy Study of Reactions of C3 Oxygenates on Heteroatom (Sn, Mo, and W) doped BEA Zeolites and the Effect of Co‐adsorbed Water

“…Hydroxyacetone, which has both a carbonyl and an α‐hydroxyl group, is a good model compound to investigate the reaction mechanism for a 1‐2 hydride shift due to the structural similarities to sugars [49] . Our study shows that hydroxyacetone undergoes a 1,2 hydride shift to form 2‐hydroxypropanal over Sn‐BEA and Mo‐BEA, but further dehydration to acrolein does not occur without water (vide infra) in contrast to the formation of mesityl oxide from acetone via self‐aldol condensation and subsequent dehydration.…”

A General User-Friendly Tool for Kinetic Calculations of Multi-Step Reactions within the Virtual Multifrequency Spectrometer Project

“…The interconversion reaction between hydroxyacetone and 2-hydroxypropanal is an important reaction in the context of atmospheric chemistry because the hydroxyacetone represent the simplest form of photochemically oxidised volatile organic compounds [26]. In a recent study, Sun et al [27] have considered the interconversion mechanisms on several hydroxycarbonyl compounds, and much attention has been focused on the interconversion reaction between hydroxyacetone and 2-hydroxypropanal. This isomerization reaction can occur through three different mechanisms, 2 high-barrier multistep processes and, as shown in Figure 3, a direct mechanism via double hydride shift involving a low-barrier concerted transition state.…”