Formation mechanisms and yields of small imidazoles from reactions of glyoxal with NH₄⁺ in water at neutral pH

Abstract

Imidazoles have numerous applications in pharmacology, chemistry, optics and electronics, making the development of their environmentally-friendly synthetic procedures worthwhile. In this work, the formation of imidazole, imidazole-2-carboxaldehyde, and 2,2-bis-1H-imidazole was investigated in the self-reaction of glyoxal and its cross-reactions with each of these compounds in aqueous solutions of inorganic ammonium salts at pH =7. Such conditions are relevant both as cheap and environmentally-friendly synthetic procedures and for the chemistry of natural environments where NH4(+) is abundant, such as in atmospheric aerosols. These reactions were investigated both by (1)H-NMR and UV-Vis absorption spectroscopy at room temperature with the objective to determine the formation pathways of the three imidazoles and the parameters affecting their yields, to identify the optimal conditions for their synthesis. The results show that only the simplest imidazole is produced by the self-reaction of glyoxal and that imidazole-2-carboxaldehyde and 2,2-bis-1H-imidazole are produced by cross-reactions of glyoxal with imidazole and imidazole-2-carboxaldehyde, respectively. The yields of imidazole-2-carboxaldehyde and 2,2-bis-1H-imidazole formed by the cross-reactions were close to unity, but the yield of imidazole formed by the self-reaction of glyoxal, YIm, was small and varied inversely with the initial glyoxal concentration, [G]0: YIm > 10% only for [G]0 < 0.1 M. The latter result was attributed to the kinetic competition between the imidazole-forming condensation pathway and the acetal/oligomer formation pathway of the glyoxal self-reaction and constitutes a bottleneck for the formation of higher imidazoles. Other parameters such as pH and the NH4(+) concentration did not affect the yields. Thus, by maintaining small glyoxal concentrations, high imidazole yields can be achieved in environmentally-friendly aqueous ammonium solutions at neutral pH. Under the same conditions, higher yields are expected expected from substituted carbonyl compounds, regardless of their concentration, as they produce less acetals.

Chemical Composition and Reaction Mechanisms for Aged p‐Xylene Secondary Organic Aerosol in the Presence of Ammonia

“…This result provides further evidence that NH 3 can react with gaseous organic acids, generating carboxylates as suggested by Na et al 33 The emergence of the absorption peak at 280 nm indicates the formation of new products in aged p-xylene SOA with NH 3 and NO. This band was also observed in evaporating droplets containing glyoxal and (NH 4 ) 2 SO 4 conducted by Lee et al 34 and in previous bulk studies 35,36 using solute concentrations of glyoxal and (NH 4 ) 2 SO 4 at the molar level. The potential chromophores were likely imidazole compounds, such as imidazole and imidazole-2-carbaldehyde, which were formed from the reactions between NH 3 and carbonyl-containing SOA.…”

“…This band was also observed in evaporating droplets containing glyoxal and (NH 4 ) 2 SO 4 conducted by Lee et al . and in previous bulk studies using solute concentrations of glyoxal and (NH 4 ) 2 SO 4 at the molar level. The potential chromophores were likely imidazole compounds, such as imidazole and imidazole‐2‐carbaldehyde, which were formed from the reactions between NH 3 and carbonyl‐containing SOA.…”

Secondary brown carbon formation via the dicarbonyl imine pathway: nitrogen heterocycle formation and synergistic effects

“…Most dicarbonyls, again with the exception of AcAc, develop absorbance maxima in the actinic flux wavelength region. 20,21,23,24,27,29,33,35,44 Formation of mixed imidazoles from Gly/MGly cross-reactions was observed and confirmed by mass spectrometry (see Section 3.3). The absorbance maximum in the visible wavelength range of HD is more pronounced and shifted to around 460 nm compared to the feature at 450 nm appearing in AS containing solutions.…”

“…2 and 11 and references therein). 16,17,[19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] Recent modeling studies suggest that BrC could contribute significantly or sometimes dominate overall aerosol absorption at specific wavelengths. 16,17,[19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] Recent modeling studies suggest that BrC could contribute significantly or sometimes dominate overall aerosol absorption at specific wavelengths.…”

“…Thus, they occur ubiquitously in the atmosphere. 23,25,27,35,50 Recently, browning of SOA from biogenic (isoprene, mono-, and sesquiterpenes) and anthropogenic (alkane, aromatic, PAH) precursors collected on filters was observed upon exposure to ammonia gas. [41][42][43][44][45] Inorganic ammonium salts, or ammonium ions respectively, have been shown to catalyze carbonyl compound self-reactions like hemiacetal oligomer formation and aldol condensation.…”