Competing Mechanistic Channels in the Oxidation of Aldehydes by Ozone

Abstract

The reaction of ozone with aldehydes has been studied intermittently for over 100 years, but its mechanism remains uncertain. Experimental results support two reaction channels: radical abstraction of the acyl hydrogen and addition to form a five-membered ring tetroxolane. We have studied the aldehyde-ozone reaction by DFT and CCSD(T) calculations. CCSD(T)/6-311+G(d,p)//M05-2X)/6-311+G(d,p) calculations predict two competitive pathways for the oxidation of formaldehyde by ozone. Abstraction of the acyl hydrogen by ozone has a barrier of 16.2 kcal/mol, leading to a radical pair, which can combine to form a hydrotrioxide; this species may subsequently decompose to a carboxylic acid and singlet oxygen. In the second reaction channel, addition of ozone to the carbonyl is stepwise, with barriers of 19.1 and 23.0 kcal/mol, leading to a five-membered ring tetroxolane intermediate. This process may be reversible, consistent with earlier observations of isotopic exchange. The two channels connect by an intramolecular hydrogen abstraction. Ring opening of the tetroxolane by an alternate O-O bond cleavage, followed by spin inversion in the resulting diradical intermediate, can give a carbonyl oxide plus (3)O(2). It is also possible that reaction of triplet oxygen with carbonyl oxides can produce ozone by the reverse route. These two stepwise reaction channels, hydrogen abstraction and addition to the C=O bond, explain much of what has been observed in the long history of ozone-aldehyde chemistry. Known reaction rates and the substantial barriers to both channels support an earlier conclusion that aldehyde oxidation by ozone is too slow to be of importance in atmospheric chemistry.

Theoretical study on the atmospheric transformation mechanism of pirimiphos-methyl initiated by O3

“…Comparing these seven channels, abstractions of H 20 and H 22 have lower barriers and require lower reaction heats. The energy barriers of these two H-abstraction reactions by O 3 molecules are lower than for alkanes or aldehydes (Timerghazin et al, 1999;Yang et al, 2008;Voukides et al, 2009). Moreover, DG for the H 20 and H 22 abstraction processes is less than zero, while for other processes it is larger than zero, as shown in Tables 1 and S1.…”

“…Studies have shown that ozonation of the CH bond in saturated hydrocarbons initiates with H abstraction by O 3 , followed by the formation of an R Å Å OOOH radical pair (Cerkovnik et al, 2002;Yang et al, 2008;Voukides et al, 2009). Within the structure of PMM, seven different kinds of hydrogen can be abstracted by O 3 , as shown in Fig.…”

Comparison on Reduction of VOCs Emissions from Radiata Pine (Pinus Radiata D. Don) between Sodium Bicarbonate and Ozone Treatments

“…Researchers have found that when cannabis leaves are treated with sodium bicarbonate, the aldehyde contents are reduced compared with untreated leaves [22]. In a reaction study of ozone and aldehydes, it was found that the slow reaction of ozone and aldehydes occurs in the atmosphere [48]. In this experiment, water was used as the reaction medium, and the long-term exposure of a high concentration of ozone had a significant impact on the aldehydes in the sample, resulting in a decline in their contents.…”

Aldehydes gas ozonation monitoring: Interest of SIFT/MS versus GC/FID

“…Based on the reactional mechanism proposed by Martinez (1982) and Voukides et al (2009), carboxylic acids are potential by-products of aldehydes gas ozonation. Hence, butanoic acid was also monitored during the ozonation experiments along with BA and O 3 .…”