Atmospheric Ring-Closure and Dehydration Reactions of 1,4-Hydroxycarbonyls in the Gas Phase: The Impact of Catalysts.

Abstract

1,4-Hydroxycarbonyls can potentially undergo sequential reactions involving cyclization followed by dehydration to form dihydrofurans. As dihydrofurans contain a double bond, they are highly reactive toward atmospheric oxidants such as OH, O3, and NO3. In the present study, we use ab initio calculations to examine the impact of various atmospheric catalysts on the energetics and kinetics of the gas-phase cyclization and dehydration reaction steps associated with 4-hydroxybutanal, a prototypical 1,4-hydroxycarbonyl molecule. The cyclization step transforms 4-hydroxybutanal into 2-hydroxytetrahydrofuran, which can subsequently undergo dehydration to form 2,3-dihydrofuran. As the barriers associated with the cyclization and dehydration steps for 4-hydroxybutanal are, respectively, 34.8 and 63.0 kcal/mol in the absence of a catalyst, both reaction steps are inaccessible under atmospheric conditions in the gas phase. However, the presence of a suitable catalyst can significantly reduce the reaction barriers, and we have examined the impact of a single molecule of H2O, HO2 radical, HC(O)OH, HNO3, and H2SO4 on these reactions. We find that H2SO4 reduces the reaction barriers the greatest, with the barrier for the cyclization step being reduced to -13.1 kcal/mol and that for the dehydration step going down to 9.2 kcal/mol, measured relative to their respective separated starting reactants. Interestingly, our kinetic study shows that HNO3 gives the fastest rate due to the combined effects of a larger atmospheric concentration and a reduced barrier. Thus, our study suggests that, with acid catalysis, the cyclization reaction step can readily occur for 1,4-hydroxycarbonyls in the gas phase. Because the dehydration step exhibits a significant barrier even with acid catalysis, the 2-hydroxytetrahydrofuran products, once formed, are likely lost through their reaction with OH radicals in the atmosphere. We have investigated the reaction pathways and the rate constant for this bimolecular reaction in the presence of excess molecular oxygen (3O2), as it would occur under tropospheric conditions, using computational chemistry over the 200-300 K temperature range. We find that the main products from these OH-initiated oxidation reactions are succinaldehyde + HO2 and 2,3-dihydro-2-furanol + HO2.

• Authors: Parandaman Arathala, Chanin B Tangtartharakul, Amitabha Sinha
• Journal: The journal of physical chemistry. A (J Phys Chem A) Vol. 125 Issue 27 Pg. 5963-5975 (Jul 15 2021) ISSN: 1520-5215 [Electronic] United States
• PMID: 34191509 (Publication Type: Journal Article)