Tropospheric alkene ozonolysis chemistry: an extended computational chemistry assessment of structural effects†

Abstract

Nineteen structurally different alkene ozonolysis reactions studied herein proceed via a 1,3-cycloaddition step to produce a short-lived primary ozonide, which then breaks down to form a Criegee intermediate (CI) and an aldehyde/ketone co-product. Both steps of each ozonolysis reaction are examined here using a high-level computational chemistry approach (DF-HF/DF-LCCSD(T)-F12a//B3LYP/aug-cc-pVTZ), and a rate constant and product branching ratio are produced for each reaction. The reactions are then categorized into broadly defined taxonomic groups on the basis of how the ozonolysis chemistry is affected by functional groups, steric bulk and the spatial arrangement of the substituent groups. The five alkene taxonomic groups used for classification are monosubstituted alkenes, trisubstituted alkenes, E-2-alkenes, Z-2-alkenes and haloalkenes. The general cycloaddition reactivity trend identified for these alkene groups is k_THEO (haloalkenes) < k_THEO (monosubstituted alkenes) < k_THEO (E-2-alkenes) ∼ k_THEO (Z-2-alkenes) < k_THEO (trisubstituted alkenes). Within these categories, one secondary trend was that if one or more substituents was small and rich in hyperconjugative α-H atoms, such as a methyl group, a higher alkene rate and a higher CI yield would be induced, compared to a bulky and α-H-poor substituent, such as a tert-butyl (^tBu) group. Furthermore, bulky or electronegative substituents were also shown to prompt a reduction in syn-CI yields. Also highlighted in the study is the theoretical mechanism of how the ozonolysis of haloalkenes generates significant yields of tropospheric CF₃CHO, a species which can undergo photolysis to produce the strong greenhouse gas fluoroform (CHF₃).