Halocarbenes May Deplete Atmospheric Ozone

Abstract

Photooxidation of tetrachloroethylene (PERC) and trichloroethylene (TCE) in simulated tropospheric smog chamber studies occurs with a time delay, accelerating simultaneous decreasing O₃/chlorinated ethylene (CE) concentrations along with increasing CCl₂O, which is attributed to CCl₂ in the case of PERC and CCl₂ or CHCl for TCE. The carbenes, chlorinated acetyl chlorides and CCl₂O products may result from the rearrangement of the oxidised and/or excited oxidised CE, e.g. an epoxide. Analyses indicate scavenging experiments have not proved the existence of Cl atoms as being responsible for chlorinated acetyl chloride formation. Halocarbenes may form complexes with O₃ which can undergo electron transfer (ET) and lead to dissociation of O₃ to O₂ and O and regeneration of carbene, resulting in a chain reaction. The direction of ET may be determined by the smallest differential HOMO–LUMO energy between the carbene and O₃ which results in greater transition state stabilisation. Similarities in the reactions of O₃ with carbenes and simple alkenes, nucleophilic carbenes with electron-poor alkenes and electrophilic carbene PhCCl with alkyl-substituted alkenes, i.e. (1) complex formation, (2) very low or negative activation energies and (3) the ability to undergo ET reactions with alkylalkenes are discussed. The possibility of the world-wide used perhalocarbons, e.g. perfluorinated carbons, hydroperhalocarbons, their halogenated replacements and starting materials degrading to halocarbenes which may degrade O₃, is analysed.