Mailing List

For all the latest news and features, sign up to receive our FREE updates by email:

A Free Review from Progress in Reaction Kinetics and Mechanism: Halocarbenes may deplete atmospheric ozone

Posted on 6. March, 2018.

Bookmark and Share

The stratospheric ozone layer protects biological processes on Earth by absorbing the more harmful solar UV radiation. The discovery of the loss of stratospheric O3 led to investigations to find the cause. It was found that chlorofluorocarbons had long atmospheric lifetimes and could reach the stratosphere where they photocleaved to yield Cl atoms which destroyed O3 in a catalytic chain reaction.

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 electronpoor 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.

Read the full article in Progress in Reaction Kinetics and Mechanism, Volume 42, Number 4, 2017, pp. 307-333


Author: Andrew Mamantov
Formerly of: US Environmental Protection Agency, Washington, DC, USA

Keywords: photooxidation, haloethylene, halocarbene, chain reaction, atmospheric ozone depletion, perhalocarbons, stratosphere 

Image: Absorption cross section of tetrachloroethylene CCl2=CCl2 at room temperature.