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Halocarbenes may deplete atmospheric ozone - Free review article

Posted on 1. February, 2018.

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The stratospheric ozone (O3) layer protects biological processes on Earth by absorbing the more harmful solar UV radiation, thereby preventing this radiation from reaching Earth’s surface. It was found that chlorofluorocarbons (CFCs), had long atmospheric lifetimes and could reach the stratosphere where they photocleaved to yield Cl atoms which destroyed O3 in a catalytic chain reaction.

This important discovery was followed by the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer, an international treaty which took place in Vienna. The agreement to phase-out and ban ozone-depleting substances (ODS), e.g. CFCs, was adopted in 1989. Amendments and Adjustments to the Protocol in the 1990s added more ODS to the phase-out list. The role of other halocarbons, e.g. fluorocarbons (FCs), hydro- and bromo-FCs, hydro- and bromo-CFCs and radical species, e.g. bromine atoms, fluorine atoms, RO radicals (R = CF3CO2, CF3O, FCO2, FO) have been investigated for their ozone depleting potential (ODP).
This review investigates the gas phase photooxidation of haloethylenes, with an emphasis on the chloroethylenes (CEs) and introduces them as precursors of possible new ozone-depleting reactive intermediates. Even though the CEs, perchloroethylene (PERC), trichloroethylene (TCE), 1,1-dichloroethylene (1,1-DCE), cis-  and trans-1,2-dichloroethylene (cis-, trans-1,2-DCE) and vinyl chloride (VC), may not have sufficient tropospheric lifetime to reach the stratosphere to deplete stratospheric ozone, they may serve as models which uncover reactive ODS which may be generated by longer-lived atmospheric molecules. The current state of analysis of CE photooxidation is provided by Franklin, Itoh et al. and Sidebottom and co-workers.
The conclusion section includes analyses which point out the similarities in four types of reactions: (1) O3 with carbenes, (2) O3 with simple alkenes, (3) nucleophilic carbenes with electron-poor alkenes and (4) electrophilic carbene PhCCl with alkyl-substituted alkenes. These similarities provide additional evidence that support the new proposals. Analyses also indicate that more investigations are needed of the mechanisms of atmospheric degradation of the world-wide used perhalogenated and hydroperhalogenated carbons; e.g. perfluorinated carbons (PFCs), hydroperfluorinated carbons (HPFCs), perchlorofluorocarbons (PClFCs), hydroperchlorofluorocarbons (HPClFCs), their starting materials and waste products which may possibly be precursors of ODS and ozone-depleting reactive intermediates.

Read the full article in:
Progress in Reaction Kinetics and Mechanism, Volume 42, Number 4, 2017, pp. 307-333
DOI: https://doi.org/10.3184/146867817X14954764850360

Progress in Reaction Kinetics and Mechanism is an international journal which considers the dynamics of processes in physical, chemical, biochemical, biological and environmental sciences with a focus on their rates and mechanisms.
Available quarterly in print and online.
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