Criegee intermediate
{{Short description|Class of chemical compounds}}
{{distinguish|Criegee oxidation|Criegee rearrangement}}
File:Carbonyl oxide (Criegee zwitterion).svg]]
A Criegee intermediate (also called a Criegee zwitterion or Criegee biradical) is a carbonyl oxide with two charge centers. These chemicals may react with sulfur dioxide and nitrogen oxides in the Earth's atmosphere, and are implicated in the formation of aerosols, which are an important factor in controlling global climate.{{cite journal |first1= Oliver |last1= Welz |first2= John D. |last2= Savee |first3= David L. |last3= Osborn |first4= Subith S. |last4= Vasu |first5= Carl J. |last5= Percival |first6= Dudley E. |last6= Shallcross |first7= Craig A. |last7= Taatjes |title= Direct Kinetic Measurements of Criegee Intermediate (CH2OO) Formed by Reaction of C2I with O2 |journal= Science |date= 13 January 2012 |volume= 335 |issue= 6065 |pages= 204–207 |doi= 10.1126/science.1213229 |pmid= 22246773 |bibcode = 2012Sci...335..204W |s2cid= 26810853 }}{{cite news| last=Castro| first=Joseph| title=How mysterious molecules may help cool Earth|url=https://www.nbcnews.com/id/wbna45975663|work=NBC News|access-date=2012-01-12 |date= January 12, 2012 }} Criegee intermediates are also an important source of OH (hydroxyl radicals).{{Cite journal|last1=Heard|first1=Dwayne E.|last2=Whalley|first2=Lisa K.|last3=Stone|first3=Daniel|date=2012|title=Tropospheric OH and HO2 radicals: field measurements and model comparisons|journal=Chemical Society Reviews|language=en|volume=41|issue=19|pages=6348–6404|doi=10.1039/C2CS35140D|pmid=22907645|doi-access=free}} OH radicals are the most important oxidant in the troposphere,{{Cite book|title=Chemistry of the upper and lower atmosphere : theory, experiments, and applications|last1=Finlayson-Pitts|first1=Barbara J.|last2=Pitts|first2=James N.|date=2000|publisher=Academic Press|isbn=9780080529073|location=San Diego|oclc=162128929}} and are important in controlling air quality and pollution.
The formation of this sort of structure was first postulated in the 1950s by Rudolf Criegee,{{cite news| url=https://www.sciencedaily.com/releases/2012/01/120112142232.htm | title= Offsetting Global Warming: Molecule in Earth's Atmosphere Could 'Cool the Planet' | date= January 12, 2012| publisher= Science Daily | access-date= 2012-01-14}} for whom it is named. It was not until 2012 that direct detection of such chemicals was reported.{{Cite journal|last1=Taatjes|first1=Craig A.|last2=Shallcross|first2=Dudley E.|last3=Percival|first3=Carl J.|last4=Vasu|first4=Subith S.|last5=Osborn|first5=David L.|last6=Savee|first6=John D.|last7=Welz|first7=Oliver|date=2012-01-13|title=Direct Kinetic Measurements of Criegee Intermediate (CH2OO) Formed by Reaction of CH2I with O2|journal=Science|language=en|volume=335|issue=6065|pages=204–207|doi=10.1126/science.1213229|issn=1095-9203|pmid=22246773|bibcode=2012Sci...335..204W|s2cid=26810853}} Infrared spectroscopy suggests the electronic structure has a substantially zwitterionic character rather than the biradical character that had previously been proposed.{{cite journal |title= Infrared Absorption Spectrum of the Simplest Criegee Intermediate CH2OO |journal= Science |date= 12 April 2013 |volume= 340 |issue= 6129 |pages= 174–176 |doi= 10.1126/science.1234369 |pmid= 23580523 |last1= Su |first1= Yu-Te |last2= Huang |first2= Yu-Hsuan|authorlink3=Henryk Witek |last3= Witek |first3= Henryk A.|authorlink4=Yuan-Pern Lee |last4= Lee |first4= Yuan-Pern |bibcode = 2013Sci...340..174S |s2cid= 31499893 }}
Formation
{{Main|Ozonolysis}}
File:Ozonolysis CarbonylOxide Formation.svg
Criegee intermediates are formed by the gas-phase reactions of alkenes and ozone in the Earth's atmosphere. Ozone adds across the carbon–carbon double bond of the alkene to form a molozonide, which then decomposes to produce a carbonyl (RR'CO) and a carbonyl oxide. The latter is known as the Criegee intermediate.{{Cite journal|last1=Marston|first1=George|last2=Johnson|first2=David|date=2008-03-25|title=The gas-phase ozonolysis of unsaturated volatile organic compounds in the troposphere|journal=Chemical Society Reviews|language=en|volume=37|issue=4|pages=699–716|doi=10.1039/B704260B|pmid=18362978|issn=1460-4744}}
The alkene ozonolysis reaction is extremely exothermic, releasing about {{convert|50|kcal/mol|kJ/mol}} of excess energy. Therefore, the Criegee intermediates are formed with a large amount of internal energy.
Removal
When Criegee intermediates are formed, some portion of them will undergo prompt unimolecular decay, producing OH radicals and other products. However, they may instead become stabilized by interactions with other molecules or react with other chemicals to give different products.
Criegee intermediates may be collisionally stabilized via collisions with other molecules in the atmosphere. These stabilized Criegee intermediates may then undergo thermal unimolecular decay to OH radicals and other products, or may undergo bimolecular reactions with other atmospheric species.
In the ozonolysis reaction sequence, the Criegee intermediate reacts with another carbonyl compound (generally the aldehyde or ketone byproduct of the Criegee-intermediate formation reaction itself) to form an ozonide (1,2,4-trioxolane).