A Criegee intermediate (also called a Criegee

zwitterion In chemistry, a zwitterion ( ; ), also called an inner salt or dipolar ion, is a molecule that contains an equal number of positively- and negatively-charged functional groups. : With amino acids, for example, in solution a chemical equilibrium wil ...

or Criegee biradical) is a

carbonyl In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containi ...

oxide with two charge centres. 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. Criegee intermediates are also an important source of OH (

hydroxyl radical The hydroxyl radical is the diatomic molecule . The hydroxyl radical is very stable as a dilute gas, but it decays very rapidly in the condensed phase. It is pervasive in some situations. Most notably the hydroxyl radicals are produced from the ...

s). OH radicals are the most important oxidant in the troposphere, 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, for whom it is named. It was not until 2012 that direct detection of such chemicals was reported.

Infrared spectroscopy Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or function ...

suggests the

electronic structure In quantum chemistry, electronic structure is the state of motion of electrons in an electrostatic field created by stationary nuclei. The term encompasses both the wave functions of the electrons and the energies associated with them. Electr ...

has a substantially zwitterionic character rather than the biradical character that had previously been proposed.

Formation

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 A molozonide (or "molecular ozonide") is a 1,2,3-trioxolane, which can also be thought of a cyclic dialkyl trioxidane. Molozonides are formed by cycloaddition of ozone and an alkene during ozonolysis, as a transient intermediate which quickly rearr ...

, which then decomposes to produce a carbonyl (RR'CO) and a carbonyl oxide. The latter is known as the Criegee intermediate. The alkene ozonolysis reaction is extremely exothermic, releasing about 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 In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...

or ketone byproduct of the Criegee-intermediate formation reaction itself) to form an

ozonide Ozonide is the polyatomic anion . Cyclic organic compounds formed by the addition of ozone () to an alkene are also called ozonides. Ionic ozonides Inorganic ozonides are dark red salts. The anion has the bent shape of the ozone molecule. Ino ...

(1,2,4-trioxolane).

References

{{reflist Free radicals Chemical bonding Environmental chemistry Climate change mitigation