Peroxyoxalate

Peroxyoxalate chemiluminescence is a chemical reaction of hydrogen peroxide with derivatives of oxalic acid is emitted to the light, capable of fluorescence when the compounds are present.

Discovery

At Bell Telephone Laboratories EA Chandross discovered at the beginning of the 1960s that in the reaction of hydrogen peroxide and oxalyl chloride in aqueous solvent mixtures a faint bluish- white glow (luminescence ) occurs. The costs incurred in the violent reaction gases brought impregnated with anthracene filter paper to fluoresce. , The reaction of the reactants in the presence of dissolved anthracene, as appeared a bright luminescence, which corresponded to the Fluorenszenzlicht this compound. 9,10- diphenyl anthracene and N- methyl acridone were also excited to fluoresce.

Reaction mechanisms

Hydrogen peroxide and oxalyl chloride are bifunctional reactants, therefore, in principle, the possibility to form oligomers. The nature of the intermediates occurring has long been controversial. About is the first step of the reaction, no doubt: In analogy to the hydrolysis of carboxylic acid chlorides, hydrogen chloride is released, and a peroxy acid ( Chlorcarbonylperameisensäure ) formed. This will decompose into carbon monoxide, oxygen, and hydrogen chloride. However, since the peroxy acid still contains the structural element of a carboxylic acid chloride, can be discussed as a result of reaction, the formation of Diperoxyoxalsäure. In the presence of water also monoperoxyoxalic acid could be formed.

It could also provide the four-membered ring molecule products are 1,2 - Dioxetandion on the Cl -C = O carbon atom by intramolecular nucleophilic attack of the HO group. Despite a long search for this suspected unstable intermediate, only succeeded in more recent times by marking the oxalyl chloride with 13C isotopes to show that in the reaction with hydrogen peroxide, 1,2- dioxanedione occurs.

Certainly compete in the reaction of oxalyl chloride with hydrogen peroxide several reactions. Which of the reactive intermediates ( " key intermediate ," High -Energy Intermediate, HEI) causes the chemiluminescence is controversial; 1,2- Dioxetandion, atomic oxygen and carbon dioxide electronically excited ( singlet ) have been considered. It should be noted that the HEI must be volatile because it is also active in the gas phase. But possibly different molecules for triggering luminescence are responsible in solution and in the gas phase.

Oxalic acid diaryl esters

Chandross ' discovery was the impetus for a comprehensive study of the luminescence phenomenon with other derivatives of oxalic acid in the laboratories of the American Cyanamid Company under the direction of MM Rauhut. Instead of aggressive, highly reactive with water oxalyl chloride oxalic acid diphenyl ( diphenyl ) was studied. Thus the formation of hydrochloric acid is avoided, however, the phenoxy group may be used as relatively good leaving group, the acylation of the hydrogen peroxide also make possible. This idea is based on the development of peroxyoxalate system which has proved to be one of the most efficient chemiluminescence processes. In analogy to perhydroloysis of oxalyl chloride (see above) it was assumed that the first reaction step arises Peroxyoxalsäurephenylester ( phenyl peroxyoxalate ). In an intramolecular ring-closing reaction of this primary product could cyclize to 1,2- Dioxetandion.

The research group led by WJ Baader (University of Sao Paulo) succeeded, a derivative of the above Synthesize primary product: 4 - chlorophenyl peroxyoxalate. The study of this labile compound had the surprising result that the addition of bases is necessary for the chemiluminescence in the presence of fluorescent compounds. The peroxy acid itself, that is, 4-chlorophenyl - peroxyoxalate, can therefore not be responsible for the light emission. Therefore, it was postulated that the peroxyacid is deprotonated by bases and the anion undergoes cyclization to 1,2- Dioxetandion. This should cause the luminescence, but is not yet explained how an electronically excited state arises from the molecule in the ground state. It has been postulated that electronically excited CO2 molecules in the singlet their energy to molecules capable of fluorescence ( fluorophores, Fluoreszer ) transferred as anthracene. These are electronically excited and emit when returning to the ground state depending on a light quantum. One can classify this case as an example of a sensitized chemiluminescence. Kinetic studies, however, make a more complicated mechanism likely is also postulated for other organic peroxides. The key intermediate (HEI ) or its excited fragment of its energy not directly to the fluorescent component from, but it actively intervenes in the process; it acts as chemiluminescence activator ( ACT). Specifically, you can create the following picture:

First, create a " Encounter Complex " from the key intermediate (eg Dioxetandion ) and the ACT (eg anthracene ). ACT then an electron is transferred to the peroxide; it results in a pair of radical anion and radical cation. The O-O bond of the peroxide is weakened, which eventually leads to rupture of the bond. From the radical anion part of CO2 is eliminated. In the new radical pair, the electron on the ACT radical cation part is transmitted back, in the electronically excited state (ACT * ) transitions. This emits the fluorescent light. The whole process is referred to as chemically initiated electron -exchange luminescence ( CIEEL, Chemically Initiated Electron Exchange Luminescence ).

Before Baader investigation, it was found that a variation of the phenyl moiety with further chlorine, nitro, and other groups introduced by substitution of an increase in the luminous efficiency. Particularly suitable aryl esters, bis ( 2,4- dinitrophenyl ) oxalate ( DNPO ) and bis (2,4,6- trichlorophenyl ) oxalate ( TCPO ). The aryloxy this "activated esters" are even better leaving groups and should therefore both the nucleophilic attack of hydrogen peroxide to the carbonyl groups of the diaryl oxalate and the postulated cyclization to 1,2- Dioxetandion favor.

Applications

Peroxyoxalate chemiluminescence are used as the light source, such as light bars. Also in the analysis they are used. Porphyrins can be detected by the PCL test after Brandl and Albrecht, which for the diagnosis of some metabolic diseases ( porphyrias ) is useful in the urine and in the faeces.