Glyoxal

Glyoxal is an organic compound with the formula OCHCHO. This yellow-coloured liquid is the smallest dialdehyde (two aldehyde groups). Paper coatings and textiles use large amounts of glyoxal as a crosslinker for starch-based formulations and as a starting material with ureas for wrinkle-resistant chemical treatments. It is used as a solubilizer and cross-linking agent in polymer chemistry. It is a valuable building block in organic synthesis, especially in the synthesis of heterocycles such as imidazoles. Commercial glyoxal is prepared by either the gas phase oxidation of ethylene glycol in the presence of a silver or copper catalyst or by the liquid phase oxidation of acetaldehyde with nitric acid. Global nameplate capacity is ~220,000 tons, with production rates less due to over capacity mostly in Asia. The first commercial source was in Lamotte, France, started in 1960 and currently owned by Clariant. The single largest commercial source is by BASF in Ludwigshafen, Germany at ~60,000 tons/annum. Significant recent capacity has been added in China. Commercial bulk glyoxal is made and reported as a 40% strength solution. Glyoxal is prepared in the lab by oxidation of acetaldehyde with selenious acid.

Speciation of glyoxal in solution
Glyoxal is supplied typically as a 40% aqueous solution. Like other small aldehydes, glyoxal forms hydrates. Furthermore the hydrates condense to give a series of oligomers, the structures of which remain uncertain. For most applications, the exact nature of the species in solution is inconsequential. At least two hydrates of glyoxal are sold commercially:
 * glyoxal dimer, dihydrate: [(CHO)2]2[H2O]2, 1,4-dioxane-trans-2,3-diol (CAS# 4845-50-5, m.p. 91-95 C)
 * glyoxal trimer, “dihydrate”: [(CHO)2]3(H2O)4 (CAS# 4405-13-4)

It is estimated that at concentrations less than 1 M, glyoxal exists predominantly as the monomer or hydrates thereof, i.e. OCHCHO, OCHCH(OH)2 or (HO)2CHCH(OH)2. At concentrations >1 M, dimers predominate. These dimers are probably dioxalanes, with the formula [(HO)CH]2O2CHCHO.

Anhydrous glyoxal
The preparation of anhydrous glyoxal entails heating solid glyoxal hydrate(s) with phosphorus pentoxide. A quote from this paper is instructive of the chemistry of that era “Man erhitzt nun das Glyoxal-Phosphorpentoxyd-Gemisch mit freier Flamme und beobachtet bald, dass sich unter Schwarzfärbung des Kolbeninhalte ein flüchtiges grünes Gas bildet, welches sich in der gekühlten Vorlage zu schönen Krystallen von gelber Farbe kondensiert.” (one heats the mixture of (crude) glyoxal and P4O10 with an open flame and soon observes that upon blackening of the contents, a mobile green gas which condenses in the cooled flask as beautiful yellow crystals).

Other occurrences
Glyoxal is an inflammatory compound formed when cooking oils and fats at high temperatures.