Oxides of Carbon - Overview

Overview

Carbon dioxide (CO₂) occurs widely in nature, and was incidentally manufactured by humans since pre-historical times, by the combustion of carbon-containing substances and fermentation of foods such as beer and bread. It was gradually recognized as a chemical substance, formerly called spiritus sylvestre ("forest spirit") or "fixed air", by various chemists in the 17th and 18th centuries.

Carbon monoxide may occur in combustion, too, and was used (though not recognized) since antiquity for the smelting of iron from its ores. Like the dioxide, it was described and studied in the West by various alchemists and chemists since the Middle Ages. Its true composition was discovered by William Cruikshank in 1800.

Carbon suboxide was discovered by Brodie in 1873, by passing electric current through carbon dioxide.

The fourth "classical" oxide, mellitic anhydride (C₁₂O₉), was apparently obtained by Liebig and Wöhler in 1830 in their study of mellite ("honeystone"), but was characterized only in 1913, by Meyer and Steiner.

Brodie also discovered in 1859 a fifth compound called graphite oxide, consisting of carbon and oxygen in ratios varying between 2:1 and 3:1; but the nature and molecular structure of this substance remained unknown until a few years ago, when it was renamed graphene oxide and became a topic of research in nanotechnology.

Notable examples of unstable or metastable oxides that were detected only in extreme situations are dicarbon monoxide radical (:C=C=O), carbon trioxide (CO₃), carbon tetroxide (CO₄), carbon pentoxide (CO₅), carbon hexoxide (CO₆) and 1,2-dioxetanedione (C₂O₄). Some of these reactive carbon oxides were detected within molecular clouds in the interstellar medium by rotational spectroscopy.

Many hypothetical oxocarbons have been studied by theoretical methods but have yet to be detected. Examples include oxalic anhydride (C₂O₃ or O=(C₂O)=O), ethylene dione (C₂O₂ or O=C=C=O) and other linear or cyclic polymers of carbon monoxide (-CO-)_n (polyketones), and linear or cyclic polymers of carbon dioxide (-CO₂-)_n, such as the dimer 1,3-dioxetanedione (C₂O₄) and the trimer 1,3,5-trioxanetrione (C₃O₆).