Approximately 1 year ago, a letter in this journal proposed a chemical solution for the 100-year-old problem of the chemical origin of engine knock (OGJ, Sept. 2, 2013, p. 18).
The element oxygen can be represented several ways depending on the dominant interests of the investigator. For the spectroscopist and the photochemist, the representation of singlet oxygen is . For the organic and the physical chemist, where the anticipated reactivity and general chemistry are important, the representation is different. For the ground state, triplet oxygen, the representation is , where "." represents unpaired and orthogonal electrons. The resultant chemistry is immediately suggestive for free radical chain chemistry. For activated oxygen, singlet oxygen, the simplest representation is 0=0, with the two electrons paired. The resultant chemistry is non-free radical and immediately suggestive of covalent and Diels-Alder chemistry. Thus, the chemistry of triplet and that of singlet oxygen are expected to be completely different and phase-dependent.
This different reactivity for singlet oxygen becomes important for oxidation chemistry of water purification, in oil and gas transportation, in polymer preparation and transportation, in the biological chemistry of aging, and in medical and psychological therapy. Singlet oxygen is also important in Diels-Alder reactions for the synthesis of active medical intermediates (e.g. 1,2-dioxene-4, previously unknown), the novel synthesis of unsaturated alcohols (e.g. allyl alcohol), the preparation of ethylene oxide, and of phenols.
Edgar J. Smutny
Senior Staff, Shell Development Co. (retired)
Houston