Semidione
Semidiones are radical anions analogous to semiquinones, obtained from the one-electron reduction of non-quinone conjugated dicarbonyls.{{cite book |editor-last1=Kevan |editor-first1=Larry |editor-last2=Kaiser |editor-link1=Larry Kevan |editor-first2=Emil T. |editor-link2=Emil T. Kaiser |title=Radical Ions |publication-date=1968 |publisher=New York Interscience |isbn=978-0-470-45490-9 |pages=87–150 |chapter=Chapter 3: Semidione Radical Anions |last=Russell |first=Glen A.}}
The simplest possible semidiones are derived from 1,2-dicarbonyls and have structure {{chem2|R\sC(\sO-)\dC(\sO•)\sR' ↔ R\sC(\sO•)\dC(\sO-)\sR'}}, making them the second member of a homologous series starting with ketyl radicals and continuing with semitriones.
They are often transient intermediates, appearing in reactions such as the final reduction step of the acyloin condensation.
{{cite book |editor-last1=Moss |editor-first1=Robert A. |editor-last2=Platz |editor-first2=Matthew |editor-last3=Jones |editor-first3=Maitland |editor-link3=Maitland Jones Jr. |title=Reactive Intermediate Chemistry |publication-date=2004 |publisher=Wiley-Interscience |isbn=978-0-471-23324-4 |chapter=Chapter 6: Organic Radical Ions |last=Roth |first=Heinz D. |page=260}}
Benzil semidione ({{chem2|Ph\sC(\sO-)\dC(\sO•)\sPh ↔ Ph\sC(\sO•)\dC(\sO-)\sPh}}), synthesized by Auguste Laurent in 1836, is believed to have been the first radical ion ever characterized.
{{cite book |series=Organic Chemistry |volume=26 |date=1973 |title=Organic Reactive Intermediates |editor-last1=McManus |editor-first1=Samuel P. |last1 = Russell |first1 = Glen A. |last2 = Norris |first2 = Robert K. |chapter = Chapter 6: Radical Ions |pages = 423–448 |publisher=Elsevier |issn=0078-611X |doi=10.1016/B978-0-12-485450-5.50011-7}}
{{Citation |last=Laurent |first=Auguste |author-link=Auguste Laurent |title=Benzoyl und Benzimid. |journal=Annalen der Pharmacie |volume=17 |issue=1 |year=1836 |pages=88–94 |doi=10.1002/jlac.18360170116 |hdl=2027/mdp.39015026322357 |url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015026322357&seq=106}}
Semidehydroascorbate is a relatively stable semitrione produced by hydrogen abstraction from ascorbate (Vitamin C).{{cite journal |last1=Njus |first1=David |last2=Kelley |first2=Patrick M. |last3=Tu |first3=Yi-Jung |last4=Schlegel |first4=H. Bernhard |title=Ascorbic acid: The chemistry underlying its antioxidant properties |journal=Free Radical Biology and Medicine |date=November 2020 |volume=159 |pages=37–43 |doi=10.1016/j.freeradbiomed.2020.07.013}}