semiquinone
File:Semiquinon mesomeric_xtd.png of a semiquinone]]
Semiquinones (or ubisemiquinones, if their origin is ubiquinone) are free radicals resulting from the removal of one hydrogen atom with its electron during the process of dehydrogenation of a hydroquinone, such as hydroquinone itself or catechol, to a quinone or alternatively the addition of a single hydrogen atom with its electron to a quinone.{{cite journal|last=Song|first=Y|author2=Buettner, GR |title=Thermodynamic and kinetic considerations for the reaction of semiquinone radicals to form superoxide and hydrogen peroxide|journal=Free Radical Biology & Medicine|date=Sep 15, 2010|volume=49|issue=6|pages=919–62|doi=10.1016/j.freeradbiomed.2010.05.009|pmid=20493944|pmc=2936108}} Semiquinones are highly unstable.
E.g. ubisemiquinone is the first of two stages in reducing the supplementary form of CoQ10 (ubiquinone) to its active form ubiquinol.