Electrochemistry of ubiquinones

First and second half‐wave reduction potentials of a series of 1,4‐benzo‐ and 1,4‐naphtho‐quinones related to the naturally occurring ubiquinones, plastoquinones and menaquinones are correlated with substituent effects. Notably, E of 2,3‐dimethoxy‐1,4‐benzoquinone is positive of the values for the 2,5‐ and 2,6‐dimethoxy isomers, and of the value for methoxy‐1,4‐benzoquinone. This phenomenon is attributed to steric inhibition of resonance when two methoxy groups occupy adjacent positions, and the significance of this orientation in the ubiquinone series is highlighted.