Reactions Of Substituted Quinones -

) on the quinone accelerate the reaction by lowering the LUMO energy.

This reversible redox cycle is how Coenzyme Q (Ubiquinone) transports electrons in the mitochondrial respiratory chain. 4. Nucleophilic Substitution ( SNArcap S sub cap N cap A r reactions of substituted quinones

Electron-withdrawing groups make the quinone a stronger oxidant (easier to reduce). Electron-donating groups (like −OMenegative cap O cap M e −CH3negative cap C cap H sub 3 ) make the quinone more stable and harder to reduce. ) on the quinone accelerate the reaction by

Usually, the initial product is a hydroquinone. In the presence of excess quinone or air, this often oxidizes back into a new, substituted quinone. 2. Diels-Alder Cycloaddition Substituted quinones act as powerful dienophiles . Electronic Effects: Electron-withdrawing groups (like −CNnegative cap C cap N −CO2Rnegative cap C cap O sub 2 cap R Nucleophilic Substitution ( SNArcap S sub cap N

If the quinone is unsymmetrically substituted, the nucleophile typically attacks the less hindered carbon or the carbon with the lowest electron density.