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.
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
Substituted quinones are some of the most versatile electrophiles in organic chemistry. Because the quinone core is electron-deficient, their reactivity is largely governed by the nature and position of the substituents ( -groups) attached to the ring. 1. Nucleophilic Conjugate Addition (Michael Addition) reactions of substituted quinones
If the quinone has a good leaving group (like a halogen in p-chloranil ), a nucleophile can displace it directly. This is a common route for synthesizing complex dyes and bioactive molecules. 5. Photochemical Reactions
This is the most common reaction for substituted quinones. A nucleophile (like an amine, thiol, or alcohol) attacks the double bond. In the presence of excess quinone or air,
The "ortho/para" rule applies here; substituents on the diene and the quinone will orient themselves to maximize electronic stabilization in the transition state. 3. Redox Chemistry (Reduction) Quinones are easily reduced to hydroquinones.
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 the initial product is a hydroquinone.
If the quinone is unsymmetrically substituted, the nucleophile typically attacks the less hindered carbon or the carbon with the lowest electron density.