Química Orgánica 1 · Reacciones radicalarias y selectividad
Phenyl radical and CO₂ from benzoyl peroxide
Condiciones: heat (80–100 °C)
Dibuja este problema tú mismo
Arrastra desde los electrones que se mueven hasta donde van, o toca uno y luego el otro. Cada flecha se revisa; Pista te ayuda cuando te atascas. No necesitas cuenta.
El mecanismo, paso a paso
Siguiente reproduce las flechas curvas de cada paso y las explica. Los pasos escritos están abajo.
Leer los 4 pasos escritos
Split the peroxide O–O bond
Heating breaks the weak O–O bond of dibenzoyl peroxide evenly, giving two benzoyloxyl radicals (PhCO2•). This is why benzoyl peroxide is a common initiator for radical polymerization.
Las flechas curvas
- One electron of the O–O bond stays on the first atom.
- The other electron of the O–O bond stays on the second atom, so the bond splits evenly into two radicals.
Lose CO₂ from a benzoyloxyl radical
The benzoyloxyl radical fragments (β-scission): the radical oxygen forms a C=O π bond while the bond from the carbonyl carbon to the ring breaks evenly. Losing very stable CO₂ drives this step and leaves the reactive phenyl radical.
Las flechas curvas
- The unpaired electron on oxygen starts a second bond (a π bond) to the carboxyl carbon.
- One electron of the ring–carboxyl C–C bond completes that new C=O π bond, making CO₂.
- The other C–C electron stays on the ring carbon, which becomes the phenyl radical.
Where is the unpaired electron?
The phenyl radical is a σ radical: its odd electron sits in an sp² orbital in the ring plane, perpendicular to the π system, so it is not stabilized by resonance and is very reactive.
Phenyl radical, CO₂ and a benzoyloxyl radical
Heating has produced a phenyl radical and carbon dioxide from one half of the peroxide; the second benzoyloxyl radical remains. Either radical can now add to a monomer to start a chain.
Qué vas a practicar
Draw O–O homolysis of dibenzoyl peroxide and the fragmentation of one benzoyloxyl radical into a phenyl radical and carbon dioxide.
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