Química Orgánica 2 · Repaso de sustitución aromática para el examen
Friedel–Crafts acetylation of chlorobenzene to 4-chloroacetophenone
Condiciones: CH3COCl, AlCl3
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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 7 pasos escritos
Chlorine of acetyl chloride binds aluminium chloride
A lone pair on the acyl chloride's chlorine bonds to the empty orbital of aluminium chloride, making chloride a much better leaving group.
Las flechas curvas
- A chlorine lone pair fills the empty aluminium orbital.
The acylium ion forms
The C–Cl bond breaks with its electrons going to chlorine, giving tetrachloroaluminate, while an oxygen lone pair forms a C≡O⁺ triple bond. The resonance-stabilised acylium ion is the electrophile.
Las flechas curvas
- The C–Cl pair leaves onto chlorine.
- An oxygen lone pair forms a third C–O bond, so carbon keeps an octet.
The carbon para to chlorine attacks the acylium ion
A ring π pair at the carbon para to chlorine forms the new C–C bond and one C≡O π pair moves onto oxygen. This slow step gives the arenium ion.
Las flechas curvas
- The ring π pair forms the C–C bond at the para carbon.
- One C≡O π pair moves onto oxygen, leaving a neutral ketone.
Chlorine shares the positive charge
After para attack the charge can sit on the carbon bearing chlorine, where a chlorine lone pair forms a C=Cl⁺ π bond and every atom has an octet. This is why chlorine directs ortho/para even though it deactivates the ring.
Las flechas curvas
- The ring π bond shifts toward the cationic carbon.
- A chlorine lone pair forms a π bond to the ring.
Why para and not ortho?
Both ortho and para attack are stabilised by chlorine's lone pair; the bulky acylium–AlCl4⁻ electrophile strongly prefers the open para position.
Tetrachloroaluminate removes the ring proton
A chloride of AlCl4⁻ takes the proton from the sp³ carbon while its Al–Cl pair stays on chlorine; the C–H pair, the ring π bond and the C=Cl⁺ π pair shift to restore the aromatic ring.
Las flechas curvas
- A chlorine lone pair takes the ring proton.
- The Al–Cl pair stays on chlorine, releasing neutral AlCl3.
- The C–H pair re-forms a ring π bond.
- The neighbouring π bond shifts along the ring.
- The C=Cl⁺ π pair returns to chlorine.
4-Chloroacetophenone
4-Chloroacetophenone forms with HCl; AlCl3 is released but stays complexed to the ketone until aqueous workup, so a full equivalent is needed.
Qué vas a practicar
Draw acylium ion formation from acetyl chloride and AlCl3, para attack by chlorobenzene and deprotonation to 4-chloroacetophenone.
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