Química Orgánica 2 · Sustitución electrofílica aromática
Friedel–Crafts alkylation of benzene to sec-butylbenzene
Condiciones: AlCl3 (cat.), 2-chlorobutane
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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
2-Chlorobutane binds aluminium chloride
A chlorine lone pair of 2-chlorobutane coordinates to the empty orbital of AlCl3. The C–Cl bond is now polarised toward the positively charged chlorine and much weaker.
Las flechas curvas
- The chlorine lone pair forms a dative bond to the Lewis-acidic aluminium.
The secondary carbocation forms
The C–Cl bond breaks and chlorine leaves as part of AlCl4⁻, giving the secondary 2-butyl cation. No hydride or methyl shift can give a more substituted cation here, so it attacks the ring as formed.
Las flechas curvas
- The C–Cl bonding pair moves onto chlorine, which stays on aluminium as AlCl4⁻.
Benzene attacks the carbocation
A ring π bond forms the new C–C bond to the cationic carbon. The ring loses its aromaticity and becomes an arenium ion; this is the slow step.
Las flechas curvas
- The ring π electrons bond to the empty p orbital of the sec-butyl cation.
The arenium ion is delocalised
The positive charge is shared by the carbons ortho and para to the new sp3 carbon. Shifting a ring π bond shows the para contributor, from which the proton loss is drawn.
Las flechas curvas
- A ring π bond moves toward the cationic carbon, relocating the positive charge para to the sp3 carbon.
Why no rearrangement?
A carbocation rearranges only when a shift gives a more stable cation.
Tetrachloroaluminate removes the ring proton
AlCl4⁻ takes the proton from the sp3 carbon. The C–H pair restores the aromatic ring, HCl is released and AlCl3 is regenerated, so it is a true catalyst.
Las flechas curvas
- A chlorine lone pair of tetrachloroaluminate takes the proton on the sp3 ring carbon.
- The Al–Cl pair returns to that chlorine, so HCl leaves and neutral AlCl3 is regenerated.
- The C–H pair re-forms the ring π bond next to the sp3 carbon.
- The neighbouring π bond shifts onto the cationic carbon, restoring the aromatic sextet.
sec-Butylbenzene
sec-Butylbenzene forms as a racemate, because the planar cation is attacked equally on both faces; HCl and AlCl3 are the by-products.
Qué vas a practicar
Draw the AlCl3 activation of 2-chlorobutane, attack of benzene on the 2-butyl cation, and deprotonation of the arenium ion to sec-butylbenzene.
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