Química Orgánica 2 · Herramientas de oxidación y reducción
LiAlH4 reduction of ethyl benzoate to benzyl alcohol
Condiciones: LiAlH4, THF; then H2O
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 7 pasos escritos
First hydride adds to the ester carbon
The Al–H bond delivers a hydride to the ester carbonyl carbon, and the C=O π pair moves onto oxygen, giving a tetrahedral alkoxide.
Las flechas curvas
- The Al–H bond pair carries the hydride to the ester carbonyl carbon.
- The C=O π pair moves onto oxygen.
Ethoxide leaves; benzaldehyde forms
The alkoxide lone pair re-forms the C=O bond and pushes out ethoxide, the leaving group of the ester. The product of this collapse is benzaldehyde.
Las flechas curvas
- An oxygen lone pair re-forms the C=O π bond.
- The C–O bond to the ethoxy group breaks and ethoxide leaves.
Why not stop at the aldehyde?
Aldehydes are more electrophilic than esters, whose carbonyl is stabilised by the alkoxy oxygen lone pair.
Second hydride adds to benzaldehyde
A fresh aluminohydride delivers a second hydride to the aldehyde carbon; the π pair moves onto oxygen, giving the benzyl alkoxide.
Las flechas curvas
- The Al–H bond pair delivers the hydride to the aldehyde carbon.
- The C=O π pair moves onto oxygen.
Water protonates the benzyl alkoxide
In the aqueous workup, water gives a proton to the benzyl alkoxide, forming benzyl alcohol and hydroxide.
Las flechas curvas
- The alkoxide lone pair takes a proton from water.
- The O–H pair stays on oxygen as hydroxide.
Water protonates ethoxide
A second water molecule protonates ethoxide, releasing ethanol, the alcohol half of the ester.
Las flechas curvas
- The ethoxide lone pair takes a proton from water.
- The O–H pair stays on oxygen as hydroxide.
Benzyl alcohol and ethanol
The ester carbon received two hydrides: ethyl benzoate has been reduced to benzyl alcohol, and the ethoxy group was released as ethanol.
Qué vas a practicar
Draw two hydride additions to ethyl benzoate with loss of ethoxide in between, then protonation of both alkoxides in the aqueous workup to give benzyl alcohol and ethanol.
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