Química Orgánica 2 · Adición y condensación aldólica
Mesityl oxide from acetone in sulfuric acid
Condiciones: H2SO4 (cat.), Δ
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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.
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Protonation of acetone
Sulfuric acid protonates the carbonyl oxygen of one acetone. In acid there is no enolate; the protonated carbonyl makes the α-hydrogens acidic enough to be removed by a weak base.
Las flechas curvas
- A carbonyl lone pair takes the proton of sulfuric acid.
- The O–H pair stays on oxygen as hydrogen sulfate.
Hydrogen sulfate removes an α-hydrogen: the enol
Hydrogen sulfate removes an α-hydrogen from the protonated ketone, giving the neutral enol, prop-1-en-2-ol. This tautomer is the nucleophile under acidic conditions.
Las flechas curvas
- A hydrogen sulfate oxygen takes an α-hydrogen.
- The C–H pair forms the enol C=C bond.
- The C=O+ π pair returns to oxygen, neutralizing it.
Protonation of a second acetone
Sulfuric acid protonates a second acetone, making its carbonyl carbon a strong electrophile for the weakly nucleophilic enol.
Las flechas curvas
- The carbonyl lone pair takes the acid proton.
- The O–H pair stays on oxygen.
The enol attacks the protonated ketone
The oxygen lone pair of the enol pushes its π electrons onto the α-carbon, which bonds to the protonated carbonyl carbon. The new C–C bond gives a protonated β-hydroxy ketone.
Las flechas curvas
- The enol oxygen lone pair forms a C=O+ bond.
- The C=C π pair forms the new C–C bond.
- The C=O+ π pair moves onto oxygen, giving the alcohol.
Deprotonation gives diacetone alcohol
Hydrogen sulfate removes the proton from the protonated ketone, giving the aldol product, diacetone alcohol.
Las flechas curvas
- Hydrogen sulfate takes the O–H proton.
- The O–H pair stays on the carbonyl oxygen.
Protonation of the hydroxyl group
In hot acid the tertiary OH is protonated, turning it into water, a good leaving group.
Las flechas curvas
- The hydroxyl lone pair takes the acid proton.
- The O–H pair stays on oxygen.
Water leaves: tertiary cation
Water departs, leaving a tertiary carbocation stabilized by its two methyl groups (an E1 ionization).
Las flechas curvas
- The C–O pair leaves with oxygen as water.
Which hydrogen is lost?
The cation can lose a proton from a methyl group or from the CH2 between the cation and the carbonyl.
Loss of the α-hydrogen gives the enone
Hydrogen sulfate removes a hydrogen from the CH2 next to the cation; the C–H pair forms the C=C bond in conjugation with the ketone.
Las flechas curvas
- Hydrogen sulfate takes the hydrogen next to the cation.
- The C–H pair forms the new C=C bond.
Mesityl oxide
Mesityl oxide (4-methylpent-3-en-2-one) forms with water, and sulfuric acid is regenerated. The conjugated C=C–C=O system is what drives the dehydration at high temperature.
Qué vas a practicar
Draw the sulfuric acid-catalysed self-condensation of acetone: enol formation, attack on a protonated acetone, and E1 dehydration of the aldol to mesityl oxide.
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