Química Orgánica 2 · Aminoácidos y conectividad de los péptidos
2-aminopropanenitrile from acetaldehyde (Strecker synthesis)
Condiciones: NH3, HCN (then H3O+, Δ converts the nitrile to alanine)
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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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Ammonia adds to the aldehyde
Ammonia and HCN form ammonium cyanide in solution (assumed), so the medium holds NH3, NH4+ and cyanide. The neutral ammonia nitrogen adds to the electrophilic aldehyde carbon, giving a zwitterionic tetrahedral adduct.
Las flechas curvas
- The nitrogen lone pair bonds to the partially positive carbonyl carbon.
- The C=O π pair moves onto oxygen, which becomes an alkoxide.
Protonate the alkoxide
The ammonium ion is the strongest acid present, so it protonates the alkoxide oxygen and is converted back to ammonia.
Las flechas curvas
- An alkoxide lone pair takes a proton from the ammonium ion.
- The N–H pair stays on nitrogen, releasing ammonia.
Deprotonate the nitrogen
Ammonia removes a proton from the positively charged nitrogen, giving the neutral carbinolamine (hemiaminal). The two proton moves are relayed through the solvent bases, not done intramolecularly.
Las flechas curvas
- The ammonia lone pair takes the N–H proton.
- The N–H pair returns to nitrogen, making it a neutral amine.
Protonate the hydroxyl group
Hydroxide is a poor leaving group, so the ammonium ion protonates the carbinolamine OH, turning it into water, a good leaving group.
Las flechas curvas
- An oxygen lone pair takes a proton from ammonium.
- The N–H pair stays on nitrogen as ammonia.
Lose water to form the iminium ion
The nitrogen lone pair pushes out water as it forms a C=N π bond, giving an iminium ion; no free carbocation forms because nitrogen stabilizes the carbon in the same step.
Las flechas curvas
- The nitrogen lone pair forms the C=N π bond.
- The C–O pair leaves with oxygen as neutral water.
Where cyanide attacks
The iminium carbon is the electrophile.
Cyanide adds to the iminium carbon
Cyanide's carbon lone pair forms a new C–C bond to the iminium carbon and the C=N π pair returns to nitrogen. The new stereocentre forms from either face of the planar iminium ion, so the aminonitrile is racemic.
Las flechas curvas
- The carbon lone pair of cyanide attacks the electrophilic iminium carbon.
- The C=N π pair moves onto nitrogen, giving a neutral amine.
2-aminopropanenitrile
The product is racemic 2-aminopropanenitrile, with water and ammonia. Heating it in aqueous acid hydrolyses the nitrile to the carboxylic acid, giving racemic alanine.
Qué vas a practicar
Draw imine formation from acetaldehyde and ammonia and cyanide addition to the iminium ion that gives 2-aminopropanenitrile, the Strecker precursor of alanine.
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