Química Orgánica 2 · Aminoácidos y conectividad de los péptidos
Glycine and alanine from acid hydrolysis of Gly-Ala
Condiciones: 6 M HCl, 110 °C
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 10 pasos escritos
Protonate the N-terminal amine
In 6 M HCl the hydronium ion protonates the basic N-terminal amine first; it stays as an ammonium group throughout.
Las flechas curvas
- The amine lone pair takes a proton from hydronium.
- The O–H pair stays on oxygen as water.
Protonate the amide oxygen
Amides protonate on oxygen, not nitrogen, because the cation is resonance-stabilized by the nitrogen lone pair. Protonation makes the very unreactive amide carbonyl electrophilic.
Las flechas curvas
- The carbonyl oxygen lone pair takes a proton from hydronium.
- The O–H pair stays on oxygen.
Water adds to the carbonyl carbon
Water attacks the protonated amide carbon and the C=O π pair moves onto the protonated oxygen, giving a tetrahedral intermediate.
Las flechas curvas
- A water lone pair attacks the carbonyl carbon.
- The C=O π pair moves onto oxygen.
Water removes a proton
A solvent water molecule removes the extra proton from the added oxygen, giving a neutral tetrahedral intermediate.
Las flechas curvas
- A water lone pair takes the O–H proton.
- The O–H pair stays on oxygen.
Protonate the nitrogen
Now the nitrogen is an amine-like sp3 nitrogen, no longer delocalized, so it is the most basic site; hydronium protonates it to make the alanine nitrogen a neutral leaving group.
Las flechas curvas
- The nitrogen lone pair takes a proton from hydronium.
- The O–H pair stays on oxygen.
Leaving group
A neutral amine leaves, not an amide anion.
The C–N bond breaks
An OH lone pair re-forms the C=O bond and pushes out the neutral alanine nitrogen, giving the protonated carboxylic acid of glycine.
Las flechas curvas
- An oxygen lone pair forms the C=O π bond.
- The C–N pair stays on nitrogen as the amine lone pair.
Deprotonate the carboxylic acid
Water removes the proton from the protonated carbonyl, giving glycine's neutral carboxylic acid group.
Las flechas curvas
- A water lone pair takes the O–H proton.
- The O–H pair stays on the carbonyl oxygen.
Protonate the alanine nitrogen
In 6 M HCl the freed alanine amine is protonated too.
Las flechas curvas
- The amine lone pair takes a proton from hydronium.
- The O–H pair stays on oxygen.
Glycine and alanine hydrochlorides
Hydrolysis gives glycine and alanine, both as ammonium salts in the hot acid; the alanine stereocentre is untouched. Amino-acid analysis of such a hydrolysate gives a peptide's composition but not its sequence.
Qué vas a practicar
Draw the acid-catalysed hydrolysis of the Gly-Ala peptide bond, from N-terminal and carbonyl protonation to loss of alanine.
Acabas de dibujar cada flecha de este problema.
Cada uno de los 5349 problemas de Orgánica 1 y 2 funciona así: comentarios en cada flecha, pistas cuando te atascas y un modo examen sin pistas. Gratis con una cuenta, sin tarjeta.
¿Atascado con tu propia tarea?
Tómale una foto: el primer escaneo es gratis, sin registrarte
¿Escaneas cada semana? Scan Standard: por unos problemas