Química Orgánica 2 · Estructura y equilibrios de los carbohidratos
Glyceraldehyde phenylhydrazone, first stage of osazone formation
Condiciones: PhNHNH2, AcOH
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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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Phenylhydrazine adds to the aldehyde
The terminal NH2 of phenylhydrazine is the nucleophile: its lone pair is not delocalized into the ring, unlike the NH attached to phenyl.
Las flechas curvas
- The NH2 lone pair bonds to the aldehyde carbon.
- The C=O π pair moves onto oxygen.
Acetic acid protonates the alkoxide
Acetic acid gives its proton to the alkoxide of the zwitterionic adduct.
Las flechas curvas
- The alkoxide lone pair takes the acetic acid proton.
- The O–H pair stays on oxygen as acetate.
Acetate removes an N–H proton
Acetate deprotonates the ammonium nitrogen, giving the neutral carbinolamine.
Las flechas curvas
- An acetate lone pair takes the N–H proton.
- The N–H pair becomes the nitrogen lone pair.
Protonate the carbinolamine OH
Acetic acid protonates the OH so it can leave as water rather than hydroxide.
Las flechas curvas
- The hydroxyl lone pair takes the acetic acid proton.
- The O–H pair stays on acetate oxygen.
Nitrogen pushes out water
The nitrogen lone pair forms C=N+ as water leaves.
Las flechas curvas
- The N lone pair forms the C=N π bond.
- The C–O bond breaks and water leaves.
What drives the loss of water?
Carbinolamine dehydration.
Acetate removes the iminium proton
Acetate takes the N–H proton of the iminium ion, giving the neutral C=N of the hydrazone and regenerating acetic acid.
Las flechas curvas
- An acetate lone pair takes the N–H proton.
- The N–H pair becomes the nitrogen lone pair.
Glyceraldehyde phenylhydrazone
With more phenylhydrazine and heat the neighbouring C2–OH is oxidized (aniline and NH3 are released) and C2 condenses with a second phenylhydrazine, giving the osazone; that is why glucose, mannose and fructose give the same osazone.
Qué vas a practicar
Draw phenylhydrazine adding to the aldehyde of glyceraldehyde, the acetic acid-mediated proton transfers and loss of water to the phenylhydrazone.
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