Química Orgánica 2 · Sustitución de acilo, aminas y sales de diazonio
Aniline from benzoyl chloride by the Curtius rearrangement
Condiciones: NaN3; Δ; then H2O
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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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Azide adds to the acid chloride
A terminal nitrogen of the azide ion attacks the carbonyl carbon of benzoyl chloride and the C=O π pair moves to oxygen.
Las flechas curvas
- The azide nitrogen lone pair forms the C–N bond.
- The C=O π pair moves onto oxygen.
Chloride leaves
The alkoxide re-forms C=O and expels chloride, giving benzoyl azide.
Las flechas curvas
- The oxygen lone pair re-forms the carbonyl.
- Chloride leaves with the C–Cl pair.
The reacting azide contributor
Benzoyl azide is a resonance hybrid. The contributor with a negative nitrogen next to the carbonyl and an N≡N unit shows the nitrogen lone pair that forms C=N and the ready N2 leaving group.
Las flechas curvas
- A lone pair on the terminal nitrogen forms a third N–N bond.
- One N=N π pair moves onto the nitrogen bonded to the carbonyl.
Curtius rearrangement
On heating, the phenyl group migrates from carbon to the adjacent nitrogen with its bonding pair as N2 leaves, all in one concerted step; the nitrogen lone pair forms the C=N bond of phenyl isocyanate.
Las flechas curvas
- The C–phenyl bonding pair carries the phenyl group to nitrogen.
- The N–N bond breaks and stable N2 departs.
- The nitrogen lone pair forms the C=N π bond of the isocyanate.
Carbon count
Carbon count.
Water adds to the isocyanate
Water attacks the central carbon of phenyl isocyanate and the C=N π pair moves onto nitrogen.
Las flechas curvas
- A water lone pair bonds to the isocyanate carbon.
- The C=N π pair moves onto nitrogen.
Water removes the O–H proton
A second water molecule takes a proton from the oxonium oxygen.
Las flechas curvas
- Water takes the proton.
- The O–H pair stays on oxygen.
Protonate nitrogen
Hydronium protonates the negatively charged nitrogen, giving phenylcarbamic acid.
Las flechas curvas
- The nitrogen lone pair takes the proton.
- The O–H pair returns to water.
Ionize the carbamic acid
Water removes the carbamic acid O–H proton, giving the carbamate anion.
Las flechas curvas
- Water takes the acidic O–H proton.
- The O–H pair stays on oxygen.
Protonate the carbamate nitrogen
Hydronium protonates the nitrogen, giving a zwitterion whose nitrogen is now a neutral amine leaving group.
Las flechas curvas
- The nitrogen lone pair takes the proton.
- The O–H pair returns to water.
Decarboxylation
The carboxylate lone pair forms a second C=O bond and the C–N bond breaks, releasing CO2 and aniline.
Las flechas curvas
- The oxygen lone pair forms the second C=O of CO2.
- The C–N pair goes to nitrogen, giving neutral aniline.
Aniline
Aniline is formed with CO2 and N2 as by-products. The amine nitrogen sits on the ring carbon that was bonded to the carbonyl carbon, with retention at the migrating group.
Qué vas a practicar
Draw benzoyl azide formation, its Curtius rearrangement to phenyl isocyanate with loss of N2, and the hydrolysis and decarboxylation that release aniline.
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