Química Orgânica 2 · Substituição acílica, aminas e sais de diazônio
Releasing benzylamine from N-benzylphthalimide with hydrazine
Condições: N-Benzylphthalimide, hydrazine hydrate (1.5 equiv), EtOH, reflux
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Hydrazine adds to an imide carbonyl
One nitrogen of hydrazine attacks a carbonyl carbon of the imide and the C=O π pair moves onto oxygen.
As setas curvas
- The hydrazine nitrogen lone pair forms the new N–C bond.
- The C=O π pair moves onto oxygen.
A second hydrazine removes the N–H proton
Hydrazine takes a proton from the positively charged nitrogen that has just bonded to carbon.
As setas curvas
- The hydrazine lone pair takes the N–H proton.
- The N–H pair stays on nitrogen.
The ring opens
The alkoxide re-forms C=O and the C–N bond to the imide nitrogen breaks. The nitrogen leaves as an amide anion, stabilised by the other carbonyl group, so the five-membered imide ring opens.
As setas curvas
- The oxygen lone pair re-forms the C=O π bond.
- The C–N bond breaks; the pair stays on the imide nitrogen.
Hydrazinium protonates the amide anion
The amide nitrogen anion takes a proton from hydrazinium, giving a neutral secondary amide and regenerating hydrazine.
As setas curvas
- The nitrogen lone pair takes the hydrazinium proton.
- The N–H pair stays on the hydrazine nitrogen.
The terminal NH2 closes a six-membered ring
The free NH2 of the acyl hydrazide attacks the remaining amide carbonyl carbon. The ring formed contains the two carbonyl carbons, both hydrazine nitrogens and two benzene carbons: six atoms.
As setas curvas
- The terminal nitrogen lone pair forms the ring N–C bond.
- The C=O π pair moves onto oxygen.
Hydrazine removes the ring N–H proton
Hydrazine takes a proton from the positively charged ring nitrogen.
As setas curvas
- The hydrazine lone pair takes the N–H proton.
- The N–H pair stays on the ring nitrogen.
Hydrazinium protonates the benzyl nitrogen
Before it can leave, the benzylamino group is protonated by hydrazinium, so it can depart as neutral benzylamine rather than as a strongly basic amide anion.
As setas curvas
- The benzylamino nitrogen lone pair takes the hydrazinium proton.
- The N–H pair stays on the hydrazine nitrogen.
Benzylamine leaves
The alkoxide re-forms C=O and the C–N bond to the protonated benzyl nitrogen breaks, releasing neutral benzylamine and the cyclic phthalhydrazide.
As setas curvas
- The oxygen lone pair re-forms the C=O π bond.
- The C–N bond breaks and benzylamine leaves with the pair.
Why hydrazine?
The imide can also be cleaved by hot aqueous acid or base.
Benzylamine and phthalhydrazide
Both carbonyl groups of the phthaloyl unit are now bonded to the hydrazine nitrogens in phthalhydrazide, and the benzyl nitrogen leaves as the free primary amine.
O que você vai praticar
Draw the two acyl substitutions by hydrazine that free benzylamine from N-benzylphthalimide and trap both carbonyls in the six-membered ring of phthalhydrazide.
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