Organic Chemistry 2 · Acyl substitution and derivative choice
Maleanilic acid from maleic anhydride and aniline
Conditions: Et2O, rt
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The mechanism, step by step
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Aniline adds to a ring carbonyl
The two carbonyls of maleic anhydride are identical, so aniline can attack either one. Its nitrogen lone pair adds to a carbonyl carbon and the C=O π pair moves onto oxygen.
The curved arrows
- The aniline nitrogen lone pair forms the new N–C bond to a ring carbonyl carbon.
- The C=O π electrons move onto oxygen as the carbon becomes tetrahedral.
The ring opens
The alkoxide re-forms C=O and the ring C–O bond breaks, so a carboxylate is the leaving group. Because the leaving group stays tethered, the ring simply opens; the alkene was cis in the ring and stays Z in the open chain.
The curved arrows
- The oxygen lone pair rebuilds the carbonyl π bond.
- The ring C–O bond breaks onto the ring oxygen, which becomes a carboxylate.
A second aniline takes the N–H proton
The opened intermediate is a zwitterion: a protonated amide nitrogen and a carboxylate at the other end. A second aniline removes the proton from nitrogen, the first half of an intermolecular relay.
The curved arrows
- The lone pair of the second aniline nitrogen bonds to the N–H proton.
- The N–H pair returns to the amide nitrogen.
The anilinium ion protonates the carboxylate
The anilinium ion (pKa about 4.6) hands its proton to the carboxylate, completing the relay. The carboxylic acid forms and aniline is returned unchanged.
The curved arrows
- A carboxylate oxygen lone pair bonds to the anilinium proton.
- The N–H pair returns to nitrogen, regenerating neutral aniline.
Maleanilic acid
Maleanilic acid, the Z amide–acid, crystallizes from ether. Heating it with acetic anhydride closes the ring again on nitrogen to give N-phenylmaleimide.
What you'll practice
Open maleic anhydride with aniline and relay the proton to the carboxylate, keeping the Z alkene, to give maleanilic acid.
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