Química Orgánica 2 · Sustitución de acilo y elección del derivado
N-Methylacetamide from acetic acid and methylamine with DCC
Condiciones: DCC, CH2Cl2
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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.
Leer los 7 pasos escritos
The acid protonates DCC
Acetic acid gives its O–H proton to a nitrogen of dicyclohexylcarbodiimide (DCC), forming acetate and a protonated carbodiimide whose central carbon is strongly electrophilic.
Las flechas curvas
- A nitrogen lone pair of DCC bonds to the acid proton.
- The O–H pair stays on oxygen, giving acetate.
Acetate adds to the carbodiimide carbon
The acetate oxygen attacks the central carbon of the protonated carbodiimide; the C=N⁺ π electrons move to the protonated nitrogen. The product is the O-acylisourea, an activated ester.
Las flechas curvas
- An acetate lone pair forms the new O–C bond.
- The C=N⁺ π electrons move onto the protonated nitrogen.
Activation
The isourea oxygen leaves as part of the very stable urea, so the acyl carbon becomes as reactive as in an anhydride.
Methylamine adds to the acyl carbon
Methylamine attacks the carbonyl carbon of the O-acylisourea; the C=O π pair moves onto oxygen, giving a tetrahedral intermediate.
Las flechas curvas
- The methylamine nitrogen lone pair forms the new N–C bond.
- The C=O π electrons move onto oxygen.
Collapse and loss of the isourea
The O⁻ re-forms C=O and the isourea oxygen leaves as an anion delocalised over O–C=N.
Las flechas curvas
- An oxygen lone pair re-forms C=O.
- The C–O bond breaks; its electrons go to the isourea oxygen.
Proton transfer gives the urea
The isourea anion takes the N–H proton of the acylammonium ion on nitrogen: the oxygen lone pair forms C=O as the C=N π pair bonds the proton. This gives neutral N-methylacetamide and dicyclohexylurea (DCU).
Las flechas curvas
- An oxygen lone pair forms the C=O π bond of the urea.
- The C=N π electrons bond nitrogen to the proton.
- The N–H pair returns to the amide nitrogen.
N-Methylacetamide
N-Methylacetamide forms with dicyclohexylurea, an insoluble solid that is filtered off. The strong C=O of the urea pays for amide formation under mild conditions.
Qué vas a practicar
Couple acetic acid with methylamine using DCC: protonate DCC, form the O-acylisourea, add the amine, and release dicyclohexylurea.
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