Química Orgânica 2 · Substituição acílica, aminas e sais de diazônio
DMAP-catalysed acetylation of cyclohexanol
Condições: Cyclohexanol, acetic anhydride (1.5 equiv), DMAP (0.05 equiv), Et3N (1.5 equiv), CH2Cl2, room temperature
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DMAP attacks the anhydride
The ring nitrogen of DMAP, the most nucleophilic site, attacks a carbonyl carbon of acetic anhydride and the C=O π pair moves onto oxygen.
As setas curvas
- The pyridine nitrogen lone pair forms the new N–C bond.
- The C=O π pair moves onto oxygen.
Acetate leaves: the N-acetylpyridinium ion
The alkoxide re-forms C=O and acetate is expelled. The N-acetylpyridinium ion is a far more reactive acylating agent than the anhydride, because its leaving group is a neutral pyridine and its positive charge makes the carbonyl carbon very electrophilic.
As setas curvas
- The oxygen lone pair re-forms the C=O π bond.
- The C–O bond breaks and acetate leaves.
Cyclohexanol adds to the acylpyridinium ion
An oxygen lone pair of cyclohexanol attacks the activated carbonyl carbon and the C=O π pair moves onto oxygen.
As setas curvas
- The alcohol oxygen lone pair forms the new O–C bond.
- The C=O π pair moves onto oxygen.
Acetate removes the O–H proton
Acetate takes the proton from the positively charged oxygen of the added alcohol.
As setas curvas
- An acetate oxygen lone pair takes the O–H proton.
- The O–H pair stays on the ester oxygen.
DMAP leaves and is regenerated
The alkoxide re-forms C=O and the C–N bond breaks. Neutral DMAP departs, ready to start another cycle.
As setas curvas
- The oxygen lone pair re-forms the C=O π bond.
- The C–N bond breaks and the pair returns to the pyridine nitrogen.
Why is DMAP such a good catalyst?
Pyridine catalyses the same acetylation, but DMAP is several thousand times more effective.
Cyclohexyl acetate
The acetyl group has been moved from the anhydride to the alcohol oxygen through DMAP, which is released unchanged.
O que você vai praticar
Draw how DMAP acts as a nucleophilic catalyst: it is acylated by acetic anhydride to an N-acetylpyridinium ion, which transfers the acetyl group to cyclohexanol and releases DMAP again.
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