Química Orgânica 2 · Substituição acílica, aminas e sais de diazônio
Why is an acid chloride so much more reactive than an amide?
Condições: Acetyl chloride reacts vigorously with water at 25 °C, giving acetic acid and HCl within seconds. Acetamide in water at 25 °C is unchanged after days.
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A pergunta
Which statement best explains the difference?
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Resposta e explicação
Resposta correta Chlorine donates little electron density into the C=O π system and leaves as the very weak base Cl⁻, whereas the amide nitrogen donates strongly and would have to leave as the very strong base NH₂⁻.
Both steps favour the acid chloride: addition to a more electrophilic carbonyl carbon, and collapse with loss of a much better leaving group.
Esta não The acid chloride is less sterically hindered than the amide.
Cl and NH₂ are similar in size; the difference is electronic, not steric.
Esta não The amide is protonated by water, and the cation repels the incoming nucleophile.
Amides are very weak bases (pKaH about −0.5) and are not protonated in neutral water; protonation would in fact activate the carbonyl.
Esta não The C–Cl bond is stronger than the C–N bond, so it pulls more electron density from carbon.
Bond strength is not the issue, and the C–Cl bond is actually weaker; what matters is π donation into C=O and the stability of the leaving group.
Solução explicada
Overlap between chlorine's 3p lone pair and the carbon 2p orbital is poor, so an acid chloride carbonyl keeps nearly all its electrophilicity, and chloride leaves easily. The amide nitrogen's 2p lone pair overlaps well with C=O, giving the C–N bond partial double-bond character and making the carbonyl carbon a weak electrophile.
O que você vai praticar
Explain the reactivity gap between an acid chloride and an amide from π donation and leaving-group ability.
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