Química Orgánica 2 · Sustitución de acilo y elección del derivado
Where does a thioester sit?
Condiciones: Compare acyl chloride, anhydride, thioester, ester, amide toward a nucleophile.
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La pregunta
Where does a thioester (RCOSR') fall on the reactivity ladder, and why?
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Respuesta y explicación
Respuesta correcta Between anhydrides and esters: weak S donation and a better leaving group than alkoxide
3p–2p overlap is poor and RS⁻ is less basic than RO⁻, but more basic than RCOO⁻.
Esta no Below amides, because sulfur is larger and donates more strongly
Size makes sulfur's overlap with carbon worse, so it donates less, not more.
Esta no Above acyl chlorides, because RS⁻ is the weakest base of all
RS⁻ (pKa ≈ 10) is far more basic than Cl⁻ (pKa ≈ −7).
Esta no Equal to esters, since S and O are in the same group
Same group does not mean same overlap or leaving-group basicity; thioesters are measurably more reactive.
Solución explicada
Sulfur's 3p lone pairs overlap poorly with carbon's 2p orbital, so a thioester's C=O gets little resonance stabilisation, and RS⁻ (thiol pKa ≈ 10) is a weaker base than RO⁻ (≈ 16). That makes thioesters more reactive than esters, which is why cells use acetyl-CoA as an acyl donor, yet thiolate is still a poorer leaving group than carboxylate, so they sit below anhydrides.
Qué vas a practicar
Place thioesters on the acyl-substitution reactivity ladder and justify it.
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