Química Orgánica 1 · Rutas de sustitución
1-Chloropropane from propan-1-ol with thionyl chloride
Condiciones: SOCl2, pyridine
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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.
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The alcohol oxygen adds to sulfur
Sulfur in SOCl2 is electron-poor because it carries an oxygen and two chlorines. The alcohol oxygen bonds to it while the S=O π pair moves onto oxygen.
Las flechas curvas
- A lone pair on the alcohol oxygen bonds to the electrophilic sulfur.
- The S=O π pair moves onto oxygen so sulfur keeps four bonds.
Chloride leaves sulfur
The oxide re-forms the S=O bond and pushes one chloride off sulfur, completing substitution at sulfur. That chloride later returns as the nucleophile.
Las flechas curvas
- The oxygen lone pair rebuilds the S=O π bond.
- An S–Cl bond pair leaves with chlorine as chloride.
Pyridine removes the O–H proton
The positively charged oxygen is a strong acid, and pyridine (pKa of pyridinium about 5) takes the proton. The neutral alkyl chlorosulfite now carries an excellent leaving group on carbon.
Las flechas curvas
- The pyridine nitrogen lone pair takes the proton on oxygen.
- The O–H bond pair stays on oxygen, which becomes neutral.
Which pathway frees the carbon?
The carbon bearing oxygen is primary.
Chloride displaces the chlorosulfite
Chloride attacks the carbon bonded to oxygen from the back side. As the C–O bond breaks its pair becomes a second S=O π bond and the last chloride leaves sulfur, so the leaving group falls apart into SO2 gas and chloride.
Las flechas curvas
- A chloride lone pair forms the new C–Cl bond from the back side.
- The C–O bond pair becomes a new S=O π bond.
- The S–Cl bond pair leaves with chlorine as chloride.
1-Chloropropane
The products are 1-chloropropane, sulfur dioxide and pyridinium chloride. Loss of SO2 gas makes the reaction irreversible; at a stereocentre this pyridine route gives inversion.
Qué vas a practicar
Draw the four arrow-pushing steps from propan-1-ol to 1-chloropropane with SOCl2 and pyridine: addition at sulfur, chloride loss, deprotonation by pyridine, and the SN2 that releases SO2 (propan-1-ol).
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