Química Orgânica 1 · Caminhos de substituição
Replace OH with Cl using thionyl chloride
Condições: Propan-1-ol with thionyl chloride (SOCl₂) and pyridine. Follow the alkyl chlorosulfite pathway to 1-chloropropane, sulfur dioxide and pyridinium chloride. Sulfur is drawn with the usual expanded-valence S=O bonds. C1 is primary, so the final displacement is SN2.
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O mecanismo, passo a passo
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Oxygen attacks sulfur
Sulfur in SOCl₂ is electron-poor, bonded to oxygen and two chlorines. Bonding the alcohol oxygen to sulfur is the first step in turning OH into a good leaving group.
As setas curvas
- A lone pair on the alcohol oxygen bonds to sulfur.
- The S=O π pair moves onto oxygen so sulfur does not gain an extra bond.
Expel chloride from sulfur
Addition to sulfur followed by loss of chloride is a substitution at sulfur. The chloride released here returns later as the nucleophile.
As setas curvas
- A lone pair on the negatively charged oxygen rebuilds the S=O π bond.
- A S–Cl pair leaves with chlorine as chloride.
Pyridine removes the O–H proton
The positively charged oxygen is a strong acid (pKa well below 0), while pyridinium has pKa approximately 5.2, so pyridine takes the proton essentially completely. The neutral product is an alkyl chlorosulfite, R–O–S(=O)Cl.
As setas curvas
- The pyridine nitrogen lone pair accepts the proton from the alcohol oxygen.
- The O–H pair stays on the alcohol oxygen, which becomes neutral.
Chloride attacks carbon as the leaving group fragments
C1 is primary, so chloride displaces the chlorosulfite group by backside SN2 rather than waiting for a carbocation. The leaving group falls apart as it leaves, into SO₂ gas and chloride; loss of the gas makes the reaction effectively irreversible.
As setas curvas
- A lone pair on free chloride bonds to the carbon bearing oxygen from the opposite side.
- The C–O pair becomes a second S=O π bond as the C–O bond breaks.
- The remaining S–Cl pair leaves as chloride, completing sulfur dioxide.
Alkyl chloride, sulfur dioxide and pyridinium chloride
Chlorine now replaces the alcohol oxygen: 1-chloropropane. That oxygen left with sulfur as part of SO₂, and the second chlorine ended as chloride next to pyridinium. Overall, the alcohol OH was replaced by Cl, with SO₂ and HCl (captured by pyridine) as byproducts. With a stereocenter at the alcohol carbon, this pyridine-assisted SN2 route would give inversion.
O que você vai praticar
Activate a primary alcohol at sulfur with SOCl₂, then replace the C–O bond by chloride SN2 while the leaving group fragments to SO₂ gas.
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