$S_Ni$ Reaction Mechanism
Substitution Nucleophilic Internal: Reaction of Alcohols with Thionyl Chloride.
By chemca Team • Updated Jan 2026
The reaction of alcohols with Thionyl Chloride ($SOCl_2$) to form alkyl chlorides typically proceeds via the $S_Ni$ mechanism (Substitution Nucleophilic Internal). This is a unique substitution reaction where the configuration of the chiral center is retained.
1. The Reaction (Darzen's Process)
Alcohol reacts with Thionyl Chloride in the absence of a base (like Pyridine).
$$ R-OH + SOCl_2 \xrightarrow{\text{Ether / Reflux}} R-Cl + SO_2 \uparrow + HCl \uparrow $$
Advantage: The byproducts ($SO_2$ and $HCl$) are gases, leaving behind pure Alkyl Chloride.
2. Mechanism: Internal Attack
Step-by-Step
Step 1: Formation of Alkyl Chlorosulfite
The alcohol acts as a nucleophile attacking the sulfur atom, displacing one chloride ion.
The alcohol acts as a nucleophile attacking the sulfur atom, displacing one chloride ion.
$$ R-OH + Cl-SO-Cl \rightarrow R-O-SO-Cl + HCl $$
Step 2: Decomposition (Internal Substitution)
The Alkyl Chlorosulfite decomposes. The chloride atom attached to sulfur attacks the alkyl group from the SAME SIDE as the leaving oxygen atom. This happens via a cyclic 4-membered transition state or a tight ion pair.
The Alkyl Chlorosulfite decomposes. The chloride atom attached to sulfur attacks the alkyl group from the SAME SIDE as the leaving oxygen atom. This happens via a cyclic 4-membered transition state or a tight ion pair.
$$ [\overset{\delta+}{R} \cdot\cdot\cdot O \cdot\cdot\cdot S(=O) \cdot\cdot\cdot \overset{\delta-}{Cl}]^\ddagger \rightarrow R-Cl + SO_2 $$
Result: Retention of Configuration
Since the nucleophile ($Cl$) originates from the leaving group itself and attacks from the front, the stereochemistry is retained.
Since the nucleophile ($Cl$) originates from the leaving group itself and attacks from the front, the stereochemistry is retained.
3. Effect of Pyridine ($S_N2$ Pathway)
Switching to Inversion
If the reaction is carried out in the presence of a base like Pyridine ($C_5H_5N$), the mechanism changes.
Reason:
- Pyridine neutralizes the $HCl$ formed in Step 1.
- This generates free Chloride ions ($Cl^-$) in the solution: $Py + HCl \rightarrow PyH^+Cl^-$.
- The free $Cl^-$ is a strong nucleophile and attacks the Alkyl Chlorosulfite from the BACKSIDE (standard $S_N2$).
$$ R-OH + SOCl_2 \xrightarrow{\text{Pyridine}} \text{Inverted } R-Cl + SO_2 + PyH^+Cl^- $$
Result: Inversion of Configuration (Walden Inversion).
4. Comparison: $S_Ni$ vs $S_N2$
| Condition | Mechanism | Nucleophile Source | Stereochemistry |
|---|---|---|---|
| No Base (Pure $SOCl_2$) | $S_Ni$ (Internal) | From $OSCl$ group (Internal) | Retention |
| With Pyridine (Base) | $S_N2$ (Bimolecular) | External $Cl^-$ (from PyHCl) | Inversion |
Knowledge Check
Test your understanding of SNi
No comments:
Post a Comment