Sulphuryl Chloride ($SO_2Cl_2$)
A convenient source of atomic chlorine for radical substitution.
Sulphuryl Chloride ($SO_2Cl_2$) is a colorless liquid that acts as a source of chlorine atoms in the presence of radical initiators (like Peroxides or Light). It is preferred over chlorine gas ($Cl_2$) for laboratory chlorination because it is a liquid and easier to handle.
1. Chlorination of Alkanes
Free Radical Substitution
In the presence of a small amount of peroxide (initiator) and heat/light, $SO_2Cl_2$ chlorinates alkanes.
- Initiator decomposes to form radicals.
- $SO_2Cl_2$ decomposes to give $SO_2$ and $Cl^\bullet$ radicals.
- $Cl^\bullet$ abstracts hydrogen from the alkane to form alkyl radical $R^\bullet$.
- $R^\bullet$ reacts with $SO_2Cl_2$ to form $R-Cl$.
2. Allylic Chlorination
Selective Substitution
Similar to NBS (N-Bromosuccinimide) which brominates allylic positions, $SO_2Cl_2$ can be used to chlorinate allylic positions in alkenes.
3. Comparison: $SO_2Cl_2$ vs $SOCl_2$
It is crucial not to confuse Sulphuryl Chloride with Thionyl Chloride.
| Feature | Sulphuryl Chloride ($SO_2Cl_2$) | Thionyl Chloride ($SOCl_2$) |
|---|---|---|
| Oxidation State of S | +6 | +4 |
| Primary Use | Chlorination of Hydrocarbons (Alkanes) | Conversion of Alcohols/Acids to Chlorides |
| Mechanism | Free Radical Substitution ($Cl^\bullet$) | Nucleophilic Substitution ($S_Ni$ / $S_N2$) |
| Substrate | $C-H$ bonds (Alkanes, Allylic) | $O-H$ bonds (Alcohols, Carboxylic Acids) |
4. Addition to Alkenes
Formation of Dichlorides
In the absence of peroxides, $SO_2Cl_2$ can act as a source of electrophilic chlorine, adding across double bonds to form vicinal dichlorides.
Knowledge Check
Test your understanding of SO2Cl2
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