PCl₃, PCl₅, and SOCl₂
The primary reagents for converting -OH groups to -Cl.
Phosphorus halides ($PCl_3, PCl_5$) and Thionyl chloride ($SOCl_2$) are essential reagents in organic synthesis. They are primarily used to replace the hydroxyl group ($-OH$) in alcohols and carboxylic acids with a chlorine atom ($-Cl$).
1. Phosphorus Trichloride ($PCl_3$)
Reaction Stoichiometry
One molecule of $PCl_3$ reacts with three molecules of alcohol or acid. The byproduct is Phosphorous Acid ($H_3PO_3$).
2. Phosphorus Pentachloride ($PCl_5$)
Vigorous Chlorination
$PCl_5$ reacts with alcohols, acids, and even carbonyl compounds. The main byproducts are Phosphoryl Chloride ($POCl_3$) and $HCl$.
3. Thionyl Chloride ($SOCl_2$)
Darzen's Process (Best Method)
$SOCl_2$ is preferred for preparing alkyl chlorides because the byproducts ($SO_2$ and $HCl$) are gases and escape, leaving pure product.
- Without Pyridine ($S_Ni$): The reaction proceeds with Retention of Configuration (Internal Nucleophilic Substitution).
- With Pyridine ($S_N2$): Pyridine neutralizes $HCl$, forming $Cl^-$ which attacks from the backside. This leads to Inversion of Configuration.
$$ R-OH + SOCl_2 \xrightarrow{\text{Pyridine}} \text{Inverted } R-Cl $$
4. Quick Comparison
| Reagent | Substrate Ratio ($ROH : Reagent$) | Byproducts | State of Byproducts |
|---|---|---|---|
| $PCl_3$ | $3 : 1$ | $H_3PO_3$ | Solid/Liquid (Non-volatile) |
| $PCl_5$ | $1 : 1$ | $POCl_3 + HCl$ | Liquid + Gas |
| $SOCl_2$ | $1 : 1$ | $SO_2 + HCl$ | Both Gases (Escape easily) |
Knowledge Check
Test your understanding of Chlorinating Agents
No comments:
Post a Comment