Finkelstein & Swarts Reactions: Halogen Exchange Methods
Direct halogenation of alkanes often results in mixtures. To synthesize specific alkyl iodides or fluorides efficiently, we use Halogen Exchange Reactions. The two most prominent methods are the Finkelstein Reaction (for Iodides) and the Swarts Reaction (for Fluorides).
1. Finkelstein Reaction
The Finkelstein reaction is used to prepare Alkyl Iodides from alkyl chlorides or alkyl bromides. It involves treating the alkyl halide with Sodium Iodide ($NaI$) in dry acetone.
Where:
- $X = Cl, Br$ (Chlorine or Bromine)
- $NaX = NaCl, NaBr$ (Precipitates out)
Mechanism & Driving Force
The reaction follows an $S_N2$ mechanism, leading to the inversion of configuration (Walden Inversion).
Why Dry Acetone?
Sodium Iodide ($NaI$) is soluble in acetone, but Sodium Chloride ($NaCl$) and Sodium Bromide ($NaBr$) are insoluble. According to Le Chatelier's Principle, the precipitation of $NaX$ shifts the equilibrium forward, driving the reaction to completion.
Examples
- $CH_3-Cl + NaI \xrightarrow{\text{Acetone}} CH_3-I + NaCl \downarrow$
- $CH_3CH_2-Br + NaI \xrightarrow{\text{Acetone}} CH_3CH_2-I + NaBr \downarrow$
2. Swarts Reaction
The Swarts reaction is the best method to prepare Alkyl Fluorides. Direct fluorination is often explosive; hence, this halogen exchange method is preferred.
Alkyl chlorides or bromides are heated in the presence of a metallic fluoride.
Common Reagents
Besides Silver Fluoride ($AgF$), other heavy metal fluorides can be used:
- Mercurous Fluoride ($Hg_2F_2$)
- Cobalt(II) Fluoride ($CoF_2$)
- Antimony(III) Fluoride ($SbF_3$)
Examples
- $CH_3-Br + AgF \rightarrow CH_3-F + AgBr$
- $2CH_3CH_2-Cl + Hg_2F_2 \rightarrow 2CH_3CH_2-F + Hg_2Cl_2$
3. Comparison Summary
| Feature | Finkelstein Reaction | Swarts Reaction |
|---|---|---|
| Product | Alkyl Iodide ($R-I$) | Alkyl Fluoride ($R-F$) |
| Reagent | $NaI$ in Dry Acetone | $AgF, Hg_2F_2, CoF_2, SbF_3$ |
| Mechanism | $S_N2$ | Halogen Exchange |
Halogen Exchange Quiz
Test your knowledge on Finkelstein & Swarts reactions. 10 MCQs with explanations.
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