NaI in Acetone: The Finkelstein Reaction
The reagent combination of Sodium Iodide ($NaI$) dissolved in Dry Acetone is primarily used for the Finkelstein Reaction. This is a classic Halogen Exchange method used to convert Alkyl Chlorides or Bromides into Alkyl Iodides.
1. General Reaction
An alkyl halide ($R-X$, where $X = Cl, Br$) reacts with Sodium Iodide in Acetone to form an Alkyl Iodide and a sodium halide precipitate.
Conditions:
- Substrate: Alkyl Chloride ($R-Cl$) or Alkyl Bromide ($R-Br$).
- Reagent: 15% solution of $NaI$ in anhydrous acetone.
- Mechanism: $S_N2$ (Substitution Nucleophilic Bimolecular).
2. The Principle: Solubility Difference
Le Chatelier's Principle
The reaction is reversible. However, it is driven to completion by the precipitation of the byproduct.
Sodium Iodide ($NaI$) is soluble in Acetone (due to higher covalent character/polarization of Iodine).
Sodium Chloride ($NaCl$) and Sodium Bromide ($NaBr$) are insoluble in Acetone and precipitate out.
This removal of product ($NaX$) shifts the equilibrium towards the right.
3. Stereochemistry ($S_N2$)
The iodide ion ($I^-$) is a very good nucleophile and attacks the alkyl halide from the backside.
If optically active $(R)-2$-Chlorobutane reacts, the product will be $(S)-2$-Iodobutane.
4. Reactivity Order
Since it follows an $S_N2$ mechanism, the reactivity depends on steric hindrance.
- Primary Halides ($1^\circ$): Most Reactive.
- Secondary Halides ($2^\circ$): Moderate Reactivity.
- Tertiary Halides ($3^\circ$): Unreactive (Steric hindrance prevents back-side attack; elimination may occur).
- Aryl/Vinyl Halides: Do not react ($C-X$ bond is strong due to resonance).
5. Use as a Qualitative Test
The reagent can be used to distinguish alkyl bromides/chlorides from alkyl iodides or other non-reactive halides.
Observation
Positive Test: Formation of a white precipitate ($NaCl$ or $NaBr$) indicates the presence of Chlorine or Bromine.
Negative Test: No precipitate indicates Fluoride (too strong bond), Iodide (no reaction), or unreactive Aryl/Vinyl halides.
6. Summary Table
| Reactant ($R-X$) | Reagent | Product | Precipitate |
|---|---|---|---|
| $R-Cl$ | $NaI$ / Acetone | $R-I$ | $NaCl \downarrow$ |
| $R-Br$ | $NaI$ / Acetone | $R-I$ | $NaBr \downarrow$ |
| $R-F$ | $NaI$ / Acetone | No Reaction | None |
Finkelstein Quiz
Test your concepts on Halogen Exchange. 10 MCQs with explanations.
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