Neutral Ferric Chloride Test: Detection of Phenols
The Neutral Ferric Chloride Test is the characteristic qualitative test for Phenols and compounds containing the Enol group ($=C(OH)-$). It relies on the formation of a colored coordination complex between the iron(III) ion and the phenolic/enolic oxygen.
1. Preparation of "Neutral" FeCl3
Ferric Chloride ($FeCl_3$) solution is acidic due to hydrolysis ($FeCl_3 + 3H_2O \rightleftharpoons Fe(OH)_3 + 3HCl$). The free HCl can interfere with the test or protonate the phenol.
How to Neutralize?
Ammonium Hydroxide ($NH_4OH$) is added dropwise to the $FeCl_3$ solution until a reddish-brown precipitate of ferric hydroxide just starts to appear. The solution is then filtered. The clear filtrate is Neutral $FeCl_3$.
2. General Reaction & Mechanism
Phenols react with neutral ferric chloride to form a water-soluble violet-colored complex.
Key Concept: The reaction is a ligand exchange where the phenol molecules (acting as ligands) replace the water/chloride ligands around the central Iron(III) ion. The colour arises from Ligand-to-Metal Charge Transfer (LMCT).
3. Color Variations (Diagnostic)
The specific color observed depends on the structure of the phenol or enol.
| Compound | Observed Color |
|---|---|
| Phenol, o-Cresol, Resorcinol | Violet / Blue |
| Catechol (1,2-Benzenediol) | Green |
| p-Cresol, Hydroquinone | Transient Blue / Green (quickly oxidizes to quinhydrone) |
| $\alpha$-Naphthol | Pink / Red |
| $\beta$-Naphthol | Green |
| Salicylic Acid | Violet |
4. Test for Enols
Compounds containing a stable enol form (like $\beta$-keto esters) also give a positive test.
Example: Ethyl Acetoacetate
Ethyl Acetoacetate exists in equilibrium between keto and enol forms. The enol form coordinates with $Fe^{3+}$ to give a Reddish-Violet color.
5. Test for Acetate/Formate Salts
While principally for phenols, neutral $FeCl_3$ reacts with neutral solutions of Acetate or Formate ions to give a Blood Red coloration.
$$ 3CH_3COO^- + Fe^{3+} \rightarrow (CH_3COO)_3Fe \text{ (Red Solution)} $$Upon boiling, this red solution forms a reddish-brown precipitate of Basic Ferric Acetate.
6. Limitations
- Aliphatic Alcohols: Do not react (Color remains yellow/orange).
- Sterically Hindered Phenols: (e.g., 2,4,6-tritert-butylphenol) may fail to form the complex.
- Some Acids: Picric Acid does not give the typical violet color. Meta and Para-nitrophenols may not give a distinct color change.
FeCl3 Test Quiz
Test your concepts on Phenol identification. 10 MCQs with explanations.
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