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Tests for Phenols and Nitro Groups

Iron (III) Chloride Test for Water-Soluble Phenols

Iron (III) Chloride - Pyridine Test for Water-Insoluble Phenols

Iron (II) Hydroxide Test for Nitro Groups


Iron (III) Chloride Test for Water-Soluble Phenols

Phenol

Standard
Phenol

Procedure (for water-soluble phenols)
The iron (III) chloride test for phenols is not completely reliable for acidic phenols, but can be administered by dissolving 15 mg of the unknown compound in 0.5 mL of water or water-alcohol mixture and add 1 to 2 drops of 1% aqueous iron (III) chloride solution.

Positive Test
A red, blue, green, or purple color is a positive test.

Cleaning up
Since the quantity of material is extremely small, the test solution can be diluted with water and flushed down the drain.

 

Iron(III) Chloride - Pyridine Test for Water-Insoluble Phenols

Phenol

Standard
Phenol

Procedure (for water-insoluble phenols or less reactive phenols)
A more sensitive test for phenols consists of dissolving or suspending 15 mg of the unknown in 0.5 mL of methylene chloride and adding 3-5 drops of a 1% solution ferric chloride in methylene chloride. Add a drop of pyridine and stir.

Positive Test (b)
Addition of pyridine and stirring will produce a color if phenols or enols are present.

 

Iron (II) Hydroxide Test for Nitro Groups

Nitro Compounds

Standard
3-Nitrobenzaldehyde

Procedure
Add about 10 mg of the compound to 1 mL of the ferrous ammonium sulfate reagent in a test tube, and then add 0.7 mL of the 2N alcoholic potassium hydroxide reagent. Stopper the tube, and shake. Note the color of the precipitate after 1 minute.
Ferrous Ammonium Sulfate Reagent: To 50 mL of recently boiled, distilled water add 2.5 g of ferrous ammonium sulfate crystals and 0.2 mL of concentrated sulfuric acid.
The Alcoholic Potassium Hydroxide Reagent will already be prepared for you.

Positive Test
A positive test is the formation of the red-brown precipitate of iron(III) hydroxide.

Complications
The red-brown to brown precipitate of iron (III) hydroxide (ferric hydroxide) is formed by the oxidation of iron(II) hydroxide (ferrous hydroxide) by the nitro compound, which in turn is reduced to the primary amine. A negative test is indicated by a greenish precipitate. In some cases partial oxidation may cause a darkening of the ferrous hydroxide. Practically all nitro compounds give a positive test in 30 seconds. The speed with which the nitro compound is reduced depends on its solubility.

Cleaning up
Pour solutions into the appropriate waste container.