Experiment 155

Objective

To systematically identify an unknown organic sample, hypothesized to be 2,4,6-trinitrophenol (picric acid), through physical analysis, solubility testing, and functional group tests.

Introduction

Physical Properties of the Sample

●      Color: Yellow

●      Physical State: Crystalline solid

●      Odor: Bitter

●      Melting Point: 122°C

This compound can be identified through elementary analysis and functional group tests, followed by comparing its melting point to known data.

Solubility Test

Materials

Chemicals

●      Supplied sample (2,4,6-trinitrophenol or picric acid)

●      Distilled water

●      5% HCl, 5% NaOH, 5% NaHCO₃

●      Ferrous sulfate, nitric acid, silver nitrate

●      Ferric chloride, 10% NaNO₂, 2% bromine solution

Apparatus

●       Test tubes

●       test tube holder

●       Bunsen burner

●       thermometer

●       500 mL flask

●       test tube stand

●       pipette

●       fusion tube

●       mortar and pestle

Reagents Preparation

●       5% HCl, 5% NaOH, and 5% NaHCO₃: Prepare in 500 mL flasks.

●       5% Ferrous Sulfate: Prepare in a 250 mL flask.

●       HNO₃ and AgNO₃: Use directly from a beaker.

●       2% Bromine and 10% NaNO₂ Solution: Prepare as needed.

Procedure

1.    Elementary Analysis

●       Preparation of Stock Solution

●       Place a clean, dry piece of sodium in a fusion tube.

●       Heat until the sodium melts, then add a small amount of the sample and heat to a dull red.

●       Drop the hot tube into a mortar containing 3-5 mL of distilled water, break the tube with the pestle, and filter the solution for testing.

●       Tests

●       Nitrogen Test: Mix 1-2 mL of stock solution with 1 mL fresh FeSO₄, boil, cool, then add H₂SO₄. (Result: Prussian blue precipitate; nitrogen present)

●       Halogen Test: Acidify with HNO₃, boil, cool, and add AgNO₃. (Result: No white, pale yellow, or yellow precipitate; halogens absent)

●       Sulfur Test: Add lead acetate after acidifying with acetic acid. (Result: No black precipitate; sulfur absent)

2.    Functional Group Tests

●       Unsaturation Test

o   Bromine in CCl₄: Dissolve 0.2 g of sample; bromine color discharges, indicating unsaturation.

o   KMnO₄ Test: Dissolve sample in acetone/water; KMnO₄ color discharges, confirming unsaturation.

●       Nitro Group Test (-NO₃)

o   Heat with granulated tin and HCl, cool, and add 10% NaNO₂. (Result: Orange-red precipitate; nitro group present)

●       Carbonyl Group Test (=C=O)

o   Dissolve in rectified spirit and add 2,4-DNPH solution. (Result: No orange-red precipitate; carbonyl group absent)

●       Amine Group Tests

o   Primary Amine (-NH₂): Dissolve sample in dilute HCl, add 10% NaNO₂, then transfer to test tube with alkaline α-naphthol solution. (Result: No orange-red precipitate; primary amine absent)

o   Secondary Amine (-NH-): Dissolve in dilute HCl, add excess 10% NaNO₂. (Result: No oily product; secondary amine absent)

o   Tertiary Amine (=N-): Dissolve in dilute HCl, add 10% NaNO₂, heat with NaOH. (Result: No green crystals; tertiary amine absent)

●       Substituted Amide (-NHCOR)

o   Boil with 20% HCl, transfer to alkaline α-naphthol solution. (Result: No orange-red precipitate; substituted amide absent)

●       Carboxylic Acid Test (-COOH)

o   Mix sample with 5% NaHCO₃. (Result: No reaction; carboxylic acid absent)

●       Phenol Test

o   Dissolve sample in rectified spirit, add 5% FeCl₃. (Result: Intense color; phenol group (Ar-OH) present)

●       Aromatic Hydrocarbon Test

o   Dissolve in alcohol, add saturated picric acid solution. (Result: Orange-red needle crystals; aromatic hydrocarbon present)

Observation

The organic sample contains nitrogen (N) and nitro (-NO₃) groups along with an -OH group, confirmed by a melting point of 122°C. These characteristics identify the compound as 2,4,6-trinitrophenol, commonly known as picric acid.

Questions

1.    What is picric acid?

2.    How is picric acid identified?

3.    What color forms in the phenol test?

4.    Which reagents are used in the phenol group test?

5.    What is phenate?