Experiment 153

Objective

To systematically identify an unknown organic sample, hypothesized to be 1,2,3-trihydroxybenzene (pyrogallol), through physical analysis, solubility testing, and functional group tests.

Introduction

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

Physical Properties of the Sample

●      Color: White

●      Physical State: Crystalline solid

●      Odor: Odorless

●      Melting Point: 133°C

Solubility Test

Materials

Chemicals

●      Supplied sample (pyrogallol)

●      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: No Prussian blue precipitate; nitrogen absent)

●      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: No orange-red precipitate; nitro group absent)

●      Carbonyl Group Test (=C=O)

o   Dissolve in rectified spirit and add 2,4-DNPH solution. (Result: No orange-red precipitate; carbonyl group 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 lacks nitrogen (N), halogens (X), and sulfur (S). It possesses an -OH group, characteristic of pyrogallol, confirmed by a melting point of 133°C, identifying the compound as 1,2,3-trihydroxybenzene (pyrogallol).

Questions

1.    What is pyrogallol?

2.    How is pyrogallol identified?

3.    How is the phenol group identified?

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