Experiment 156

Objective

To identify the presence of 3,4,5-trihydroxybenzoic acid (gallic acid) in an unknown organic sample.

Introduction

This experiment involves physical observations, solubility testing, and functional group analysis to identify the unknown compound. The melting point comparison aids in confirming the identity of the compound.

Physical Characteristics of Sample:

·       Color: White

·       State: Needle-like crystals

·       Odor: Odorless

·       Melting Point: 222°C

Solubility Test Results

Materials

Chemicals

·        Unknown organic sample (presumed 3,4,5-trihydroxybenzoic 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 and holder

·        Bunsen burner

·        Thermometer

·        500 mL flask, test tube stand

·        Pipette

·        Fusion tube

·        Mortar and pestle

Preparation of Reagents

·       Prepare solutions of 5% HCl, 5% NaOH, 5% NaHCO₃, and 5% ferrous sulfate in a 500 mL flask.

·       Prepare a 10% NaNO₂ solution and a 20% NaOH solution for specific tests.

Procedure

1.    Elementary Analysis (Stock Solution Preparation)

·        Heat a piece of sodium in a fusion tube until molten.

·        Add a small amount of the sample, heat to a dull red, then drop the tube into a mortar with 3-5 mL of distilled water.

·        Crush the tube, filter the solution, and use it for further testing.

·        Test for Nitrogen: Add 1-2 mL of the stock solution to freshly prepared ferrous sulfate, boil, cool, and add a few drops of H₂SO₄.

·       Observation: No Prussian blue indicates nitrogen is absent.

·        Test for Halogens: Acidify 1 mL of boiling solution with nitric acid, boil, cool, and add silver nitrate.

·       Observation: Absence of white, pale yellow, or yellow precipitate indicates no halogens (Cl, Br, I).

·        Test for Sulfur: Acidify 1 mL of solution with acetic acid and add lead acetate.

·       Observation: No black precipitate, indicating sulfur is absent.

2.    Functional Group Tests

·        Test for Unsaturation: Dissolve 0.2 g of the sample in CCl₄, add bromine solution.

·       Observation: No discharge of bromine color, indicating unsaturation is absent.

·        Primary Amine (-NH₂): Dissolve sample in dilute HCl, add NaNO₂, and transfer to an alkaline α-naphthol solution.

·       Observation: No orange-red precipitate, indicating absence of primary amine.

·        Secondary Amine (-NH-): Dissolve sample in dilute HCl, add excess NaNO₂.

·       Observation: No reaction, indicating absence of secondary amine.

·        Tertiary Amine (=N-): Dissolve sample in dilute HCl, add NaNO₂.

·       Observation: No green crystals form, indicating absence of tertiary amine.

·        Carbonyl Group (=C=O): Dissolve in rectified spirit, add 2,4-DNPH.

·       Observation: No orange-red precipitate, indicating carbonyl group is absent.

·        Carboxylic Acid (-COOH): Add sample to NaHCO₃ solution.

·       Observation: Effervescence of CO₂, turning limewater milky, confirms -COOH.

·        Phenol Group (Ar-OH): Dissolve in rectified spirit, add FeCl₃.

·       Observation: Intense coloration (purple, blue, red, violet, or green) confirms the presence of -OH.

·        Aromatic Hydrocarbon: Dissolve in absolute alcohol, add picric acid solution.

·       Observation: Orange-red needle-shaped crystals confirm aromatic hydrocarbon.

Observation

Identified Functional Groups:

·        Elements: None (N, X, S absent)

·        Functional Groups: Carboxylic acid (-COOH), Phenol (-OH)

Based on the melting point of 222°C and the presence of these functional groups, the sample is identified as 3,4,5-trihydroxybenzoic acid (gallic acid).

Questions

1.    What is gallic acid?

2.    How is the -COOH group identified?

3.    How is the -OH group identified?

4.    What is the Liebermann test?