Experiment 162

Objective

To identify the presence of benzoic acid in an unknown organic sample.

Introduction

This experiment combines physical observation, solubility testing, and functional group analyses to systematically identify the unknown compound. A melting point comparison with known data further confirms its identity.

Physical Characteristics of Sample:

●      Color: White

●      State: Crystalline solid

●      Odor: Pungent vapor odor

●      Melting Point: 122°C

Solubility Test Results

Materials

Chemicals

●      Unknown organic sample (presumed benzoic 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, then add bromine solution.

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

●       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: No intense coloration, indicating phenol group is absent.

●       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)

Based on the melting point of 122°C and the presence of these functional groups, the sample is identified as benzoic acid.

Questions

1.    What is benzoic acid?

2.    How is the -COOH group identified?

3.    How is the -OH group identified?

4.    What is the Liebermann test?

5.    How is benzoic acid identified?

6.    How is benzoic acid produced?