Experiment 149

Objective

To systematically identify the presence of p-aminobenzoic acid in an unknown organic sample.

Introduction

This experiment involves analyzing an unknown organic compound by observing its physical properties, performing solubility tests, and conducting functional group analyses. The compound is then identified by comparing its melting point with known standards.

Physical Characteristics of Sample:

·       Color: White

·       State: Solid

·       Odor: Odorless

·       Melting Point: 186°

The identification process includes an elementary analysis to detect elements in the sample and specific tests to confirm functional groups.

Solubility Test Results

Materials 

Chemicals

·       Unknown organic sample (presumed p-aminobenzoic 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: Prussian blue color indicates nitrogen.

·       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

·       Primary Amine (-NH₂): Add 10% NaNO₂ solution to a sample dissolved in dilute HCl, then add to alkaline α-naphthol solution.

·       Observation: Orange-red precipitate confirms -NH₂.

·       Secondary Amine (-NH-): Add excess 10% NaNO₂ to a sample dissolved in dilute HCl.

·       Observation: No change indicates secondary amine is absent.

·       Tertiary Amine (=N-): Add 10% NaNO₂ to a sample in dilute HCl, heat with 10% NaOH.

·       Observation: No green crystals form, indicating tertiary amine is absent.

·       Substituted Amide (-NHCOR): Boil sample with 20% HCl, cool, and add to alkaline α-naphthol solution.

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

·       Nitro Group (-NO₂): Boil with granulated tin and HCl, cool, dilute with water, and add NaNO₂.

·       Observation: No orange-red precipitate, indicating -NO₂ group 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 (Ar-OH): Dissolve in rectified spirit, add 5% FeCl₃ solution.

·       Observation: No intense coloration, indicating phenol 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: Nitrogen (N)Functional Groups: Primary amine (-NH₂) and carboxylic acid (-COOH)

Based on the melting point of 186°C and the presence of these functional groups, the organic sample is identified as p-aminobenzoic acid

Questions

1.    What is p-aminobenzoic acid?

2.    How are primary, secondary, and tertiary amines identified?

3.    What is the Lassaigne test?

4.    What happens when bromine is added to unsaturated carbon?

5.    What color appears in the nitrogen test?

6.    How is the -COOH group identified in carbonyl compounds?