Experiment 157 

Objective

To systematically identify the unknown organic sample, hypothesized as 3,4-dihydroxybenzaldehyde (protocatechualdehyde), using melting point and functional group tests.

Introduction

Sample Characteristics

●      Color: Colorless

●      Physical State: Crystalline solid

●      Odor: Odorless

●      Melting Point: 153°C

This compound can be identified by performing elementary analysis and functional group tests, allowing structural confirmation through melting point and chemical reactivity.

Solubility Tests

Materials

Chemicals

●      Sample (suspected 3,4-Dihydroxybenzaldehyde)

●      Distilled water, 5% HCl, 5% NaOH, 5% NaHCO₃

●      Ferrous sulfate, Nitric acid, Silver nitrate, Ferric chloride

●      10% Sodium nitrite (NaNO₂), 2% Bromine solution, 20% NaOH

Apparatus:

●      Test tubes, test tube holder, Bunsen burner, thermometer

●      500 ml flask, test tube stand, pipette, fusion tube, mortar, pestle

Reagents Preparation

●      Prepare 5% solutions of HCl, NaOH, and NaHCO₃ in 500 ml flasks.

●      Prepare 5% Ferrous sulfate and 10% NaNO₂ solutions.

●      Take HNO₃ and AgNO₃ in beakers.

●      Prepare 20% NaOH solution.

Procedure

1.    Elementary Analysis

●      Preparation of Stock Solution: Place a clean, dry piece of sodium in a fusion tube, heat until it melts, add a small amount of sample, and heat until dull-red. Drop the fusion tube into a mortar with 3-5 ml distilled water, break the tube, and filter for use as stock solution.

●      Nitrogen Test: Add 1 ml of freshly prepared ferrous sulfate to 1-2 ml of stock solution, boil, cool, then add H₂SO₄. Absence of Prussian blue color confirms nitrogen is absent.

●      Halogen Test: Acidify 1 ml of stock solution with nitric acid, boil for 6-7 minutes, cool, then add silver nitrate. No precipitate confirms halogens (chlorine, bromine, iodine) are absent.

●      Sulfur Test: Acidify 1 ml of stock solution with dilute acetic acid, then add lead acetate. Absence of black precipitate indicates sulfur is absent.

2.    Functional Group Tests

●      Unsaturation Test:

●      Dissolve 0.2 g of sample in CCl₄, add 2% bromine in CCl₄ dropwise. Bromine color discharge indicates unsaturation.

●      Dissolve 0.2 g of sample in acetone or water, add 2% KMnO₄. KMnO₄ color discharge confirms unsaturation.

●      Nitro (-NO₃) Group Test: Boil sample with tin and HCl, cool, dilute, add NaNO₂, and test with alkaline α-naphthol. Absence of color confirms -NO₃ is absent.

●      Carbonyl (=C=O) Group Test: Dissolve sample in rectified spirit, add 2,4-DNPH, and observe. Formation of orange-red precipitate confirms presence of =C=O.

o   Aldehyde vs. Ketone Test (Fehling’s Test): Heat sample with Fehling’s solution in a water bath for 10-12 minutes, cool. Pink precipitate indicates the presence of an aldehyde.

●      Carboxylic Acid (-COOH) Test: Add sample to NaHCO₃ solution. Absence of reaction confirms -COOH is absent.

●      Phenol (Ar-OH) Test: Dissolve sample in rectified spirit, add FeCl₃. Intense coloration (purple, blue, red, violet, green) confirms the presence of an -OH group.

●      Aromatic Hydrocarbon Test: Dissolve sample in alcohol, add picric acid solution. Formation of orange-red needle-shaped crystals confirms aromatic hydrocarbon.

Observations

The sample does not contain N, X, or S, but has -OH and -CHO groups as its functional groups. The melting point of 153°C and chemical properties confirm the sample as 3,4-dihydroxybenzaldehyde (Protocatechualdehyde).

Questions

1.    What is protocatechualdehyde?

2.    How is the phenol group identified?

3.    What reagent is used in the phenol group test?

4.    How is the aldehyde group identified?