Qualitative Organic Analysis

Introduction

Qualitative organic analysis is a systematic approach used to identify the principal functional group (FG) present in an unknown organic compound. This process involves a combination of solubility tests and specific classification (functional group) tests. The identification of functional groups is fundamental in organic chemistry, as it determines the chemical reactivity and properties of organic molecules.

Overview of Functional Group Classes

In this experiment, each unknown compound contains only one principal functional group, chosen from the following eleven classes:

• 1° alcohol

• 2° alcohol (not a 2-alkanol)

• 2° 2-alkanol (R-CH(OH)-CH3)

• 3° alcohol

• Aldehyde

• Amine (1°, 2°, or 3°)

• Carboxylic acid

• Ester

• 2-alkanone (methyl ketone, R-CO-CH3)

• Miscellaneous ketone (not a methyl ketone)

• Phenol

Experimental Procedure

Solubility Tests

Solubility tests are performed first to provide preliminary information about the functional group present. The solubility of a compound in various solvents can indicate the presence of acidic, basic, or neutral functional groups.

• Procedure: Add 1–2 drops of liquid unknown (or ~10 mg of solid) to 1 mL of solvent in a small test tube. Mix thoroughly and observe dissolution.

• Interpretation: Consistency in the amount of solute and solvent is crucial. Use a graduated cylinder to measure 1 mL for reference.

• Controls: Mix toluene with water (insoluble) and acetone with water (soluble) to familiarize with results.

Classification (Functional Group) Tests

Based on solubility results, select appropriate classification tests to confirm the functional group. Always perform at least one classification test per unknown, and compare results with a known compound possessing the same functional group.

Key Classification Tests by Functional Group

Alcohols (ROH)

• Jones (Chromic Acid) Oxidation: Distinguishes 1° and 2° alcohols (and aldehydes) by the disappearance of orange color and formation of a green/blue precipitate within 5 seconds. 3° alcohols do not react.

• Cerium (IV) Nitrate Test: Positive for alcohols (yellow to orange-red color change). Negative for esters, ketones, carboxylic acids, and simple aldehydes.

• Lucas Test: For water-soluble alcohols. 3° alcohols react instantly (cloudiness), 2° alcohols react in 5 seconds to ≥5 min, 1° alcohols react in ≥1 hour.

• Iodoform Test (for 2-alkanols): Positive for compounds with the R-CHOH-CH3 group, forming a bright yellow precipitate (CHI3).

Aldehydes (RCHO)

• Jones (Chromic Acid) Oxidation: Positive for aldehydes (see above).

• 2,4-Dinitrophenylhydrazine (2,4-DNP) Test: Positive for aldehydes and ketones (formation of yellow, orange, or red precipitate). Negative for esters and alcohols.

Amines (RNH2, R2NH, R3N)

• Litmus Test: Aqueous solution or water-wet litmus paper turns blue in the presence of basic amines. Compare with a known amine for accuracy.

Carboxylic Acids (RCO2H)

• Solubility in 5% NaHCO3: Positive if compound dissolves with evolution of CO2 gas.

• Wet-Litmus Test: Acidic compounds turn blue litmus red.

• Precipitation with Acid: Addition of conc. HCl or H2SO4 to the NaHCO3 solution causes the acid to precipitate out.

Esters (RCO2R)

• Alkaline Iron (III) Hydroxamate Test: Positive for esters (red-blue or magenta color upon addition of FeCl3 after reaction with hydroxylamine and acidification).

Ketones (R-CO-R)

• 2,4-Dinitrophenylhydrazine (2,4-DNP) Test: Positive for ketones (see above).

• Iodoform Test (for 2-alkanones): Positive for methyl ketones (R-CO-CH3), forming a bright yellow precipitate (CHI3).

Phenols (Ar-OH)

• Solubility in NaOH: Phenols are soluble in 5% NaOH but insoluble in 5% NaHCO3. Acidification causes the phenol to precipitate.

• Cerium (IV) Nitrate Test: Positive for phenols (yellow-orange to red color change).

• Iron (III) Chloride Test: Positive for phenols (red, blue, green, or purple color).

• Bromine in CH2Cl2 Decolorization: Phenols rapidly decolorize bromine solution, often with HBr vapor formation (acidic, test with litmus).

Summary Table: Functional Group Classification Tests

Waste Disposal Guidelines

• Chromic acid waste: Chromium (VI) Heavy Metal Waste container

• Cerium (IV) nitrate and iodoform test waste: Oxidizer Waste container

• Lucas test waste: Inorganic Acid Waste container

• Hydroxamate and bromine test waste: Halogenated Liquid Organic Waste container

• Solubility test waste: Solubility Test Waste container

• Iron (III) chloride test waste: Heavy Metal Waste container

• Used litmus paper: Household Waste container

Safety Precautions

• All chemicals should be considered toxic, irritant, or hazardous.

• Wear appropriate PPE (gloves, goggles, lab coat) at all times.

• Avoid direct inhalation of vapors and skin contact.

• Wash hands and change gloves regularly, especially if contaminated.

• Dispose of all waste according to the guidelines above.

Additional info:

• Functional group identification is a foundational skill in organic chemistry, essential for understanding reactivity and synthesis.

• Always compare test results with known standards to avoid misinterpretation.

• Do not perform unnecessary tests once a functional group is confirmed, to avoid confusion and conserve sample.