EXP 6: Solubility Test for Organic Compounds

Solubility Test for Organic Compounds

Background

Organic qualitative analysis utilizes various techniques to identify unknown compounds. Among these, solubility tests are essential for investigating the physical properties of a compound and inferring the presence of functional groups. The solubility behavior of organic compounds in different solvents provides valuable clues about their structure, especially regarding acidic or basic functional groups.

• Solubility refers to the ability of a substance to dissolve in a solvent, forming a homogeneous solution.

• Solubility tests can distinguish between polar and nonpolar compounds, as well as acidic, basic, and neutral functional groups.

• Common solvents used include water, sodium hydroxide (NaOH), sodium bicarbonate (NaHCO3), hydrochloric acid (HCl), and sulfuric acid (H2SO4).

Objective

• To identify the functional group(s) present in an unknown organic compound using a series of solubility tests.

Materials and Equipment

• Test tubes

• Water

• NaOH (1 M)

• NaHCO3 (5% solution)

• HCl (1 M)

• H2SO4 (concentrated)

• Standard samples: ethylamine, cyclohexane, benzoic acid, phenol

• Unknown sample

Solubility Test Procedures

Solubility Test in Water

This test determines whether the compound is polar enough to dissolve in water, indicating the presence of polar functional groups.

• Add approximately 6 drops of water to a test tube containing the unknown sample.

• Shake the tube and observe whether a clear solution forms.

• If the sample is soluble, a homogeneous solution will result. If not, add more water and repeat.

• Insolubility suggests a nonpolar compound or one with large hydrophobic regions.

Solubility Test in NaOH, NaHCO3, HCl, or H2SO4

These tests help identify acidic or basic functional groups by observing their reactions with strong acids or bases.

• NaOH (Sodium Hydroxide): Dissolves acidic compounds (e.g., carboxylic acids, phenols) by forming water-soluble salts.

• NaHCO3 (Sodium Bicarbonate): Reacts with strong acids (e.g., carboxylic acids) to produce CO2 bubbles.

• HCl (Hydrochloric Acid): Dissolves basic compounds (e.g., amines) by forming water-soluble ammonium salts.

• H2SO4 (Sulfuric Acid): Dissolves neutral compounds with oxygen-containing functional groups (e.g., alcohols, aldehydes, ketones).

Procedure:

• Add a few drops of the solvent to the sample and observe for dissolution, gas evolution (CO2 bubbles), or other reactions.

• Record observations for each solvent.

Interpretation of Results

• Solubility in water: Indicates polar functional groups (e.g., alcohols, amines, carboxylic acids).

• Solubility in NaOH: Indicates acidic functional groups (carboxylic acids, phenols).

• CO2 evolution in NaHCO3: Indicates strong acids (carboxylic acids).

• Solubility in HCl: Indicates basic functional groups (amines).

• Solubility in H2SO4: Indicates neutral oxygen-containing groups (alcohols, aldehydes, ketones).

Functional Groups and Their Solubility Behavior

Functional Group	Solubility in Water	Solubility in NaOH	Solubility in NaHCO3	Solubility in HCl	Solubility in H2SO4
Carboxylic Acid	Yes	Yes	Yes (CO2 bubbles)	No	Yes
Phenol	Yes	Yes	No	No	Yes
Amines	Yes	No	No	Yes	Yes
Alcohols	Yes	No	No	No	Yes
Hydrocarbons	No	No	No	No	No

Safety and Hazard Assessment

Laboratory Safety Considerations

• Always wear appropriate personal protective equipment (PPE): lab coat, gloves, goggles.

• Handle strong acids and bases (HCl, H2SO4, NaOH) with care; they are corrosive and can cause burns.

• Dispose of chemical waste according to laboratory protocols.

• Know the location of safety showers, eyewash stations, and first aid kits.

Guide Questions and Analysis

• Why is water solubility the first test run? Water solubility is a simple and rapid test that provides initial information about the polarity of the compound and the presence of polar functional groups.

• How do characteristic acidic and basic functional groups respond to solubility tests? Acidic groups dissolve in basic solutions (NaOH, NaHCO3), while basic groups dissolve in acidic solutions (HCl).

• What information can be gathered from a positive or negative solubility result? Positive results indicate the presence of specific functional groups; negative results suggest the absence or low polarity of such groups.

• Why can a specific functional group dissolve or react in an aqueous solution? Functional groups interact with solvents via hydrogen bonding, ionization, or acid-base reactions, increasing solubility.

Figures and Diagrams

Schematic Diagram for Solubility Test (Figure 1)

The schematic diagram outlines the decision process for solubility testing, starting with water and progressing through basic and acidic solvents to identify functional groups.

Molecular Structures (Figure 2)

• Ethylamine: Amine functional group

• Cyclohexane: Hydrocarbon

• Benzoic acid: Carboxylic acid functional group

• Phenol: Phenol functional group

Structures of Some Functional Groups (Figure 3)

• Alcohol: $R-OH$

• Aldehyde: $R-CHO$

• Ketone: $R-CO-R'$

• Carboxylic acid: $R-COOH$

• Amine: $R-NH_2$

• Phenol: $C_6H_5OH$

References

• University of Delaware: Identifying an Unknown Compound by Solubility, Functional Group Tests, and Spectroscopy

• McMurry, J. Organic Chemistry, 9th Edition

• Other online resources and laboratory manuals

Additional info: Academic context and explanations have been expanded for clarity and completeness. The schematic diagram and tables have been described and recreated for study purposes.