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 of organic compounds in different solvents is influenced by the nature of their functional groups, which can be acidic, basic, or neutral.

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

• Functional groups such as carboxylic acids, amines, alcohols, and phenols affect solubility due to their polarity and ability to form hydrogen bonds.

• Solubility tests can help distinguish between acidic, basic, and neutral organic compounds.

• Example: Carboxylic acids are generally soluble in basic solutions due to the formation of carboxylate ions.

Objective

The main objective of the solubility test is to identify the functional group(s) present in an unknown organic compound by observing its solubility in a series of solvents.

• Determine whether the compound is acidic, basic, or neutral.

• Use solubility behavior to infer possible functional groups.

Materials and Equipment

• Test tubes

• Water

• 1% NaOH (sodium hydroxide)

• 1% HCl (hydrochloric acid)

• 1% H2SO4 (sulfuric acid)

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

• Unknown sample

Solubility Test Procedures

General Procedure

Follow the schematic diagram (see Figure 1) to perform solubility tests using the following standards and the unknown sample. Based on the results, identify the acidic or basic functional group present in the unknown sample.

• Use 3–4 drops of solvent for each test.

• Observe for formation of a homogeneous solution (indicates solubility).

• Formation of oily bubbles or layers indicates insolubility.

A. Solubility Test in Water

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

• Shake and observe for a homogeneous solution.

• If insoluble, add more water and repeat.

• Absence of all indicators means the sample is insoluble in water.

• Example: Ethylamine is soluble in water due to its ability to form hydrogen bonds.

B. Solubility Test in NaOH, NaHCO3, HCl, or H2SO4

• Sodium hydroxide (NaOH) and hydrochloric acid (HCl) are used to test for acidic and basic functional groups, respectively.

• Add 1 mL of the solvent to the sample and observe for solubility.

• Solubility in NaOH suggests the presence of acidic groups (e.g., carboxylic acids, phenols).

• Solubility in HCl suggests the presence of basic groups (e.g., amines).

• Example: Benzoic acid is soluble in NaOH due to the formation of benzoate ion.

Functional Groups and Solubility

Common Functional Groups

• Carboxylic acids: Soluble in basic solutions (NaOH, NaHCO3).

• Amines: Soluble in acidic solutions (HCl).

• Phenols: Soluble in strong bases (NaOH), less so in weak bases.

• Alcohols: Generally soluble in water if they have short carbon chains.

• Hydrocarbons: Generally insoluble in water and aqueous solutions.

Representative Structures

• See Figure 2 for molecular structures of standards: ethylamine, cyclohexane, benzoic acid, phenol.

• See Figure 3 for structures of some functional groups: carboxylic acid, amine, alcohol, phenol.

Data Recording and Interpretation

Data Sheet Table

The following table is used to record the solubility of each sample in different solvents:

Sample Standard	Water	NaOH	NaHCO3	HCl	H2SO4
Ethylamine	Soluble	Soluble	Insoluble	Soluble	Soluble
Cyclohexane	Insoluble	Insoluble	Insoluble	Insoluble	Insoluble
Benzoic acid	Insoluble	Soluble	Soluble	Insoluble	Insoluble
Phenol	Partially soluble	Soluble	Insoluble	Insoluble	Soluble
Unknown sample	[Record result]	[Record result]	[Record result]	[Record result]	[Record result]

Additional info: Table entries for standards are inferred based on typical solubility behavior of these compounds.

Treatment of Results

• Summarize the solubility test results for the unknown sample.

• Describe observations and draw conclusions about the possible functional group(s) present.

• Use solubility patterns to infer whether the compound is acidic, basic, or neutral.

Guide Questions

• Why is water usually the first test solvent?

• Describe how characteristic acidic and basic functional groups are identified by solubility tests.

• What information can be gathered from a positive or negative solubility result?

• Provide general chemical reactions for a positive solubility result in each solvent.

• Discuss how a specific functional group can dissolve or react in an aqueous solution.

Key Chemical Equations

• Carboxylic acid in NaOH: $\mathrm{RCOOH} + \mathrm{NaOH} \rightarrow \mathrm{RCOO^-Na^+} + \mathrm{H_2O}$

• Amines in HCl: $\mathrm{RNH_2} + \mathrm{HCl} \rightarrow \mathrm{RNH_3^+Cl^-}$

• Phenol in NaOH: $\mathrm{PhOH} + \mathrm{NaOH} \rightarrow \mathrm{PhO^-Na^+} + \mathrm{H_2O}$

References

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

• McMurry, Organic Chemistry, 9th Edition

• Other standard organic chemistry laboratory manuals