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Procedure for Determining Solubility of Organic Compounds

The amounts of material to use for a solubility test are somewhat flexible. Use 2-3 drops of a liquid or approximately 10 mg of a solid. Unless the solid is already a fine powder, crush a small amount of the solid on a watch glass with the back of a spatula. Do not weigh the solid; simply use enough to cover the tip of a small spatula. Your instructor will demonstrate how to estimate the correct amount. Place the appropriate amount of either your solid or liquid unknown in a small test tube and proceed with the following solubility tests.

1) Water Solubility
Add approximately 6 drops of water to the test tube containing your unknown.  Shake the tube and/or stir with a glass stirring rod. A soluble unknown will form a homogeneous solution with water, while an insoluble liquid will remain as a separate phase. A liquid which is soluble in water may be either a low molecular weight polar compound of up to 5 carbon atoms or less. You may add additional water, up to 1 mL, if your compound does not completely dissolve with the smaller amount.

Check the pH of the water to determine if your unknown is partially or completely soluble in water and whether your compound has changed the pH of the water.

  • pH paper turns red: water soluble acidic compound
  • pH paper turns blue: water soluble basic compound
  • pH paper does not change color: water soluble neutral compound or insoluble compound

An organic compound which is soluble in water is typically a low molecular weight polar compound of up to 5-6 carbon atoms or less.

Why is it unnecessary to determine the pH of the remaining acid-base solubility tests (#2-#4)?

Acid-Base Solubility Tests:
[Please write a general chemical reaction(s) for any positive solubility test result(s) that you obtain for your unknown compound. Your reaction should demonstrate how any organic compound with a specific functional group can dissolve/react in an aqueous solution.
]

2) 5% NaOH Solubility

Add approximately 1 mL of 5% NaOH in small portions of about 6 drops each to the test tube containing your unknown. Shake test tube vigorously after the addition of each portion of solvent. Solubility will be indicated by the formation of a homogeneous solution, a color change, or the evolution of gas or heat. If soluble, then your unknown is behaving as an organic acid. The most common organic acids are carboxylic acids and phenols. Carboxylic acids are usually considered stronger acids than phenols, but both of these acids will react with NaOH (a strong base).

3) 5% NaHCO3 Solubility
Add approximately 1 mL of 5% NaHCO3 in small portions of about 6 drops each to the test tube containing your unknown. Shake test tube vigorously after the addition of each portion of solvent. Solubility will be indicated by the formation of a homogeneous solution, a color change, or the evolution of gas or heat. If soluble, then it is a strong organic acid. If not, then it is a weak organic acid, if it dissolves in NaOH. The most common weak organic acid are phenols. Typically, only a carboxylic acid will react with NaHCO3
.

4) 5% HCl Solubility
Add approximately 1 mL of 5% HCl; in small portions of about 6 drops each to the test tube containing your unknown. Shake test tube vigorously after the addition of each portion of solvent. Solubility will be indicated by the formation of a homogeneous solution, a color change, or the evolution of gas or heat. If your compound is HCl-soluble, then it is an organic base. Amines are the most common organic base.

If insoluble in all solutions, then your unknown is not an acidic or basic organic compound.

pKa Values of Selected Organic and Inorganic Compounds
(For a more complete list of pKa values, see http://www2.lsdiv.harvard.edu/labs/evans/pdf/evans_pKa_table.pdf

Compound pKa
H2O 15.7
C6H5OH (phenol) 9.95
HCl (hydrochloric acid) -8.0
CH3COOH (acetic acid) 4.76
H2SO4 (sulfuric acid) -3.0
CH3CH2NH3+ (protonated ethyl amine) 10.6
CH3CONH3+ (protonated acetamide) 15.1
HCO3- (hydrogen carbonate ion) 10.3