Qualitative Analysis of Organic Compounds - Physical Constants, Preliminary Test, Solubility Tests

Qualitative Analysis of Organic Compounds

The analysis and identification of unknown organic compounds constitutes a very important aspect of experimental organic chemistry. There is no definite set procedure that can be applied overall to organic qualitative analysis. Sundry books have different approaches, but a systematic approach based on the scheme given below will give satisfactory results. Qualitative tests that require substantial quantities of a number of (often hazardous) chemicals to be stocked in the lab for experimental use are frequently being phased out of organic chemistry in favor of modern spectroscopic techniques. In order to deduce the identity of your two unknowns, you will combine one qualitative test, that for the classification of halides, with modern analytical techniques of infrared spectroscopy (IR) and mass spectrometry (MS). An elaboration on qualitative tests can be found in this Appendix and Hornback on page 291.

General Scheme of Analysis

A. Preliminary Test
Note physical characteristics: solid, liquid, color, and odor. Compounds that are yellow to red in color are often highly conjugated. Amines often have a fish-like odor, while esters usually have a pleasant fruity or floral smell. Acids have a sharp, biting odor. Odors can illicit information about your unknown; it is wise to sniff them with caution. Some compounds can have corrosive vapors or make you feel nauseous.

B. Physical Constants
Determine the boiling point or melting point. Distillation is recommended in case of liquids. It serves the dual purpose of determining the boiling point as well as purifying the liquid for subsequent tests.

C. Solubility Tests
The solubility of the unknown in the following reagents provides very useful information. In general, about 1 mL of the solvent is used with approximately 0.1 g or 0.2 mL (2-3 drops) of the unknown compound. Assistance in analyzing the results from your solubility tests can be found in the solubility flowchart given in Fieser and Williamson on page 606. (Careful, this flowchart is much more complex than our lab requires and often can make it more difficult to form conclusions from your solubility data..)

D. Group Classification Tests
After analysis of the previous tests and the compound's IR spectrum, if needed, further information can be deduced by performing carefully selected functional group classification tests.