How do we know when an Organic Compounds is Pure?

The classical criteria for determining the purity of organic compounds are correct elemental compositions  and sharpness of melting point or constancy of boiling point. Important though these analytical and physical criteria are, they can be misleading or even useless. For instance, the analytical criterion is of no help with possible mixtures of isomers because these mixtures have the same elemental composition. The simple physical criteria are not applicable to substances that decompose when one attempts to determine the melting point or boiling point. Furthermore, boiling points are not very helpful for liquids that are mixtures of substances with nearly the same boiling point or are azeotropes. Similar difficulties may be encountered with mixtures of solid substances that form mixed crystals or are eutectics.  Much sharper criteria for the purity of organic compounds now are provided through use of "super-separation" methods to see if any contaminants can be separated, or by spectroscopic techniques, as will be discussed later in this chapter. We begin here with a brief description of chromatographic methods of separation.

An azeotrope is a mixture of two or more substances that boils at a constant temperature, either higher or lower than any of its constituents. Thus an 8.5:1 mole mixture of ethanol and water boils like a pure substance, distilling at 78.2^o, which is lower than the boiling point of ethanol (78.5^o) or of water (100^o). In contrast, a 1.35:1 mole mixture of methanoic (formic) acid and water boils at 107.1o, which is higher than the boiling points of either methanoic acid (100.7^o) or water (100^o).

When solid substances are mixed, the melting point of each normally is depressed. The eutectic mixture is the mixture of the solids with the lowest melting point.