Overview of Organic Analysis

Once the purity of the unknown is established, various physical properties are determined. The melting point of a solid or boiling point of a liquid is considered essential. Occasionally, the density and/or refractive index of a liquid may be useful, and for certain compounds, either liquid or solid, the specific rotation can be determined if a substance is optically active. 

Establishing what elements other than carbon and hydrogen are present is critical for identifying the compound, and techniques for elemental analysis are described later. Molar mass, as determined by cryoscopic techniques or mass spectrometry (Sec. 8.5), or percentage by mass composition of the elements present, also provides important data. The solubility of the unknown compound in water, in dilute acids and bases, or in various organic solvents may signal the presence or absence of various functional groups. 

Final assignment of a structure to the unknown compound is achieved by one of several procedures. The classic method involves the chemical conversion of the substance into a solid derivative. The success of this technique depends on the availability of information about the unknown and its various derivatives. Of prime importance is knowledge of the melting or boiling points of possible candidates for the unknown as well as the melting points of solid derivatives. Many tabulations of organic compounds are available for this purpose, and references to two of them are provided at the end of Section 25.5. Abbreviated tables of liquid and solid organic compounds and of their solid derivatives are provided at the website associated with this textbook. 

The identification of the compound may also be completed by thoughtful analysis of the spectroscopic data for the compound. Indeed, unequivocal proof of structure based solely upon spectral data often is possible demonstrating that the IR and NMR spectra of an unknown and a known compound are identical suffices to prove that the substances are identical. 

The following sections contain descriptions of the stepwise procedures that may be used to identify an unknown compound using classic methods alone or in combination with spectroscopic methods. As a reminder, these procedures should not be performed unless the compound is pure. Since it is possible that you may be given a mixture of unknown compounds to identify, a procedure is provided in Section 25.4 for separating a mixture into its individual components so that each one can be identified. 

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