Miroslav Večeřa

Approach to Analysis

In the previous chapter we mentioned that the analyst in organic chemistry does not have a systematic procedure at his disposal for the solution of his problems. There are cases, of course, when he is able to elaborate a basic systematic procedure for a series of analyses, but in the case of one anomalous sample such a procedure might already become useless.

Carboxylic Acids and Their Derivatives

The form in which acids are present in the analyzed material is important to the analyst: whether they are free, in aqueous solution, in the form of salts or solutions of these salts, or mixed with other neutral or acid substances. The character of the sample determines the course of the analytical procedure. It is also important to bear in mind that some carboxylic acids are volatile (lower aliphatic monocarboxylic acids), some are volatilized by steam distillation (monocarboxylic acids), others are water soluble (lower monocarboxylic and certain dicarboxylic acids), and practically all are soluble in 5% sodium hydrogen carbonate. All these properties offer a possibility of separating these substances from others. As carboxylic acids do not give characteristic color reactions, all general detections of this class of substance are based on their acidity. This is determined in aqueous solutions by means of indicators, and in the case of water-insoluble acids on the basis of their solubility in 5% sodium hydrogen carbonate. In all instances, however, it is advisable to complete the proof of acidity by titration with alkali; in the case of pure acids by determining their NE (neutralization equivalent), which is characteristic for each acid. In the case of more complicated mixtures titration affords information on the approximate amount of acids present in the sample and whether they represent a substantial fraction of it. If acidity is found in the sample, it is also necessary to look for the presence of sulfur, because the acidity could be caused by the presence of sulfonic acids. However, as the detection of acidity need not be reliable in more complicated cases, it is much safer to employ paper chromatography and to carry out the acidity detection on the chromatogram. This method has the advantage that it can also be carried out with salts.

Aims and Methods of Organic Qualitative Analysis

The basic aims of organic qualitative analysis are the detection and the identification of organic compounds. As other terms for both ideas sometimes appear in the literature, we shall first give their definitions.

Tests for Elements

Tests for elements in organic substances are carried out after oxidative or reductive mineralization by common methods of inorganic qualitative analysis.

Preparation of Chemical Substances for Analysis

Some modern methods of organic analysis permit the analysis of substances which are not quite pure, or even true mixtures. This is sometimes necessary because of the nature of the sample or its quantity. Certain methods, for example, chromatography, permit simultaneous separation and identification of components. Nevertheless, the first aim of the analyst when identifying a substance remains the preparation of individual substances, using basic separation and purification operations; only then he can proceed with carrying out the analysis proper. The separation and purification methods. are met by the analyst in subsequent preparation of derivatives. Sometimes the separation and purification of the sample are more difficult and time-consuming than the analysis itself. Therefore, the mastering and the knowledge of basic separation and purification methods are prerequisites without which the analyst’s work in an organic laboratory cannot be conceived. Various individual operations are well described in the literature in connection with the techniques of organic chemistry, because, especially in the micropreparative work recommended so much in this field, organic chemistry and analysis use the same techniques for their work. It is beyond the scope of this monograph to describe the details of given operations, as these are accessible in various basic works (1, 2, 3).

Group and Classification Reactions

Organic compounds can be suitably and systematically classified by group and classification reactions. Color and precipitation reactions lead to the determination of the presence of functional groups, and the analyst can thus choose a suitable means for further analysis. For practical procedure see the general remarks on p. 51 and Table 1.

Some Nitrogen-Containing Compounds

For the detection and identification of aliphatic and aromatic nitro compounds reduction is used (catalytic; with hydrogen or with the system: metal plus HCl); amines formed on reduction can be detected and identified by methods described on p. 317.

Carbonic Acid Derivatives

A series of important compounds can be obtained by the substitution of the oxygen atom or of the OH groups in carbonic acid CO(OH)2 by groups containing sulfur, halogens, or nitrogen.