Functional analysis by colorimetry and fluorimetry

Color reactions, once described for particular chemical species, were generally of empirical origin, sometimes discovered by chance, and were often neither sensitive nor even selective. With the development of organic synthesis and instrumental methods, comprehensive studies have made it possible to isolate and identify the products formed. The evidence thus provided for reaction schemes has given new impetus to research, and knowledge of other organic chemistry reactions has led to the design of new analytical methods. For example, organic analysis does not only involve the cleavage or destruction of matter, but also the cyclization, duplication and cycloaddition of carbonaceous structural elements.

In functional analysis, any process whose mechanism has been established should be applicable, all things being equal, to all compounds benefiting from the target function. But the reactivity of the target function is not independent; it is linked to the structure of the molecule as a whole, and this correlation can even prevent reactivity.

In practical terms, functional analysis techniques using visible and ultraviolet spectrometry or fluorimetry mean that the relative value of results is not identical for all substances with the same assay function. However, within certain limits, Lambert-Beer's law, a linear relationship between absorbance and the quantity of substance used, remains satisfactory. It authorizes quantitative tests, provided calibration is performed using the chemical species itself.

M. Pesez and J. Bartos have developed numerous applications of colorimetry, fluorimetry and also ultraviolet spectrophotometry in books of international scope, indicating the appropriate wavelength values and concentration range.

In organic analysis, the use of visible colorimetric and fluorimetric methods is declining in favor of direct instrumental methods that do not require prior reactions, such as mass spectrometry, nuclear magnetic resonance or infrared, which provide an image of the entire molecule. However, functional analysis by colorimetry or fluorimetry continues to be used for rapid identification of molecules without the need for sophisticated equipment, possibly involving extraction with an immiscible solvent.

The implementation of these chemical reactions also has the advantage of materializing the reactivity of functional groups. Knowledge of this often enables us to predict the stability of organic molecules, incompatibilities...