Elucidation of Reaction Mechanisms in Organic Chemistry (overall)

Identification of Products and Intermediates - Spectroscopic Methods

The identification of stable, isolable products is carried out with the common methods used in the investigation and elucidation of organic compounds' structures. Today, the most important of these methods are the NMR spectroscopy, the mass spectrometry, and the . These methods are suitable for complete structure elucidations, especially when they are applied in combination with each other. However, in some cases, additional methods are required to determine special structural characteristics, such as the absolute configuration of an asymmetric carbon atom. The exact knowledge of the structure of the products and starting products is very important for the elucidation of the reaction mechanism, since the structures of the intermediates can often not be completely elucidated, due to their short lifetime and consequently low concentration. However, the intermediates have been developed from the starting products and are then further converted into the products. Therefore, the exact knowledge of the structures of the products and starting products is very helpful in determining the intermediate structures, or at least in excluding particular structure alternatives.

In the following, a few spectroscopic methods for the investigation of the structure of intermediate products are introduced.

Spectroscopic methods

The most common spectroscopic methods of identifying (or detecting, respectively) intermediates are the UV-VIS spectroscopy, the infrared spectroscopy, the NMR spectroscopy, and the ESR spectroscopy. The question of which method to apply in each case depends on several things:

1. Which functional group should be detected?
2. How high is the concentration of the intermediate during the reaction?
Fig.1
UV spectrum.

The UV-VIS spectroscopy is particularly suitable for detecting conjugated double bond systems, even when they contain heteroatoms (e.g. α,β-unsaturated carbonyl compounds). Isolated double bonds can be detected in addition to several special functional groups, as well. The extinction coefficient and, thus the detectability, usually considerably increase with the extent of the conjugated double bond system. Under good conditions, concentrations as small as 10-6 M may be detected.

Fig.2
Infrared spectrum.

The infrared spectroscopy is particularly suitable for detecting specific functional groups, which display a high absorbance coefficient in the infrared region and absorb in a characteristic wavenumber range in which no or hardly any overlapping with the absorption bands of other functional groups occur. The different carbonyl compounds, ketenes, ammonium ions, and nitriles are examples of functional groups that may be detected by infrared spectroscopy. The position and intensity of the absorption bands in the UV-VIS and infrared region of a large number of functional groups may be acquired from tabular compilations.

Fig.3
NMR spectrum.

In connection with the range of application, the NMR spectroscopy is the most efficient any of the mentioned methods, which have been discussed up to this point. With NMR spectroscopy, virtually all organic compounds can be detected and more easily distinguished than with any other method. However, the lower detection limit of the NMR spectroscopy is higher than with other methods. Thus, the NMR spectroscopy cannot be applied to signficantly low concentrations. Through the last years, the detection limit has been considerably lowered by the development of more and more efficient NMR spectrometers. Therefore, above all, the most sensitive NMR method, the $H1$ NMR spectroscopy, is more and more often applied in the identification of intermediates.

Fig.4
ESR spectrum.

The ESR spectroscopy is the most sensitive of any of the methods that have been discussed, that is, it displays the lowest detection limit. However, only or paramagnetic compounds, respectively, can be detected by ESR spectroscopy. Certainly, the ESR spectroscopy is the best choice for detection in these cases.

The mere detection of a compound in the reaction mixture is not sufficient for the identifiction of an intermediate product, as it could also be a side product, which has developed in small amounts. In order to identify a compound as an intermediate, further proof of the compound's conversion into a product is additionally required.

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