Reaction intermediates identification monitoring methods

Identification of the intermediates in a multi-step reaction is the major objective of studies of reaction mechanisms. It is most useful to study intermediates present in low concentrations without chemical interference with the reacting system, i.e. by rapid spectroscopic methods. The most common methods in organic chemistry include ultraviolet-visible (UV-VIS), IR, and EPR spectroscopy. In principle, all other spectroscopic methods for the detection of reaction intermediates are also applicable provided that they are fast enough to monitor the intermediate and able to provide sufficient structural information to assist in the identification of the transient species. 

In principle, the application of time-resolved techniques permits identification of intermediates by monitoring their progress to the stable products of reaction. In 1973, Lehman and Berry [25] reported the first application of time-resolved photochemical methods to the study of aryl azides. Using conventional flash photolysis, they irradiated 2-azidobiphenyl in cyclohexane solution. Time-resolved absorption spectroscopy revealed an intermediate assigned as the triplet nitrene primarily on the basis of the similarity of its spectrum to that measured by Reiser [18] in low-temperature experiments. Lehman [25] monitored the rate of carbazole formation and found it to occur by a kinetically first-order process with a lifetime of 460 /is at room temperature. These findings led them to conclude that photolysis of 2-azidobiphenyl at room temperature leads rapidly to the triplet nitrene, and that this species is the precursor to carbazole [25], However, this point of view clearly is at odds with Swenton s triplet sensitization experiments [23],... 

The possibility of solid-state NMR spectroscopy to perform the analysis of hydrocarbons in the adsorbed states on zeolites at room and higher temperatures and directly in the course of the reaction is the basis for application of this method for reaction characterization occurring on zeolites and other solid catalysts. Solid-state NMR provides in situ monitoring of the reaction proceeding identification of the reaction intermediates, analysis of the reaction kinetics. This allows establishing the mechanisms of the reaction as well as the pathways of hydrocarbon activation. Here we provide some examples to demonstrate the approaches that are used in NMR studies to identify the nature of the intermediate on zeolite surface, to follow the kinetics of the reaction and establishing the reaction mechanisms. 

Because the characterization of support-bound intermediates is difficult (see below), solid-phase reactions are most conveniently monitored by cleaving the intermediates from the support and analyzing them in solution. Depending on the loading, 5-20 mg of support will usually deliver sufficient material for analysis by HPLC, LC-MS, and NMR, and enable assessment of the outcome of a reaction. Analytical tools that are particularly well suited for the rapid analysis of small samples resulting from solid-phase synthesis include MALDI-TOF MS [3-5], ion-spray MS [6-8], and tandem MS [9]. MALDI-TOF MS can even be used to analyze the product cleaved from a single bead [5], and is therefore well suited to the identification of products synthesized by the mix-and-split method (Section 1.2). The analysis and quantification of small amounts of product can be further facilitated by using supports with two linkers, which enable either release of the desired product or release of the product covalently bound to a dye [10-13], to an isotopic label [11], or to a sensitizer for mass spectrometry [6,14,15] (e.g., product-linker-dye- analytical linker -Pol). 

In industrial processes starting materials, intermediates and end products are regularly analysed, in which GC/FT-IR may have its due share. For example, solvents are recycled after distillation or other purification steps. The purity of solvents is checked before recycling by gas chromatography, where eventual impurities, once known, are identified on the basis of retention times. In the case of unknown impurities GC/FT-IR or GC/MS methods must be used for identification, of which the former is particularly useful if aromatic isomers are to be distinguished. Another important area is monitoring reactions like catalytic oxidation or polymerization. 

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