Photochemical Biradical Formation Investigated in Micro Reactors

3 Photochemical Biradical Formation Investigated in Micro Reactors Organic synthesis 87 [OS 87] Benzopinacol formation by two-radical combination 

The pinacol formation reaction follows a radical mechanism. Benzopinacol, benzophenone and the mixed pinacol are formed jointly with many radical species [72, 74]. In the course of the reaction, first a high-energy excited state is generated with the aid of photons. Thereafter, this excited-state species reacts with a solvent molecule 2-propanol to give two respective radicals. The 2-propanol radical reacts with one molecule of benzophenone (in the ground state, without photon aid) to lengthen the radical chain. By combination of radicals, adducts are formed, including the desired product benzopinacol. Chain termination reactions quench the radicals by other paths. 

The reactor was first primed with a cleaning solution, then with the reacting solution, and fed by pumping for a longer period [72, 74]. Then, the liquid flow was set to 1 ml min The samples were analyzed typically after 48 h to ensure completion of dark follow-up reactions. 

Besides on-line UV analysis, off-line HPLC analysis was performed after about 48 h dark reaction [72, 74]. In this period, the photoinduced reaction proceeds by radical paths in the dark. 

Owing to the high concentration applied (0.5 M), the Lambert-Beer law is no longer valid [72, 74]. Therefore, absorption coefficients were derived experimentally as described in detail in [72]. These measurements took advantage of having the thin layers in the micro reactor, i.e. they would not have been feasible in conventional cuvettes. 

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