Benzophenone mixed pinacol

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. 

If the primary chemical process is still abstraction of a hydrogen atom to produce the benzophenone ketyl radical, one would expect to observe some mixed pinacol product arising from coupling of the benzophenone ketyl and acetone ketyl radicals,... 

A number of different mechanisms have been proposed to account for the fact that this product is not observed. Recently, however, a report appeared that described the formation of the mixed pinacol from the photoreduction of benzophenone with isopropyl alcohol and the photoreduction of acetone with benzhydrol. The data from this study are presented in Table 3.10.(73)... 

We saw earlier that when benzophenone is photoreduced in isopropyl alcohol only benzpinacol and acetone are produced although one would also expect to observe the formation of a mixed pinacol<73) ... 

The photoreduction of benzophenone in methanol and ethanol gives benzopinacol and a cross-coupled product.Moreover, Weiner reported that photoreduction of benzophenone in 2-propanol yields a pinacol-type compound and the mixed pinacol (QH5)2C(OH)C(OH)(CH3)2-5 These reactions occur by the free ketyl radicals. 

Table 3.10. Ratio of Mixed to Normal Pinacol in the Photoreduction of Benzophenone with Isopropyl Alcohol and Acetone with Benzhydrol...

---