PHOTOCHEMISTRY: PHOTO- REDUCTION

The excited triplet states of carbonyl compounds abstract hydrogen atom from the reacting substrate and forms free radicals which then undergo coupling disproportionation to yield the products. The photo-reduction of benzophenone may be cited as a example. This reduction has been extensively studied. Normally this reduction is carried out by taking the solutions of benzophenone in isopropyl alcohol and irradiating with a radiation of 345nm. Isopropyl alcohol is inert at this radiation but n→π* transition occurs in benzophenone. S₁ state of benzophenone drops to T₁ which abstracts hydrogen atom from isopropyl alcohol as indicated below to form ketyl radicals. 
It may be mentioned that the hydrogen abstration reaction takes place in the n→π* state. For this reason ketones doesn't undergo photo-reduction on irradiation in isopropanol because conjugation of lone pair of electrons on nitrogen atom with the aromatic ring lowers the energy required for π→π* transition occurs more easily than n→π* transition. 

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