2-Pentanone, photoelimination

Fig. 6. Stern-Volmer plot of the quenching of type II photoelimination of 2-pentanone and 2-hexanone.

Total quantum yields in the absence of quencher for 2-pentanone and 2-hexanone are 0.20 and 0.25, respectively.326 Subtraction of the quantum yields for singlet reaction from the total quantum yields furnishes values of triplet quantum yields. Stern-Volmer plots of reciprocals of the triplet quantum yields versus piperylene concentration indicate that triplet 2-hexanone is five times as reactive as triplet 2-pentanone.273 Rough measurements have been made with methoxyacetone which suggest that at least 60% of its total photoelimination occurs from an unquenchable singlet state.273... 

The only type of triplet reaction for which rate constants have been measured for various kinds of ketones is type II photoelimination. The rate constants for 2-pentanone and 2-hexanone have been found to be 2 x I08 and 1 x 10 sec-1, respectively.215 The corresponding rates for triplet butyrophenone and valerophenone are 8 x 10 and 1.4 x 10s sec-1, appreciably slower. Neither p-phenylbutyrophenone366 nor a-valeronaphthone273 undergo photoelimination in a readily measurable quantum yield. These results can be rationalized by a reasonable set of hypotheses such as follow. 

Interestingly, essentially all the quenching by biacetyl in the case of 2-pentanone has been attributed to excitation transfer from excited singlet states of the donor. Since 2-pentanone possesses 7-hydrogens, one of its primary photoreactions is the photoelimination shown in Eq. (46). 

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