Acidified methanol, proton

The UV irradiation of quinoxaline in methanol yields radicals, not by hydrogen abstraction, but by protonation of the first singlet excited state, followed by exiplex formation.82 Irradiation of quinoxaline in acidified methanol furnishes 2-methylquinoxaline, and the reaction is suggested to go through a pathway involving electron-transfer from the solvent to an excited state of the protonated quinoxaline (Scheme 3).83... 

The influence of acid on these additions is considerable. In ethanol, for example, ethyl nicotinate is converted into the corresponding 6a-hydroxyethylpyridine, whereas in acidified solution the 6-ethylpyridine derivative is obtained.147 The function of the acid may, of course, simply be to promote dehydration of the intermediate 6a-hydroxyethyldihydropyridine. Phthalazine and quinoxaline are converted by irradiation in acidified methanol into 1-methylphthalazine and 2-methylquinoxaline, respectively.148 In this case, however, there is evidence to suggest that the alkylations proceed by way of electron transfer from the solvent to an excited state of the protonated diazines in neutral solution, a hydrogen... 

This also reflects the fact that proton transfer in a solvent such as methanol involves the species CH3OH2 However, in the alcohols there is only one site for hydrogen bond formation, whereas the water molecule has two. Thus orientational effects are more important in acidified alcohol systems, so that the enhancement of proton mobility with respect to other monovalent cations is not as great as it is in water. 

Possible side-reactions, such as the formation of allyl hydroperoxides from compounds having an allylic hydrogen (a competitive ene reaction Section 6.7.3) or 1,4-endoperoxides from 1,3-dienes, may be suppressed by changing the experimental conditions.1359,1438 For example, the extent of [2 + 2] and [4 + 2] photooxygenation in styrenes, such as the propenylanisole 522 (Scheme 6.253), is controlled by solvent polarity and pH, possibly due to protonation of a perepoxide/zwitterion intermediate.1421,1439 The [4 + 2] product 523 is preferentially produced in non-polar benzene or chloroform, whereas the 1,2-dioxetane 524 is almost exclusively formed in methanol or acidified non-polar solvents. 

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