gem-Dinitro and gem-nitronitroso dianion radicals formed in the reaction between sodium nitrite–ascorbate and carbonyl compounds such as aldehydes, alicyclic ketones, lactams and peptides. An EPR study

Abstract

gem-Dinitro and gem-nitronitroso radical dianion R–C˙(NO₂)₂^2– or R–C˙(NO₂)(NO)^2–(R = alkyl, aryl) are formed in the reaction between sodium nitrite and carbonyl compounds such as acetaldehyde, benzaldehyde, furfural, cyclopentanone, cyclohexanone, lactams, some dipeptides or insulin dissolved in aqueous acid containing a small amount of ascorbic acid. The radicals appeared in the dark at room temperature when the reaction mixtures had been made alkaline. The EPR spectra indicated an interaction of the unpaired electron with two nitrogen nuclei, and also with either a hydrogen or a further nitrogen nucleus in the parent substances. The two nitrogens were non-equivalent in the radicals derived from substances such as acetaldehyde or glycylglycine (nine-line system) owing to the formation of R–C˙(NO₂)(NO)^2–, but equivalent in the radicals from substances such as cyclopentanone (five-line system) owing to the formation of R–C˙(NO₂)₂^2–. The reaction involves nitration/nitrosation of the carbonyl carbon in acid solution with cleavage of C–C bonds or dissociation of a hydrogen ion leading to intermediate nitronitroso or dinitro compounds which are reduced by ascorbic acid to give the observed radicals when the reaction mixtures are made alkaline.