Differences in the reactivity of phthalic hydrazide and luminol with hydroxyl radicals

The reactivity of 5-amino-2,3-dihydro-phthalazine-1,4-dione (luminol) and p hthalic hydrazide with hydroxyl radicals was studied. HO.-radicals were gen erated by the Fenton reaction as well as by water radiolysis. Both luminol and phthalic hydrazide react with hydroxyl radicals under intense chemilumi nescence (CL) emission. However, exclusively the CL arising from phthalic h ydrazide oxidation can be quenched by competition (e.g. by the addition of carbohydrates), whereas luminol CL is enhanced. The reactivities of both compounds with HO. radicals were further studied b y time-resolved spectroscopy (pulse radiolysis), competition methods, NMR s pectroscopy and mass spectrometry. Whereas only slight differences were det ectable by pulse radiolysis, the analysis of competition kinetics in the pr esence of p-nitroso-dimethylaniline (NDMA) gave a two-fold enhanced reactiv ity for luminol (4.8 x 10(9) l mol(-1) s(-1)) in comparison to phthalic hyd razide (2.0 x 10(9) l mol(-1) s(-1)). NMR and mass spectrometric analyses revealed significant differences in the reactivity of HO.-radicals: whereas in luminol solutions hydroxylation of the aromatic ring system predominated, hydroxylated products were not detec table upon irradiation of phthalic hydrazide. A hypothetical mechanism is p roposed which may explain the observed differences.