LUMINOL'S SYNYHESIS[12, 13]
The first step in the preparation of luminol involves the reaction of 3-nitrophthalic acid, 1, with hydrazine, 2. This reaction is most conveniently carried out in the high-boiling solvent triethylene glycol, which is added to an aqueous solution of the hydrazine salt.
The excess water is then boiled off, and the temperature is raised to the point where the dehydration has gone to completion within a few minutes.
The nitro- group of the resulting compound, nitrophthalhydrazide, 3, is then reduced to an amine group using sodium dithionite to form luminol, 4. Compound 3 is soluble in alkali due to enolization, thus the reduction is carried out in basic solution.
In neutral solution, luminol is zwitterionic:
When dissolved in a basic solution, luminol becomes dianionic: