Question

1. Address the acid/base nature of luminol and its intermediate, phthalhydrzide.

2. When testing light emission with luminol, why do we perform it in basic solutions?

Answer 1

Luminol (C₈H₇N₃O₂) is a chemical that exhibits chemiluminescence, with a blue glow, when mixed with an appropriate oxidizing agent. Luminol is a white-to-pale-yellow crystalline solid that is soluble in most polar organic solvents, but insoluble in water.

Since Luminol consist amino group it is having basic nature.

Luminol may be synthesized by two-step process. It begins from 3-nitrophthalic acid^, First, hydrazine (N₂H₄) is heated with the 3-nitrophthalic acid in a high-boiling solvent such as triethylene glycol. An acyl substitution condensation reaction occurs, with loss of water, forming 3-nitrophthalhydrazide. Reduction of the nitro group to an amino group with sodium dithionite (Na₂S₂O₄), via a transient hydroxylamine intermediate, produces luminol.

To exhibit its luminescence, the luminol must be activated with an oxidant. Usually, a solution containing hydrogen peroxide (H₂O₂) and hydroxide ions in water is the activator. In the presence of a catalyst such as an iron or periodate compound, the hydrogen peroxide decomposes to form oxygen and water:

2 H₂O₂ ? O₂ + 2 H₂O

Luminol reacts with the hydroxide ion, forming a dianion. The oxygen produced from the hydrogen peroxide then reacts with the luminol dianion. The product of this reaction—an unstable organic peroxide—is made by the loss of a nitrogen molecule, the change of electrons from excited state to ground state, and the emission of energy as a photon. This emission produces the blue glow.

Performance will be done in basic solutions. Because the base which abstarcts proton from NH to generate dianion. Hence here Basic solution is must to abstract proton from Luminol.