catalisi

[Catalysis] [Spectrochemical Analysis] [Reaction's Movie]

LUMINOL'S REACTION [10, 11]

The luminol's chemiluminescence manifests with emission of light when it comes oxidized to 3-aminoftalic acid with H2O2. The basic reaction is given below.

Mechanism of the Reaction

STEP 1
3-aminophthalhydrazide (luminol)

3-aminophthalhydrazide combines with a basic solution, which removes the hydrogen protons from the nitro groups to release two water molecules. A catalyst decomposes the hydrogen peroxide (or other oxidizing molecule).


STEP 2
diazachinon

The resulting molecule undergoes keto-enol tautomerization to move the negative formal charges onto the carbonyl oxygens.


STEP 3
3-APA (3-amino-phthalate)

The oxygen next performs a cyclic addition to the two (previously) carbonyl carbons. Nitrogen is an excellent leaving group because its own bonds are so strong (and as a gas, it is entropically favoured too) so the charge on the oxygens come back down to form carboxylate anions by expelling nitrogen gas. This leaves 3-APA*.
An important point to take note of is the formation of the cyclic peroxide which converts directly to 3-APA*. Chemiluminescent reactions often involve the cleavage of an organic peroxide because this bond is particularly weak and there is much energy to be gained by cleavage and subsequent reorganisation of bonds. This works with the leaving group effect of the nitrogen (above) to form a particularly efficient reaction.


[Catalysis] [Spectrochemical Analysis] [Reaction's Movie]