Mechanism of the Reaction

A sensible mechanism of the reaction is as follows. Base removes the nitrogen protons leaving a negative charge which moves onto the carbonyl oxygen to form what is known as an enolate (a charged version of the keto-enol tautomerisation you should be familiar with). 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.

This sequence of events has been confirmed by using radiolabelled ¹⁸O in the oxygen source which was incorporated into the product as predicted.

e.g. in sodium hydroxide,

Luminol + 2NaOH + O₂ N₂ + Na₂APA + hv

White et al have shown that the chemiluminescence of luminol has an emission spectra with two peaks indicating two similar species that emit light. This has been attributed to a 3-APA* hydrogen bonded to water or protonated fully which emits at 424 nm. The other peak arises from a 3-APA* not bonded in this way (485 nm).

The same team have also shown, through comparison with other known fluorescence and phosphorescence reactions that the chemiluminescence of luminol proceeds from a singlet state like fluorescence. The strength of emission has been shown to directly correlate with the addition of electron donating substituents (e.g. CH₃O-) on the benzene ring.

Molecular oxygen is in a triplet state (to find out why follow this Link). It has been suggested that adducts of oxygen exist as triplet states. Loss of nitrogen leads to a vibrationally excited triplet state of 3-APA which undergoes intersystem crossing to form a singlet state from which it can emit a photon.

Note that if you are using fluorescers then this changes the reaction kinetics drastically as two main types of reaction occur and the introduction of third bodies complicate matters somewhat (in this case there are more than ten rate constants to deal with! This paper discusses the mechanism of “photosenitised” chemiluminescence of luminol but the conclusions reached are well beyond the scope of any A-level project.

An example of the colourful reactions possible with fluorescers is shown here.