Question

In the chemiluminesence reaction, a diene is generated on the luminol molecule. Why does the reaction not progress from that point forward as a Diels-Alder reaction with molecular oxygen acting as the dienophile? How does ground state molecular oxygen exist and does this affect the possibility of a Diels-Alder mechanism with O2?

Answer 1

The luminol produce diene in presence of a base accroding to following scheme.

Dioxygen (O₂) in its ground will be in triplet state with two unpaired electrons . To react with diene molecule the electrons in ground-state dioxygen need to be spin paired (to gnerate singlet oxygen), a process that requires energy. Generally this activation energy is relatively high ( around 23 kcal/mole), making the non-catalyzed reactions of dioxygen very slow at physiological temperatures. So, in the chemiluminesence reaction, the reaction does not progress (after diene formation) as a Diels-Alder reaction with O_2.

Singlet oxygen is an excited form in which both electrons are paired in the same LUMO. This displays double bond character between the oxygens. So this will readily participate in Diels-Alder reaction.