Synthesis of Luminol

Ordinarily, when reactants change to products, the difference in their energy is known as the enthalpy of the reaction, which can be either exothermic, where heat is released, or endothermic, where heat is absorbed.

In a chemiluminescent reaction, the reactants form an intermediate, which is in an electronically excited state. This intermediate is unstable. As it relaxes down to the ground state, the electrons in the intermediate releases energy in the emission of visible light rather than in the form of heat.

This phenomenon is similar to fluorescence. Recall that the electrons in a molecule occupy the ground state. When they absorb energy, such as when exposed to a certain wavelength of light, they become excited and transition to a higher, excited energy level. As the excited state relaxes back down to the ground state, the excess energy is emitted in the form of visible light.

However, it is important to distinguish between the two. The difference is how the electrons of the molecules are excited. In chemiluminescence, energy generated from a chemical reaction excites the electrons…whereas in fluorescence, the electrons are excited by direct absorption of light energy or other electromagnetic radiation. As the electrons relax down to ground state, both types of luminescence release the energy as emitted light.

Let’s take a look at an example of each. When you shine UV light on a bottle of tonic water, the electrons in quinine absorb the energy and are excited to a higher energy level. When the electrons relax, they release energy in the form of blue light.

Now, consider a glow stick, which has an outer tube containing diphenyl oxalate and a dye, and an inner tube containing hydrogen peroxide. When you bend the glow stick, the inner tube breaks, allowing the reactants to mix. The hydrogen peroxide oxidizes the diphenyl oxalate to form phenol and cyclic peroxide. The cyclic peroxide decomposes to carbon dioxide, releasing energy that excites the dye to a higher energy level. As the dye relaxes to the ground state, a photon of light is released. Here, the process is chemiluminescence because the energy provided to excite the dye comes from a chemical reaction.

In this lab, you will synthesize 3-aminophthalhydrazide, also known as luminol, by performing a series of reactions. You will then react the luminol with dimethyl sulfoxide over solid potassium hydroxide to observe its chemiluminescence.