Luminol Synthesis and Chemiluminescence

Purpose of the Lab

To synthesize luminol and investigate the chemiluminescence reaction that luminol is known for

What does this lab demonstrate

It shows a multistep process of using a starting material, converting it to a product, then using the product as a starting material for a different reaction

What is luminol best known for its use in?

Crime scene investigations to detect blood

What is the active ingredient in blood that causes it to glow

The iron in the hemoglobin

Chemiluminescence can defined as?

- As the release of light without heat due to a chemical reaction

- It comes from the energy released from breaking, forming, or reconstructing of chemical bonds

Describe what happens in the first portion of the experiment

- Use 5-nitro-2,3- dihydrophthalazine- 1,4- dione as a the starting material

- The nitro functionality group will be reduced using sodium hydrosulfite with a mix of NaOH in water

- Reaction utilizes a single reduction pathway

- The result is an anime formation and will be used as the starting material to investigate the chemiluminescent properties of luminol

Describe what happens in the second portion of the experiment

- Crude limnol will react with NaOH, hydrogen peroxide (H2O2), potassium ferricyanide to cleave the dihydrophthalazine ring and form a phthalate salt along with the release of light

Hydrogen peroxide and the iron catalyst provide what

A molecular oxygen source for the reaction

(T/F): Ground state molecular oxygen (O2) exists in a triplet stage, meaning two unpaired electrons of the same spin

True

What happens when a triplet oxygen is added to luminol by a complex mechanism catalyzed by a change in iron oxidation states

- Phthalate salt is forced into a T1 (triplet excited, excited spin-unpaired state) stage due to the addition

- Product molecule, 5-aminophthalate salt, converts to S1 excited state (excited spin-paired state)

- Electronic state relaxes to the S0 ground state

How does T1 convert to S1

Due to intersystem crossing

what happens when the reaction reaches ground state (S0)

It releases a photon which is seen as blue light

When first starting the lab, when adding NaOH to the starting material what color did it change into?

deep red/brown color

By adding this chemical it allowed the solid to precipitate into luminol after it was vacuumed

Acetic acid

During the chemiluminescence experiment how many solutions were made

Four

What is the compound being oxidized in the luminol synthesis reaction?

The compound that is being oxidized is: sodium hydrosulfite since it's the reducing reagent in the reaction

What is the purpose of acetic acid in the luminol synthesis reaction? How would the amount of the luminol product obtained at the end of this reaction be affected if the number of moles of acetic acid added were less than the moles of NaOH added at the beginning of the experiment?

The purpose of the acetic acid is to protonate the luminol in order for it to precipitate out of the aqueous layer. If the number of moles of acetic acid added were less than the moles of NaOH, then less luminol would be precipitated out. Since sodium hydroxide is used to deprotonate the luminol in order for it to enter the aqueous layer, where the reaction occurs. There would also be less product for the acid to protonate, if deprotonation is limited in moles.

In the luminol synthesis reaction, why was COLD water used for rinsing the filtration step? How would the efficiency of the filtration be affected if HOT water was used instead?

The reason cold water was used for rinsing and filter the precipitate was due to the fact that it inhibits the solids from dissolving back into the solution. If hot water were to be used instead, some of the luminol produced would dissolve and the product would be lost.

What is the difference between chemiluminescence and fluorescence? Where does the ENERGY for light emission come from in each case?

The difference between fluorescence and chemiluminescence, is that fluorescence gets its energy from external uses while chemiluminescence obtains its energy through means of chemical reaction. Fluorescence's energy for light emission comes from absorbing light from another source. Chemiluminescence receives its energy for light emission through chemical that causes a molecule to become excited and then return back to its ground state, releasing energy.

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

The reason the reaction does not progress from via a Diels-Alder reaction with molecular oxygen (O2), is due to the fact that oxygen is presented as a diradical in a triplet state. In a Diels-Alder reaction it requires an oxygen double bond in a singlet state. The ground state of oxygen exists as diradical, because there are not enough electrons on oxygen to make 2 sigma bonds. Thus, it affects the possibility of a Diels-Alder mechanism with oxygen to occur.

When isolating the solid through precipitation and vacuum filtration, why was the solid not rinsed with a solvent like ethyl acetate or dichloromethane?

Since dichloromethane and ethyl acetate are both polar, thus like would dissolve like. Meaning it decrease the product yield and dissolve the luminol that was created for precipitation and filtration.