Luminoll - lab report PDF

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Chemiluminescence of Luminol – Report Written by: Katie Banas Reference: Experimental Organic Chemistry - A Miniscale and Microscale Approach, Sixth Edition, Gilbert and Martin Discussion The purpose of this experiment was to synthesize luminol and demonstrate its chemiluminesent characteristic. Before synthesis can occur, the process by which it will take place has to be determined first. Fischer Esterification cannot be used in its production because the reagent 3-nitrophthalic acid is a strong base, due to its amine group, and will transform into an ammonium salt, which reacts with alcohol and thereby destroying the nucleophile. Luminol is a unique compound, so it requires unique conditions to be produced. Specifically, its activation energy is very high making its initiation difficult to accommodate. Because the luminol reaction requires the system to occur at 230 degrees Celsius, a normal solvent cannot withstand such high temperatures, so another mechanism had to be found. One possible solution is pressurizing the system, but it is quite dangerous, rendering it an unsuitable solution for a teaching lab. However, there was a more favorable solution, which was to use a solvent with a high boiling point. Initially, 3-nitrophthalic acid was added to hydrazine and were then subsequently heated on a flameless hot plate, creating a creamy white solution. Following the addition of ethylene glycol to the solution, the color of the solution changed to a dull yellow. This solvent was chosen for the experiment because of its high boiling point. As the solution continued to heat, the color grew increasingly brown while the consistency became thicker. Some boiling water was added to the solution and then was vacuum filtered out to isolate the crude precipitate, nitrohydrazide. Rather than drying it, the crude was immediately returned to a beaker containing aqueous sodium hydroxide, which decreased the intensity of the solution’s color to a beige. Once the precipitate had been thoroughly mixed into dissolution, sodium hydrosulfite was added to the solution. The mixture was brought to just below a boil and held there for five minutes. The beaker was placed in a cold bath after the addition of acetic acid. The solution underwent vacuum filtration to isolate the luminol precipitate. Yet again, the unique characteristics of luminol inhibit normal product analysis. The melting point of luminol exceeded the maximum temperature the provided melting point

machines were able to read. Another solution was needed to confirm the true identity of the final precipitate. The product was added to a potassium ferricyanide solution. If the precipitate formed was in fact luminol, the iron from the compound would oxidize the luminol, causing it to glow. This is known as chemiluminescene, which is when glowing light is a product of a reaction. Specifically, photons are produced, which are responsible for the glowing effect. The oxidation reaction provides energy that the electrons absorb, moving them to an excited state. When an electron releases that energy on its return to its ground state, a photon is produced. This phenomenon is referred to as fluorescence. These electrons make a fast, direct return to the ground state from their excited states. This uninterrupted route classifies these electrons as being in a “singlet” state. However, the electrons can make a detour on their return trips. This detour is called an inner system crossing, in which an electron can travel from the singlet state to a triplet state before making its way back to their ground state. This transition makes the electron flip its spin, which requires energy. This event illustrates Hund’s Rule that the triplet state is of a lower energy than that of the singlet state. It is important to note that an electron that switched to the triplet state must reverse its spin to its original rotation before a photon is emitted and return to the ground state. This particular process is known as phosphorescence. To identify which process happened, fluorenscence or phosphorescence, two possibilities are available. The first option is the use of electron resonance spintroscopy, which is unreasonable to use in an undergraduate lab. The second, much simpler possibility is to look at the color that the photon emits. If the glowing color is red, phosphorescence was the electron process chosen because it’s at a lower energy state. Conversely, the high energy state that occurs during fluorenscence emits a blue light. While the luminol electrons begin in the triplet state, they migrated to the singlet state before returning to the ground state, making a blue glowing color.

Conclusion This experiment was designed to perform a luminol synthesis and observe its chemiluminescent qualities. In this experiment, 3-nitrophthalic acid, hydrazine, ethylene glycol, boiling water, sodium hydroxide, sodium hydrosulfite, and acetic acid were added

together in various steps to synthesize luminol. A unique approach was required during this lab because of several factors that eliminated the use of typical reaction conditions. Primarily, the highly elevated temperature at which the luminol reaction occurs required a different technique to be utilized. Specifically, the use of a solvent with a very high boiling point. In addition to causing problems with the initiation of the reaction, the temperature at which luminol melts exceeded the range of the melting point machines provided in the laboratory. Rather than using electron resonance spintroscopy, the identity of the final product was determined by the glowing color emitted after it had been added to a potassium ferricyanide solution. The mixture glowed a blue light, therefore confirming that luminol was synthesized. The luminol synthesis began with 0.202 grams of 3-nitrophthalic acid. After the addition of boiling water, the solution was vacuum filtered. The crude product collected weighed 0.684 grams. Since the precipitate was not dried before moving forward with the experiment, some product was lost when it was transferred back to a conical vial. Aqueous sodium hydroxide and 0.607 grams of fresh sodium hydrosulfite were then added to the product. Once the acetic acid had been added, the solution was vacuum filtered again. The final product was 0.326 grams. The percent yield was calculated by dividing the mass of the final sample by the mass of the crude sample and then multiplying by 100%. The percent yield was found to be 47.66%. This yield could have increased had the crude sample been dried first because a visible amount remained on the paper that was unsalvagable. Overall, this experiment fulfilled its objective of demonstrating the proper preparation of luminol and its chemiluminescent abilities. Since the addition of the final precipitate to the potassium ferricynaide produced a bluish glow confirming the identity as luminol, the experiment was a success....