Name: luminol chemiluminescence in forensics
Uploaded: 2020-03-26T07:22:29.000Z
Duration: 3 min 2 s
Channel: JoVE

Wenn sich Reaktanten in Produkte verwandeln, wird die Differenz in ihrer Energie normalerweise als Enthalpie der Reaktion bezeichnet, die entweder exotherm sein kann, bei der Wärme freigesetzt wird, oder endotherm, bei der Wärme absorbiert wird.
Bei einer Chemilumineszenzreaktion bilden die Reaktanten ein Zwischenprodukt, das sich in einem elektronisch angeregten Zustand befindet. Dieses Zwischenprodukt ist instabil. Beim Entspannen in den Grundzustand geben die Elektronen im Zwischenstadium Energie nicht in Form von Wärme, sondern in Form von sichtbarem Licht ab.
Dieses Phänomen ähnelt der Fluoreszenz. Erinnern wir uns daran, dass die Elektronen in einem Molekül den Grundzustand einnehmen. Wenn sie Energie absorbieren, z. B. wenn sie einer bestimmten Wellenlänge des Lichts ausgesetzt werden, werden sie angeregt und gehen auf ein höheres, angeregtes Energieniveau über. Wenn sich der angeregte Zustand wieder in den Grundzustand entspannt, wird die überschüssige Energie in Form von sichtbarem Licht abgegeben.
Es ist jedoch wichtig, zwischen den beiden zu unterscheiden. Der Unterschied besteht darin, wie die Elektronen der Moleküle angeregt werden. Bei der Chemilumineszenz werden die Elektronen durch die durch eine chemische Reaktion erzeugte Energie angeregt, während bei der Fluoreszenz die Elektronen durch direkte Absorption von Lichtenergie oder anderer elektromagnetischer Strahlung angeregt werden. Wenn sich die Elektronen in den Grundzustand entspannen, geben beide Arten der Lumineszenz die Energie als emittiertes Licht ab.
Werfen wir einen Blick auf ein Beispiel für beide. Wenn du eine Flasche Tonic Water mit UV-Licht bestrahlst, absorbieren die Elektronen im Chinin die Energie und werden auf ein höheres Energieniveau angeregt. Wenn sich die Elektronen entspannen, geben sie Energie in Form von blauem Licht ab.
Stellen Sie sich nun einen Leuchtstab vor, der ein äußeres Rohr mit Diphenyloxalat und einem Farbstoff und ein inneres Rohr mit Wasserstoffperoxid hat. Wenn Sie den Leuchtstab biegen, bricht das Innenrohr, wodurch sich die Reaktanten vermischen können. Das Wasserstoffperoxid oxidiert das Diphenyloxalat zu Phenol und cyclischem Peroxid. Das zyklische Peroxid zersetzt sich zu Kohlendioxid und setzt Energie frei, die den Farbstoff auf ein höheres Energieniveau anregt. Wenn sich der Farbstoff in den Grundzustand entspannt, wird ein Lichtphoton freigesetzt. Hier handelt es sich um Chemilumineszenz, da die Energie, die zur Anregung des Farbstoffs bereitgestellt wird, aus einer chemischen Reaktion stammt.
In diesem Labor synthetisieren Sie 3-Aminophthalhydrazid, auch bekannt als Luminol, indem Sie eine Reihe von Reaktionen durchführen. Sie werden dann das Luminol mit Dimethylsulfoxid über festem Kaliumhydroxid reagieren, um seine Chemilumineszenz zu beobachten.

synthese von luminol

Chemische Reaktionen können als endotherm oder exotherm klassifiziert werden, je nachdem, ob die Reaktionen beim Übergang zu Produkten Wärme absorbieren oder abgeben. Die Energiedifferenz zwischen Reaktanten und Produkten wird als Enthalpie der Reaktion bezeichnet. Dies ergibt sich aus der Berechnung der Energiedifferenz zwischen den Produkten und den Reaktanten oder ΔH. Ist ΔH positiv, ist die Reaktion endotherm; Ist sie negativ, ist die Reaktion exotherm. Während chemische Reaktionen traditionell als Umwandlung von Reaktanten in Produkte angesehen werden, laufen viele Reaktionen in mehreren Schritten ab und bilden Zwischenprodukte. Bei Chemilumineszenzreaktionen gehen diese Zwischenprodukte von einem Hochenergiezustand in den Grundzustand über und setzen ein Lichtphoton frei, das im Verlauf der Reaktion sichtbar sein kann.
<h4>Atomare Energie</h4>
Niels Bohr, ein dänischer Physiker, schlug die Theorie vor, dass ein Elektron, das den Atomkern umkreist, nur bestimmte Umlaufbahnen oder Energieniveaus einnehmen kann. Ein Atom, dessen gesamte Elektronen sich auf dem niedrigstmöglichen Energieniveau befinden, befindet sich in seinem Grundzustand. Wenn ein Elektron ein Energieniveau einnimmt, das höher ist als der Grundzustand, befindet sich das Atom in seinem angeregten Zustand.
Eine zentrale Annahme in Bohrs Theorie ist, dass ein Elektron so lange im Grundzustand verbleibt, bis es gestört wird. Ein Elektron wird also nur durch die Absorption von Energie in einen angeregten Zustand gehoben. Wenn sich das Elektron wieder in den Grundzustand entspannt, gibt es diese Energie wieder ab. Oft geschieht dies in Form eines Lichtphotons, dessen Wellenlänge in direktem Zusammenhang mit der Energiedifferenz zwischen dem angeregten und dem Grundzustand steht.
Wenn ein Elektron durch die Absorption von Licht einer bestimmten Energie oder Wellenlänge auf ein höheres Energieniveau angeregt wird, wird das Phänomen als Fluoreszenz bezeichnet. Dieser Effekt ist zu sehen, wenn Sie ein gewaschenes weißes T-Shirt Schwarzlicht oder UV-Licht aussetzen. Der charakteristische Glanz wird durch die Fluoreszenz des Waschmittels und der Bleichmittel im Hemd verursacht, die es bei normalem Licht weiß erscheinen lassen.
<h4>Chemilumineszenz</h4>
Bei einer Chemilumineszenzreaktion wird ein Elektron angeregt, indem es Wärme absorbiert, die während einer Reaktion freigesetzt wird. Dann wird Licht freigesetzt, wenn sich das Elektron wieder in den Grundzustand entspannt. Der Hauptunterschied zwischen Chemilumineszenz und Fluoreszenz besteht darin, dass die Energie, die die Elektronen bei der Chemilumineszenz anregt, direkt aus der Reaktion stammt.
Eine praktische Anwendung der Chemilumineszenz ist in kommerziellen Leuchtstäben. Ein Leuchtstab enthält zwei getrennte Lösungen: eine mit Peroxid und die andere mit Diphenyloxalat und einem Farbstoff. Wenn die beiden Lösungen gemischt werden, was bei der Aktivierung des Leuchtstabs geschieht, erzeugt die resultierende Reaktion zwischen dem Peroxid und dem Diphenyloxalat Energie, die den Farbstoff in einen höheren Energiezustand anregt. Wenn die Reaktanten erschöpft sind, kehrt der Farbstoff in den Grundzustand zurück und gibt Licht ab. Das ist es, was den Leuchtstäben ihre charakteristische Farbe verleiht.
<h4>Luminol</h4>
Luminol ist eine Chemikalie, die chemilumineszierende Eigenschaften aufweist und in einer Vielzahl von Anwendungen eingesetzt wird, vor allem in der Forensik. Luminol (C8H7N3O2) emittiert blau, wenn es mit einem Oxidationsmittel gemischt wird. In der Forensik reagiert Luminol mit dem Eisen im Hämoglobin, wodurch Forensiker sehr kleine Blutspuren identifizieren können.
Luminol wird durch die Dehydratisierungsreaktion von 3-Nitrophthalsäure mit Hydrazin synthetisiert. Die Reaktion wird erhitzt, um Wasser zu entfernen, und Triethylenglykol wird hinzugefügt, um die Temperatur weiter zu erhöhen. Die Nitrogruppe des 3-Nitrophthalhydrazids wird dann mit Natriumdithionit reduziert, um bei hohem pH-Wert eine Aminogruppe zu bilden. Unter basischen Bedingungen ist das 3-Nitrophthalhydrazid löslich. Die Zugabe von Eisessig fällt das Luminol aus.
Mit Kaliumhydroxid bildet Luminol ein Dianion. Die Hydroxid-Anionen deprotonieren die beiden Wasserstoffatome, die an die Stickstoffe im Luminol gebunden sind. Sauerstoffgas oxidiert Luminol in seinen angeregten Zustand. Wenn es sich wieder in den Grundzustand entspannt, gibt es ein blau-weißes Licht ab.
<h4>Referenzen</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr., P.M., Townsend, J.R. (2015). Chemie und chemische Reaktivität. Belmont, Kalifornien: Brooks/Cole, Cengage Lernen.</li>
</ol>

Synthese von Luminol

Chemische Reaktionen können als endotherm oder exotherm klassifiziert werden, je nachdem, ob die Reaktionen beim Übergang zu Produkten Wärme absorbieren oder abgeben. ...

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Synthesis of Luminol

luminol chemiluminescence in forensics

Luminol chemiluminescence shows how a chemical reaction can produce light and help with blood detection. Chemical reactions may be endothermic or exothermic, depending on whether they absorb or release heat as reactants become products. The enthalpy of a reaction is the energy difference between reactants and products, or ΔH. A positive ΔH means the reaction is endothermic. A negative ΔH means it is exothermic.

Many reactions do not happen in a single step. They can pass through intermediates along the way. In chemiluminescent reactions, these intermediates move from a high-energy state to the ground state and release a photon of light. That light can be seen as the reaction continues.

Atomic energy levels help explain why this glow happens. Niels Bohr proposed that electrons can occupy only certain orbits, or energy levels, around the nucleus. An atom is in the ground state when its electrons are in the lowest possible energy level. When an electron moves to a higher level, the atom is in an excited state. The electron must absorb energy to move up, and it releases that energy when it falls back down. This release is often a photon of light, and the light wavelength depends on the energy gap. When light causes this kind of excitation, the process is called fluorescence, such as the glow seen under a black light on a white T-shirt washed with detergent and bleach compounds.

Chemiluminescence is related to fluorescence, but the energy comes from the chemical reaction itself. In glow sticks, one solution contains peroxide and the other contains diphenyl oxalate and a color dye. When the solutions mix, the reaction produces energy that excites the dye. As the reactants are used up, the dye returns to the ground state and gives off light. This is why glow sticks have their characteristic color.

Luminol is a chemiluminescent chemical with an important forensic use. It emits blue light when mixed with an oxidizing agent. In forensic science, luminol reacts with the iron in hemoglobin, which helps detect very small traces of blood. Luminol is made by dehydrating 3-nitrophthalic acid with hydrazine. The reaction is heated to remove water, and triethylene glycol is added to raise the temperature further. Next, the nitro group in 3-nitrophthalhydrazide is reduced with sodium dithionite to form an amino group at high pH. The compound is soluble in basic conditions, and glacial acetic acid is added to precipitate the luminol. With potassium hydroxide, luminol forms a dianion because hydroxide ions remove the two hydrogens attached to the nitrogens. Oxygen gas then oxidizes luminol to its excited state. When it relaxes back to the ground state, it gives off a blue-white light.

Luminol Chemiluminescence in Forensics

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synthesis of luminol

Luminol chemiluminescence shows how a chemical reaction can produce light and help with blood detection. Chemical reactions may be endothermic or exothermic, depending on whether they absorb or release heat as reactants become products. The enthalpy of a reaction is the energy difference between reactants and products, or ΔH. A positive ΔH means the reaction is endothermic. A negative ΔH means it is exothermic. Many reactions do not happen in a single step. They can pass through intermediates...

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Luminol Chemiluminescence in Forensics

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