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

Normalmente, cuando los reactivos se convierten en productos, la diferencia en su energía se conoce como entalpía de la reacción, que puede ser exotérmica, donde se libera calor, o endotérmica, donde se absorbe el calor.
En una reacción quimioluminiscente, los reactivos forman un intermediario, que se encuentra en un estado excitado electrónicamente. Este intermediario es inestable. A medida que se relaja hasta el estado fundamental, los electrones en el intermedio liberan energía en la emisión de luz visible en lugar de en forma de calor.
Este fenómeno es similar a la fluorescencia. Recordemos que los electrones de una molécula ocupan el estado fundamental. Cuando absorben energía, como cuando se exponen a una cierta longitud de onda de luz, se excitan y pasan a un nivel de energía excitado más alto. A medida que el estado excitado se relaja hasta el estado fundamental, el exceso de energía se emite en forma de luz visible.
Sin embargo, es importante distinguir entre los dos. La diferencia es cómo se excitan los electrones de las moléculas. En la quimioluminiscencia, la energía generada a partir de una reacción química excita los electrones, mientras que en la fluorescencia, los electrones se excitan por absorción directa de energía luminosa u otra radiación electromagnética. A medida que los electrones se relajan hasta el estado fundamental, ambos tipos de luminiscencia liberan la energía en forma de luz emitida.
Echemos un vistazo a un ejemplo de cada uno. Cuando iluminas una botella de agua tónica con luz ultravioleta, los electrones de la quinina absorben la energía y se excitan a un nivel de energía más alto. Cuando los electrones se relajan, liberan energía en forma de luz azul.
Ahora, considere una barra luminosa, que tiene un tubo exterior que contiene oxalato de difenilo y un tinte, y un tubo interior que contiene peróxido de hidrógeno. Cuando doblas la barra luminosa, la cámara de aire se rompe, permitiendo que los reactivos se mezclen. El peróxido de hidrógeno oxida el oxalato de difenilo para formar fenol y peróxido cíclico. El peróxido cíclico se descompone en dióxido de carbono, liberando energía que excita el tinte a un nivel de energía más alto. A medida que el tinte se relaja hasta el estado fundamental, se libera un fotón de luz. Aquí, el proceso es quimioluminiscencia porque la energía proporcionada para excitar el tinte proviene de una reacción química.
En este laboratorio, sintetizará 3-aminoftalhidrazida, también conocida como luminol, mediante la realización de una serie de reacciones. A continuación, hará reaccionar el luminol con dimetilsulfóxido sobre hidróxido de potasio sólido para observar su quimioluminiscencia.

síntesis de luminol

Synthesis of Luminol

Las reacciones químicas se pueden clasificar como endotérmicas o exotérmicas dependiendo de si las reacciones absorben o liberan calor cuando se transforman en productos. La diferencia de energía entre los reactivos y los productos se conoce como entalpía de la reacción. Esto se encuentra calculando la diferencia de energías entre los productos y los reactivos, o ΔH. Si ΔH es positivo, la reacción es endotérmica; Si es negativo, la reacción es exotérmica. Si bien tradicionalmente se piensa en las reacciones químicas como una conversión de reactivos en productos, muchas reacciones tienen lugar en múltiples pasos, formando intermedios. En las reacciones quimioluminiscentes, estos intermediarios pasan de un estado de alta energía al estado fundamental, liberando un fotón de luz que puede ser visible a medida que avanza la reacción.
<h4>Niveles de energía atómica</h4>
Niels Bohr, un físico danés, propuso la teoría de que un electrón que orbita alrededor del núcleo del átomo solo puede ocupar ciertas órbitas o niveles de energía. Se dice que un átomo con todos sus electrones en el nivel de energía más bajo posible está en su estado fundamental. Cuando un electrón ocupa un nivel de energía superior al estado fundamental, el átomo se encuentra en su estado excitado.
Una suposición clave en la teoría de Bohr es que un electrón permanece en el estado fundamental hasta que es perturbado. Por lo tanto, un electrón solo se eleva a un estado excitado mediante la absorción de energía. Cuando el electrón se relaja de nuevo al estado fundamental, libera esa energía. A menudo, esto es en forma de un fotón de luz, cuya longitud de onda está directamente relacionada con la diferencia de energía entre los estados excitado y fundamental.
Cuando un electrón se excita a un nivel de energía más alto al absorber luz de una cierta energía o longitud de onda, el fenómeno se llama fluorescencia. Este efecto se puede ver si expones una camiseta blanca lavada a una luz negra o a una luz ultravioleta. El brillo característico es causado por la fluorescencia del detergente y los compuestos blanqueadores en la camisa, que la hacen lucir blanca bajo la luz regular.
<h4>quimioluminiscencia</h4>
En una reacción quimioluminiscente, un electrón se excita absorbiendo el calor que se libera durante una reacción. Luego, la luz se libera cuando el electrón se relaja de nuevo al estado fundamental. La principal diferencia entre la quimioluminiscencia y la fluorescencia es que la energía que excita los electrones en la quimioluminiscencia proviene directamente de la reacción.
Una aplicación práctica de la quimioluminiscencia es en las barras luminosas comerciales. Una barra luminosa contiene dos soluciones separadas: una que contiene peróxido y la otra que contiene oxalato de difenilo y un tinte de color. Cuando se mezclan las dos soluciones, lo que ocurre al activar la barra luminosa, la reacción resultante entre el peróxido y el oxalato de difenilo produce energía que excita el tinte a un estado de energía más alto. A medida que se agotan los reactivos, el tinte vuelve al estado fundamental y libera luz. Esto es lo que le da a las barras luminosas su color característico.
<h4>Luminol</h4>
El luminol es un producto químico que exhibe propiedades quimioluminiscentes y se utiliza en una amplia gama de aplicaciones, sobre todo en medicina forense. El luminol (C, 8, H,7, N,3, O, 2) emite azul cuando se mezcla con un agente oxidante. En el caso de la medicina forense, el luminol reacciona con el hierro de la hemoglobina, lo que permite a los científicos forenses identificar rastros muy pequeños de sangre.
El luminol se sintetiza mediante la reacción de deshidratación del ácido 3-nitroftálico con la hidracina. La reacción se calienta para eliminar el agua y se agrega trietilenglicol para aumentar aún más la temperatura. A continuación, el grupo nitro de la 3-nitroftalhidrazida se reduce utilizando ditionito de sodio para formar un grupo amino a pH alto. En condiciones básicas, la 3-nitroftalhidrazida es soluble. La adición de ácido acético glacial precipita el luminol.
Con el hidróxido de potasio, el luminol forma un dianión. Los aniones hidróxido desprotonan los dos hidrógenos que están unidos a los nitrógenos en el luminol. El gas oxígeno oxida el luminol a su estado excitado. A medida que se relaja y vuelve al estado fundamental, libera una luz azul y blanca.
<h4>Referencias</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2015). Química y reactividad química. Belmont, CA: Brooks/Cole, Cengage Learning.</li>
</ol>

Síntesis de luminol

Las reacciones químicas se pueden clasificar como endotérmicas o exotérmicas dependiendo de si las reacciones absorben o liberan calor cuando se transforman en productos. ...

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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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Luminol Chemiluminescence in Forensics

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