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

Normalmente, quando os reagentes se transformam em produtos, a diferença em sua energia é conhecida como entalpia da reação, que pode ser exotérmica, onde o calor é liberado, ou endotérmica, onde o calor é absorvido.
Em uma reação quimioluminescente, os reagentes formam um intermediário, que está em um estado eletronicamente excitado. Este intermediário é instável. À medida que relaxa até o estado fundamental, os elétrons no intermediário liberam energia na emissão de luz visível, e não na forma de calor.
Este fenômeno é semelhante à fluorescência. Lembre-se de que os elétrons em uma molécula ocupam o estado fundamental. Quando eles absorvem energia, como quando expostos a um certo comprimento de onda de luz, eles ficam excitados e fazem a transição para um nível de energia mais alto e excitado. À medida que o estado excitado relaxa de volta ao estado fundamental, o excesso de energia é emitido na forma de luz visível.
No entanto, é importante distinguir entre os dois. A diferença é como os elétrons das moléculas são excitados. Na quimioluminescência, a energia gerada a partir de uma reação química excita os elétrons ... enquanto na fluorescência, os elétrons são excitados pela absorção direta de energia luminosa ou outra radiação eletromagnética. À medida que os elétrons relaxam até o estado fundamental, ambos os tipos de luminescência liberam a energia como luz emitida.
Vamos dar uma olhada em um exemplo de cada um. Quando você ilumina uma garrafa de água tônica, os elétrons do quinino absorvem a energia e são excitados para um nível de energia mais alto. Quando os elétrons relaxam, eles liberam energia na forma de luz azul.
Agora, considere um bastão luminoso, que tem um tubo externo contendo oxalato de difenila e um corante, e um tubo interno contendo peróxido de hidrogênio. Quando você dobra o bastão luminoso, o tubo interno se quebra, permitindo que os reagentes se misturem. O peróxido de hidrogênio oxida o oxalato de difenila para formar fenol e peróxido cíclico. O peróxido cíclico se decompõe em dióxido de carbono, liberando energia que excita o corante para um nível de energia mais alto. À medida que o corante relaxa para o estado fundamental, um fóton de luz é liberado. Aqui, o processo é quimioluminescência porque a energia fornecida para excitar o corante vem de uma reação química.
Neste laboratório, você sintetizará a 3-aminoftalhidrazida, também conhecida como luminol, realizando uma série de reações. Você então reagirá o luminol com dimetilsulfóxido sobre hidróxido de potássio sólido para observar sua quimioluminescência.

síntese de luminol

Synthesis of Luminol

As reações químicas podem ser classificadas como endotérmicas ou exotérmicas, dependendo se as reações absorvem ou liberam calor quando mudam para produtos. A diferença de energia entre reagentes e produtos é conhecida como entalpia da reação. Isso é encontrado calculando a diferença de energias entre os produtos e os reagentes, ou ΔH. Se ΔH for positivo, a reação é endotérmica; se for negativo, a reação é exotérmica. Embora as reações químicas sejam tradicionalmente consideradas como uma conversão de reagentes em produtos, muitas reações ocorrem em várias etapas, formando intermediários. Nas reações quimioluminescentes, esses intermediários fazem a transição de um estado de alta energia para o estado fundamental, liberando um fóton de luz que pode ser visível à medida que a reação progride.
<h4>Níveis de energia atômica</h4>
Niels Bohr, um físico dinamarquês, propôs a teoria de que um elétron orbitando o núcleo do átomo pode ocupar apenas certas órbitas ou níveis de energia. Diz-se que um átomo com todos os seus elétrons no nível de energia mais baixo possível está em seu estado fundamental. Quando um elétron ocupa um nível de energia superior ao estado fundamental, o átomo está em seu estado excitado.
Uma suposição fundamental na teoria de Bohr é que um elétron permanece no estado fundamental até ser perturbado. Assim, um elétron só é elevado a um estado excitado absorvendo energia. Quando o elétron relaxa de volta ao estado fundamental, ele libera essa energia. Muitas vezes, isso ocorre na forma de um fóton de luz, cujo comprimento de onda está diretamente relacionado à diferença de energia entre os estados excitado e fundamental.
Quando um elétron é excitado a um nível de energia mais alto absorvendo luz de uma determinada energia ou comprimento de onda, o fenômeno é chamado de fluorescência. Este efeito pode ser visto se você expor uma camiseta branca lavada a uma luz negra ou luz UV. O brilho característico é causado pela fluorescência do detergente e dos compostos branqueadores da camisa, que a fazem parecer branca sob luz normal.
<h4>Quimioluminescência</h4>
Em uma reação quimioluminescente, um elétron é excitado absorvendo o calor que é liberado durante uma reação. Então, a luz é liberada quando o elétron relaxa de volta ao estado fundamental. A principal diferença entre quimioluminescência e fluorescência é que a energia que excita os elétrons na quimioluminescência vem diretamente da reação.
Uma aplicação prática da quimioluminescência é em bastões luminosos comerciais. Um bastão luminoso contém duas soluções separadas: uma contendo peróxido e a outra contendo oxalato de difenila e um corante colorido. Quando as duas soluções são misturadas, o que ocorre ao ativar o bastão luminoso, a reação resultante entre o peróxido e o oxalato de difenila produz energia que excita o corante para um estado de energia mais alto. À medida que os reagentes se esgotam, o corante retorna ao estado fundamental e libera luz. Isso é o que dá aos bastões luminosos sua cor característica.
<h4>Luminol</h4>
O luminol é um produto químico que exibe propriedades quimioluminescentes e é utilizado em uma ampla gama de aplicações, principalmente em ciência forense. O luminol (C8H7N3O2) emite azul quando é misturado com um agente oxidante. No caso da ciência forense, o luminol reage com o ferro da hemoglobina, permitindo que os cientistas forenses identifiquem traços muito pequenos de sangue.
O luminol é sintetizado pela reação de desidratação do ácido 3-nitroftálico com hidrazina. A reação é aquecida para remover a água e o trietilenoglicol é adicionado para aumentar ainda mais a temperatura. O grupo nitro da 3-nitroftalhidrazida é então reduzido usando ditionito de sódio para formar um grupo amino em pH alto. Em condições básicas, a 3-nitroftalhidrazida é solúvel. A adição de ácido acético glacial precipita o luminol.
Com hidróxido de potássio, o luminol forma um dianion. Os ânions hidróxido desprotonam os dois hidrogênios que estão ligados aos nitrogênios no luminol. O gás oxigênio oxida o luminol ao seu estado excitado. À medida que relaxa de volta ao estado fundamental, ele libera uma luz azul-esbranquiçada.
<h4>Referências</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2015). Química e Reatividade Química. Belmont, CA: Brooks / Cole, Cengage Learning.</li>
</ol>

Síntese de Luminol

As reações químicas podem ser classificadas como endotérmicas ou exotérmicas, dependendo se as reações absorvem ou liberam calor quando mudam para produtos. ...

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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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