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

Habituellement, lorsque les réactifs se transforment en produits, la différence de leur énergie est connue sous le nom d’enthalpie de la réaction, qui peut être soit exothermique, où la chaleur est libérée, soit endothermique, où la chaleur est absorbée.
Dans une réaction chimiluminescente, les réactifs forment un intermédiaire, qui est dans un état d’excitation électronique. Cet intermédiaire est instable. Lorsqu’il se détend jusqu’à l’état fondamental, les électrons de l’intermédiaire libèrent de l’énergie dans l’émission de lumière visible plutôt que sous forme de chaleur.
Ce phénomène est similaire à la fluorescence. Rappelons que les électrons d’une molécule occupent l’état fondamental. Lorsqu’ils absorbent de l’énergie, par exemple lorsqu’ils sont exposés à une certaine longueur d’onde de lumière, ils deviennent excités et passent à un niveau d’énergie plus élevé. Lorsque l’état excité se détend pour revenir à l’état fondamental, l’énergie excédentaire est émise sous forme de lumière visible.
Cependant, il est important de faire la distinction entre les deux. La différence réside dans la façon dont les électrons des molécules sont excités. Dans la chimiluminescence, l’énergie générée à partir d’une réaction chimique excite les électrons... tandis que dans la fluorescence, les électrons sont excités par l’absorption directe de l’énergie lumineuse ou d’autres rayonnements électromagnétiques. Lorsque les électrons se détendent jusqu’à l’état fondamental, les deux types de luminescence libèrent l’énergie sous forme de lumière émise.
Jetons un coup d’œil à un exemple de chacun. Lorsque vous faites briller une lumière UV sur une bouteille d’eau tonique, les électrons de la quinine absorbent l’énergie et sont excités à un niveau d’énergie plus élevé. Lorsque les électrons se détendent, ils libèrent de l’énergie sous forme de lumière bleue.
Considérons maintenant un bâton lumineux, qui a un tube extérieur contenant de l’oxalate de diphényle et un colorant, et un tube intérieur contenant du peroxyde d’hydrogène. Lorsque vous pliez le bâton lumineux, le tube intérieur se brise, permettant aux réactifs de se mélanger. Le peroxyde d’hydrogène oxyde l’oxalate de diphényle pour former du phénol et du peroxyde cyclique. Le peroxyde cyclique se décompose en dioxyde de carbone, libérant de l’énergie qui excite le colorant à un niveau d’énergie plus élevé. Lorsque le colorant se détend jusqu’à l’état fondamental, un photon de lumière est libéré. Ici, le processus est de chimiluminescence car l’énergie fournie pour exciter le colorant provient d’une réaction chimique.
Dans ce laboratoire, vous synthétiserez le 3-aminophtalhydrazide, également connu sous le nom de luminol, en effectuant une série de réactions. Vous ferez ensuite réagir le luminol avec du sulfoxyde de diméthyle sur de l’hydroxyde de potassium solide pour observer sa chimiluminescence.

synthèse du luminol

Synthesis of Luminol

Les réactions chimiques peuvent être classées comme endothermiques ou exothermiques selon qu’elles absorbent ou libèrent de la chaleur lorsqu’elles se transforment en produits. La différence d’énergie entre les réactifs et les produits est connue sous le nom d’enthalpie de la réaction. On le trouve en calculant la différence d’énergie entre les produits et les réactifs, ou ΔH. Si ΔH est positif, la réaction est endothermique ; S’il est négatif, la réaction est exothermique. Alors que les réactions chimiques sont traditionnellement considérées comme la conversion de réactifs en produits, de nombreuses réactions se déroulent en plusieurs étapes, formant des intermédiaires. Dans les réactions chimiluminescentes, ces intermédiaires passent d’un état de haute énergie à l’état fondamental, libérant un photon de lumière qui peut être visible au fur et à mesure que la réaction progresse.
<h4>Niveaux d’énergie atomique</h4>
Niels Bohr, un physicien danois, a proposé la théorie selon laquelle un électron en orbite autour du noyau de l’atome ne peut occuper que certaines orbites ou certains niveaux d’énergie. On dit qu’un atome dont tous les électrons sont dans le niveau d’énergie le plus bas possible est à son état fondamental. Lorsqu’un électron occupe un niveau d’énergie supérieur à l’état fondamental, l’atome est dans son état excité.
Une hypothèse clé de la théorie de Bohr est qu’un électron reste dans l’état fondamental jusqu’à ce qu’il soit perturbé. Ainsi, un électron n’est élevé à un état excité qu’en absorbant de l’énergie. Lorsque l’électron se détend jusqu’à l’état fondamental, il libère cette énergie. Souvent, cela se présente sous la forme d’un photon de lumière, dont la longueur d’onde est directement liée à la différence d’énergie entre l’état excité et l’état fondamental.
Lorsqu’un électron est excité à un niveau d’énergie plus élevé en absorbant la lumière d’une certaine énergie ou longueur d’onde, le phénomène est appelé fluorescence. Cet effet peut être observé si vous exposez un t-shirt blanc lavé à une lumière noire ou à une lumière UV. L’éclat caractéristique est causé par la fluorescence du détergent et des composés de blanchiment dans la chemise, qui lui donnent un aspect blanc sous une lumière régulière.
<h4>Chimiluminescence</h4>
Dans une réaction chimiluminescente, un électron est excité en absorbant la chaleur qui est libérée au cours d’une réaction. Ensuite, la lumière est libérée lorsque l’électron se détend pour revenir à l’état fondamental. La principale différence entre la chimiluminescence et la fluorescence est que l’énergie qui excite les électrons dans la chimiluminescence provient directement de la réaction.
Une application pratique de la chimiluminescence est dans les bâtons lumineux commerciaux. Un bâton lumineux contient deux solutions distinctes : l’une contenant du peroxyde et l’autre contenant de l’oxalate de diphényle et un colorant. Lorsque les deux solutions sont mélangées, ce qui se produit lors de l’activation du bâton lumineux, la réaction résultante entre le peroxyde et l’oxalate de diphényle produit de l’énergie qui excite le colorant à un état d’énergie plus élevé. Au fur et à mesure que les réactifs sont épuisés, le colorant revient à l’état fondamental et libère de la lumière. C’est ce qui donne aux bâtons lumineux leur couleur caractéristique.
<h4>Luminol</h4>
Le luminol est un produit chimique qui présente des propriétés chimiluminescentes et est utilisé dans un large éventail d’applications, notamment en médecine légale. Le luminol (C8H7N3O2) émet du bleu lorsqu’il est mélangé à un agent oxydant. Dans le cas de la médecine légale, le luminol réagit avec le fer contenu dans l’hémoglobine, ce qui permet aux médecins légistes d’identifier de très petites traces de sang.
Le luminol est synthétisé par la réaction de déshydratation de l’acide 3-nitrophtalique avec l’hydrazine. La réaction est chauffée pour éliminer l’eau, et du triéthylène glycol est ajouté pour augmenter encore la température. Le groupe nitro du 3-nitrophthalhydrazide est ensuite réduit à l’aide de dithionite de sodium pour former un groupe amino à pH élevé. Dans des conditions basiques, le 3-nitrophthalhydrazide est soluble. L’ajout d’acide acétique glacial précipite le luminol.
Avec l’hydroxyde de potassium, le luminol forme un dianion. Les anions hydroxyde déprotonent les deux hydrogènes qui sont liés aux azotes dans le luminol. L’oxygène gazeux oxyde le luminol jusqu’à son état excité. Lorsqu’il se détend pour revenir à l’état fondamental, il libère une lumière bleu-blanc.
<h4>références</h4>
<ol>
	<li style="font-size :17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2015). Chimie et réactivité chimique. Belmont, Californie : Brooks/Cole, Cengage Learning.</li>
</ol>

Synthèse du Luminol

Les réactions chimiques peuvent être classées comme endothermiques ou exothermiques selon qu’elles absorbent ou libèrent de la chaleur lorsqu’elles se transforment en produits. ...

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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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