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

Di solito, quando i reagenti si trasformano in prodotti, la differenza nella loro energia è nota come entalpia della reazione, che può essere esotermica, dove viene rilasciato calore, o endotermica, dove il calore viene assorbito.
In una reazione chemiluminescente, i reagenti formano un intermedio, che si trova in uno stato eccitato elettronicamente. Questo intermedio è instabile. Mentre si rilassa fino allo stato fondamentale, gli elettroni nell'intermedio rilasciano energia nell'emissione di luce visibile piuttosto che sotto forma di calore.
Questo fenomeno è simile alla fluorescenza. Ricordiamo che gli elettroni in una molecola occupano lo stato fondamentale. Quando assorbono energia, ad esempio quando sono esposti a una certa lunghezza d'onda della luce, si eccitano e passano a un livello di energia più elevato. Quando lo stato eccitato si rilassa tornando allo stato fondamentale, l'energia in eccesso viene emessa sotto forma di luce visibile.
Tuttavia, è importante distinguere tra i due. La differenza sta nel modo in cui gli elettroni delle molecole vengono eccitati. Nella chemiluminescenza, l'energia generata da una reazione chimica eccita gli elettroni... mentre nella fluorescenza, gli elettroni vengono eccitati dall'assorbimento diretto di energia luminosa o di altre radiazioni elettromagnetiche. Quando gli elettroni si rilassano fino allo stato fondamentale, entrambi i tipi di luminescenza rilasciano l'energia sotto forma di luce emessa.
Diamo un'occhiata a un esempio di ciascuno. Quando si accende la luce UV su una bottiglia di acqua tonica, gli elettroni nel chinino assorbono l'energia e vengono eccitati a un livello di energia più elevato. Quando gli elettroni si rilassano, rilasciano energia sotto forma di luce blu.
Ora, considera un bastoncino luminoso, che ha un tubo esterno contenente ossalato di difenile e un colorante e un tubo interno contenente perossido di idrogeno. Quando si piega il bastoncino luminoso, la camera d'aria si rompe, permettendo ai reagenti di mescolarsi. Il perossido di idrogeno ossida il difenilossalato per formare fenolo e perossido ciclico. Il perossido ciclico si decompone in anidride carbonica, rilasciando energia che eccita il colorante a un livello di energia più elevato. Quando il colorante si rilassa allo stato fondamentale, viene rilasciato un fotone di luce. Qui, il processo è la chemiluminescenza perché l'energia fornita per eccitare il colorante proviene da una reazione chimica.
In questo laboratorio, sintetizzerai la 3-amminoftalidrazide, nota anche come luminol, eseguendo una serie di reazioni. Farai quindi reagire il luminol con dimetilsolfossido su idrossido di potassio solido per osservarne la chemiluminescenza.

sintesi del luminol

Le reazioni chimiche possono essere classificate come endotermiche o esotermiche a seconda che le reazioni assorbano o rilascino calore quando si trasformano in prodotti. La differenza di energia tra reagenti e prodotti è nota come entalpia della reazione. Questo si trova calcolando la differenza di energie tra i prodotti e i reagenti, o ΔH. Se ΔH è positivo, la reazione è endotermica; Se è negativo, la reazione è esotermica. Mentre le reazioni chimiche sono tradizionalmente pensate come una conversione di reagenti in prodotti, molte reazioni avvengono in più fasi, formando intermedi. Nelle reazioni chemiluminescenti, questi intermedi passano da uno stato ad alta energia allo stato fondamentale, rilasciando un fotone di luce che può essere visibile man mano che la reazione progredisce.
<h4>Livelli di energia atomica</h4>
Niels Bohr, un fisico danese, ha proposto la teoria secondo cui un elettrone in orbita attorno al nucleo dell'atomo può occupare solo determinate orbite o livelli di energia. Si dice che un atomo con tutti i suoi elettroni nel livello di energia più basso possibile sia allo stato fondamentale. Quando un elettrone occupa un livello di energia superiore allo stato fondamentale, l'atomo è nel suo stato eccitato.
Un assunto chiave nella teoria di Bohr è che un elettrone rimane nello stato fondamentale fino a quando non viene disturbato. Pertanto, un elettrone viene elevato a uno stato eccitato solo assorbendo energia. Quando l'elettrone si rilassa di nuovo allo stato fondamentale, rilascia quell'energia. Spesso, questo è sotto forma di un fotone di luce, la cui lunghezza d'onda è direttamente correlata alla differenza di energia tra lo stato eccitato e quello fondamentale.
Quando un elettrone viene eccitato a un livello di energia più elevato assorbendo luce di una certa energia o lunghezza d'onda, il fenomeno è chiamato fluorescenza. Questo effetto può essere visto se esponi una maglietta bianca lavata a una luce nera o a una luce UV. Il bagliore caratteristico è causato dalla fluorescenza del detersivo e dei composti sbiancanti nella maglietta, che la fanno sembrare bianca alla luce normale.
<h4>chemiluminescenza</h4>
In una reazione chemiluminescente, un elettrone viene eccitato assorbendo il calore che viene rilasciato durante una reazione. Quindi, la luce viene rilasciata quando l'elettrone si rilassa tornando allo stato fondamentale. La principale differenza tra chemiluminescenza e fluorescenza è che l'energia che eccita gli elettroni in chemiluminescenza proviene direttamente dalla reazione.
Un'applicazione pratica della chemiluminescenza è nei bastoncini luminosi commerciali. Un bastoncino luminoso contiene due soluzioni separate: una contenente perossido e l'altra contenente ossalato di difenile e un colorante. Quando le due soluzioni vengono miscelate, cosa che avviene dopo l'attivazione del glow stick, la reazione risultante tra il perossido e l'ossalato di difenile produce energia che eccita il colorante a uno stato energetico più elevato. Quando i reagenti sono esauriti, il colorante ritorna allo stato fondamentale e rilascia luce. Questo è ciò che conferisce ai bastoncini luminosi il loro colore caratteristico.
<h4>Luminol</h4>
Il luminol è una sostanza chimica che presenta proprietà chemiluminescenti e viene utilizzata in un'ampia gamma di applicazioni, in particolare in ambito forense. Il luminol (C8H7N3O2) emette blu quando viene miscelato con un agente ossidante. Nel caso della medicina legale, il luminol reagisce con il ferro nell'emoglobina, consentendo agli scienziati forensi di identificare tracce di sangue molto piccole.
Il luminol è sintetizzato dalla reazione di disidratazione dell'acido 3-nitroftalico con l'idrazina. La reazione viene riscaldata per rimuovere l'acqua e viene aggiunto glicole trietilenico per aumentare ulteriormente la temperatura. Il gruppo nitro della 3-nitroftalidrazide viene quindi ridotto utilizzando il ditionito di sodio per formare un gruppo amminico ad alto pH. In condizioni basiche, la 3-nitroftalidrazide è solubile. L'aggiunta di acido acetico glaciale fa precipitare il luminol.
Con l'idrossido di potassio, il luminol forma un dianion. Gli anioni idrossido deprotonano i due idrogeni che sono attaccati agli azoti nel luminolo. L'ossigeno gassoso ossida il luminolo al suo stato eccitato. Mentre si rilassa tornando allo stato fondamentale, rilascia una luce bianco-blu.
<h4>referenze</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2015). Chimica e reattività chimica. Belmont, CA: Brooks/Cole, Cengage Learning.</li>
</ol>

Sintesi del Luminol

Le reazioni chimiche possono essere classificate come endotermiche o esotermiche a seconda che le reazioni assorbano o rilascino calore quando si trasformano in prodotti. ...

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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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Learn how luminol chemiluminescence produces blue-white light, why glow sticks glow, and how forensic scientists use luminol to detect traces of blood.

Luminol Chemiluminescence in Forensics | JoVE K12

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Luminol Chemiluminescence in Forensics

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