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

Обычно, когда реагенты превращаются в продукты, разница в их энергии известна как энтальпия реакции, которая может быть либо экзотермической, когда выделяется тепло, либо эндотермической, когда тепло поглощается.
В хемилюминесцентной реакции реагенты образуют промежуточный продукт, который находится в электронно-возбужденном состоянии. Этот промежуточный продукт нестабилен. Когда он расслабляется до основного состояния, электроны в промежуточном звене высвобождают энергию в виде излучения видимого света, а не в виде тепла.
Это явление похоже на флуоресценцию. Вспомним, что электроны в молекуле занимают основное состояние. Когда они поглощают энергию, например, при воздействии света определенной длины волны, они возбуждаются и переходят на более высокий, возбужденный энергетический уровень. Когда возбужденное состояние расслабляется до основного состояния, избыточная энергия излучается в виде видимого света.
Тем не менее, важно различать эти два понятия. Разница заключается в том, как возбуждаются электроны молекул. При хемилюминесценции энергия, генерируемая в результате химической реакции, возбуждает электроны, тогда как при флуоресценции электроны возбуждаются за счет прямого поглощения световой энергии или другого электромагнитного излучения. Когда электроны расслабляются до основного состояния, оба типа люминесценции высвобождают энергию в виде излучаемого света.
Давайте рассмотрим пример каждого из них. Когда вы направляете ультрафиолетовый свет на бутылку с тонизирующей водой, электроны хинина поглощают энергию и возбуждаются до более высокого энергетического уровня. Когда электроны расслабляются, они высвобождают энергию в виде синего света.
Теперь рассмотрим светящуюся палочку, которая имеет внешнюю трубку, содержащую дифенилоксалат и краситель, и внутреннюю трубку, содержащую перекись водорода. Когда вы сгибаете светящуюся палочку, внутренняя трубка разрывается, позволяя реагентам смешаться. Перекись водорода окисляет дифенилоксалат с образованием фенола и циклической перекиси. Циклическая перекись разлагается до углекислого газа, высвобождая энергию, которая возбуждает краситель до более высокого энергетического уровня. Когда краситель расслабляется до основного состояния, выделяется фотон света. В данном случае процесс называется хемилюминесценцией, потому что энергия, обеспечиваемая для возбуждения красителя, происходит в результате химической реакции.
В этой лаборатории вы синтезируете 3-аминофтальгидразид, также известный как люминол, выполняя серию реакций. Затем вы прореагируете люминол с диметилсульфоксидом на твердом гидроксиде калия, чтобы наблюдать его хемилюминесценцию.

Синтез люминола

Synthesis of Luminol

Химические реакции можно классифицировать как эндотермические или экзотермические в зависимости от того, поглощают ли реакции или выделяют тепло при переходе в продукты. Разница в энергии между реагентами и продуктами известна как энтальпия реакции. Это можно найти путем вычисления разницы энергий между продуктами и реагентами, или ΔH. Если ΔH положительный, реакция является эндотермической; Если она отрицательная, то реакция является экзотермической. В то время как химические реакции традиционно рассматриваются как превращение реагентов в продукты, многие реакции протекают в несколько этапов, образуя промежуточные продукты. В хемилюминесцентных реакциях эти промежуточные продукты переходят из высокоэнергетического состояния в основное состояние, высвобождая фотон света, который может быть виден по мере развития реакции.
<h4>Уровни атомной энергии</h4>
Нильс Бор, датский физик, выдвинул теорию о том, что электрон, вращающийся вокруг ядра атома, может занимать только определенные орбиты или энергетические уровни. Атом, все электроны которого находятся на самом низком возможном энергетическом уровне, называется находящимся в своем основном состоянии. Когда электрон занимает энергетический уровень выше основного состояния, атом находится в возбужденном состоянии.
Ключевое предположение в теории Бора состоит в том, что электрон остается в основном состоянии до тех пор, пока его не потревожат. Таким образом, электрон приводится в возбужденное состояние только за счет поглощения энергии. Когда электрон расслабляется обратно в основное состояние, он высвобождает эту энергию. Часто это происходит в виде фотона света, длина волны которого напрямую связана с разницей энергий между возбужденным и основным состояниями.
Когда электрон возбуждается до более высокого энергетического уровня, поглощая свет определенной энергии или длины волны, это явление называется флуоресценцией. Этот эффект можно увидеть, если подвергнуть постиранную белую футболку воздействию черного света или ультрафиолета. Характерное свечение вызвано флуоресценцией моющего средства и отбеливающих составов в рубашке, благодаря чему она выглядит белой при обычном освещении.
<h4>Хемилюминесценция</h4>
В хемилюминесцентной реакции электрон возбуждается за счет поглощения тепла, выделяющегося во время реакции. Затем свет высвобождается, когда электрон расслабляется обратно в основное состояние. Основное различие между хемилюминесценцией и флуоресценцией заключается в том, что энергия, которая возбуждает электроны в хемилюминесценции, поступает непосредственно из реакции.
Одним из практических применений хемилюминесценции является использование в коммерческих светящихся палочках. Светящаяся палочка содержит два отдельных раствора: один содержит перекись, а другой — дифенилоксалат и цветной краситель. Когда два раствора смешиваются, что происходит при активации светящейся палочки, результирующая реакция между перекисью и дифенилоксалатом производит энергию, которая возбуждает краситель до более высокого энергетического состояния. Когда реагенты исчерпываются, краситель возвращается в исходное состояние и выделяет свет. Именно это придает светящимся палочкам характерный цвет.
<h4>Люминол</h4>
Люминол — это химическое вещество, которое проявляет хемилюминесцентные свойства и используется в широком спектре применений, в первую очередь в криминалистике. Люминол (C8H7N3O2) выделяет синий цвет при смешивании с окислителем. В случае с криминалистикой люминол вступает в реакцию с железом в гемоглобине, что позволяет судмедэкспертам идентифицировать очень небольшие следы крови.
Люминол синтезируется в результате реакции дегидратации 3-нитрофталевой кислоты с гидразином. Реакцию нагревают для удаления воды, а для дальнейшего повышения температуры добавляют триэтиленгликоль. Затем нитрогруппу 3-нитрофталгидразида восстанавливают с помощью дитионита натрия с образованием аминогруппы при высоком pH. В основных условиях 3-нитрофталгидразид растворим. Добавление ледяной уксусной кислоты выпадает в осадок люминол.
С гидроксидом калия люминол образует дианион. Анионы гидроксида депротонируют два атома водорода, которые присоединены к азотам в люминоле. Газообразный кислород окисляет люминол до возбужденного состояния. Когда он расслабляется обратно в исходное состояние, он испускает сине-белый свет.
<h4>Ссылки</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2015). Химия и химическая реакционная способность. Бельмонт, Калифорния: Брукс/Коул, Cengage Learning.</li>
</ol>

Химические реакции можно классифицировать как эндотермические или экзотермические в зависимости от того, поглощают ли реакции или выделяют тепло при переходе в продукты. ...

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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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uv-vis spectroscopy of dyes

How Dyes Absorb Visible Light

Dyes absorb visible light because their molecular structure lets them interact with specific wavelengths. When light hits a substance, some light is absorbed and the rest is reflected or transmitted. The color we see depends on the wavelength that is reflected. A blue substance reflects blue light, around 430 to 480 nm, and absorbs the complementary orange region, around 590 to 630 nm. Compounds that do not absorb in the visible range appear colorless to the human eye. Light also has energy and...

identifying alcohols

Alcohol Tests for Primary, Secondary, and Tertiary

Alcohol tests can help sort alcohols by structure and reactivity. Alcohols are organic molecules with a hydroxyl group attached to an alkyl or aryl group, written as ROH. If the carbon with the hydroxyl group has one R group, the alcohol is primary. If it has two R groups, it is secondary. If it has three R groups, it is tertiary. Alcohols can also be aromatic when they contain a benzene ring. Phenol is the simplest aromatic alcohol. To identify an alcohol, chemists can use its chemical...

chemistry

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Luminol Chemiluminescence in Forensics

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