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

בדרך כלל, כאשר מגיבים משתנים לתוצרים, ההבדל באנרגיה שלהם ידוע כאנתלפיה של התגובה, אשר יכולה להיות אקסותרמית, שבה חום משתחרר, או אנדותרמי, שבו חום נספג.
בתגובה כימילומינסנטית, המגיבים יוצרים מתווך, שנמצא במצב מעורר אלקטרונית. ביניים זה אינו יציב. כאשר הוא נרגע למצב הקרקע, האלקטרונים בביניים משחררים אנרגיה בפליטת אור נראה ולא בצורה של חום.
תופעה זו דומה לפלואורסצנטיות. נזכיר כי האלקטרונים במולקולה תופסים את מצב הקרקע. כאשר הם סופגים אנרגיה, כגון כאשר הם נחשפים לאורך גל מסוים של אור, הם הופכים נרגשים ועוברים לרמת אנרגיה גבוהה יותר, נרגשת. כאשר המצב הנרגש נרגע חזרה למצב הקרקע, האנרגיה העודפת נפלטת בצורה של אור נראה.
עם זאת, חשוב להבחין בין השניים. ההבדל הוא האופן שבו האלקטרונים של המולקולות מעוררים. בכימילומינסנציה, אנרגיה הנוצרת מתגובה כימית מעוררת את האלקטרונים... ואילו בפלואורסצנציה, האלקטרונים מעוררים על ידי בליעה ישירה של אנרגיית אור או קרינה אלקטרומגנטית אחרת. כאשר האלקטרונים נרגעים למצב הקרקע, שני סוגי ההארה משחררים את האנרגיה כאור הנפלט.
בואו נסתכל על דוגמה של כל אחד. כאשר אתם מאירים אור UV על בקבוק מי טוניק, האלקטרונים בכינין סופגים את האנרגיה ומעוררים לרמת אנרגיה גבוהה יותר. כאשר האלקטרונים נרגעים, הם משחררים אנרגיה בצורה של אור כחול.
כעת, חשבו על מקל זוהר, שיש לו צינור חיצוני המכיל דיפניל אוקסלט וצבע, וצינור פנימי המכיל מי חמצן. כאשר מכופפים את מקל הזוהר, הצינור הפנימי נשבר ומאפשר למגיבים להתערבב. מי החמצן מחמצנים את הדיפניל אוקסלט ליצירת פנול ופרוקסיד מחזורי. מי החמצן המחזוריים מתפרקים לפחמן דו חמצני, ומשחררים אנרגיה שמעוררת את הצבע לרמת אנרגיה גבוהה יותר. כאשר הצבע נרגע למצב הקרקע, פוטון של אור משתחרר. כאן, התהליך הוא כימילומינסנציה מכיוון שהאנרגיה המסופקת כדי לעורר את הצבע מגיעה מתגובה כימית.
במעבדה זו, תוכלו לסנתז 3-aminophthalhydrazide, הידוע גם בשם luminol, על ידי ביצוע סדרה של תגובות. לאחר מכן תגיבו את הלומינול עם דימתיל סולפוקסיד על אשלגן הידרוקסיד מוצק כדי לבחון את הכימילומינסנציה שלו.

סינתזה של לומינול

Synthesis of Luminol

תגובות כימיות יכולות להיות מסווגות כאנדותרמיות או אקסותרמיות, תלוי אם התגובות סופגות או משחררות חום כאשר הן משתנות למוצרים. ההבדל באנרגיה בין מגיבים לתוצרים ידוע כאנתלפיה של התגובה. ניתן למצוא זאת על ידי חישוב הפרש האנרגיות בין התוצרים לבין המגיבים, או ΔH. אם ΔH חיובי, התגובה היא אנדותרמית; אם היא שלילית, התגובה היא אקסותרמית. בעוד שתגובות כימיות נחשבות באופן מסורתי כהמרה של מגיבים לתוצרים, תגובות רבות מתרחשות במספר שלבים, ויוצרות מתווכים. בתגובות כימילומינסנטיות, מתווכים אלה עוברים ממצב אנרגיה גבוהה למצב הקרקע, ומשחררים פוטון של אור שעשוי להיראות לעין ככל שהתגובה מתקדמת.
<h4>רמות אנרגיה אטומית</h4>
נילס בוהר, פיזיקאי דני, הציע את התאוריה לפיה אלקטרון המקיף את גרעין האטום יכול לתפוס רק מסלולים מסוימים או רמות אנרגיה מסוימות. אטום שכל האלקטרונים שלו נמצאים ברמת האנרגיה הנמוכה ביותר האפשרית נמצא במצב הקרקע שלו. כאשר אלקטרון תופס רמת אנרגיה גבוהה ממצב הקרקע, האטום נמצא במצב מעורר.
הנחת מפתח בתיאוריה של בוהר היא שאלקטרון נשאר במצב הקרקע עד שהוא מופרע. לפיכך, אלקטרון מורם למצב מעורר רק על ידי ספיגת אנרגיה. כאשר האלקטרון נרגע בחזרה למצב הקרקע, הוא משחרר את האנרגיה הזו. לעתים קרובות, זה בצורה של פוטון של אור, שאורך הגל שלו קשור ישירות להפרש האנרגיה בין המצב המעורר למצב הקרקע.
כאשר אלקטרון מעורר לרמת אנרגיה גבוהה יותר על ידי בליעת אור בעל אנרגיה מסוימת או אורך גל מסוים, התופעה נקראת פלואורסצנטיות. אפקט זה ניתן לראות אם אתה חושף חולצת טריקו לבנה מכובסת לאור שחור או UV-light. הזוהר האופייני נגרם על ידי פלואורסצנטיות של חומרי הניקוי ותרכובות ההלבנה בחולצה, אשר גורמים לה להיראות לבנה תחת אור רגיל.
<h4>כימילומינסנציה</h4>
בתגובה כימילומינסנטית, אלקטרון מעורר על ידי בליעת חום המשתחרר במהלך תגובה. לאחר מכן, האור משתחרר כאשר האלקטרון נרגע בחזרה למצב הקרקע. ההבדל העיקרי בין כימילומינסנציה לפלואורסנציה הוא שהאנרגיה שמעוררת את האלקטרונים בכימילומינסנציה מגיעה ישירות מהתגובה.
יישום מעשי אחד של כימילומינסנציה הוא במקלות זוהר מסחריים. מקל זוהר מכיל שתי תמיסות נפרדות: האחת מכילה מי חמצן והשנייה מכילה דיפניל אוקסלט וצבע צבע. כאשר שתי התמיסות מעורבבות, המתרחשת בעת הפעלת מקל הזוהר, התגובה המתקבלת בין מי חמצן לבין דיפניל אוקסלט מייצרת אנרגיה המעוררת את הצבע למצב אנרגיה גבוה יותר. כאשר המגיבים מותשים, הצבע חוזר למצב הקרקע ומשחרר אור. זה מה שנותן למקלות זוהר את הצבע האופייני שלהם.
<h4>לומינול</h4>
לומינול הוא כימיקל בעל תכונות כימילומינסנטיות ומשמש במגוון רחב של יישומים, בעיקר בזיהוי פלילי. לומינול (C8H7N3O2) פולט כחול כאשר הוא מעורבב עם חומר מחמצן. במקרה של זיהוי פלילי, לומינול מגיב עם הברזל בהמוגלובין, מה שמאפשר למדעני זיהוי פלילי לזהות עקבות קטנים מאוד של דם.
Luminol מסונתז על ידי תגובת התייבשות של חומצה 3-nitrophthalic עם hydrazine. התגובה מחוממת כדי להסיר מים, וטריאתילן גליקול מתווסף כדי להגדיל עוד יותר את הטמפרטורה. קבוצת הניטרו של 3-nitrophthalhydrazide מופחתת לאחר מכן באמצעות נתרן dithionite כדי ליצור קבוצת אמינו ב- pH גבוה. בתנאים בסיסיים, 3-nitrophthalhydrazide הוא מסיס. תוספת של חומצה אצטית קרחונית מזרזת את הלומינול.
עם אשלגן הידרוקסיד, לומינול יוצר דיאניון. אניונים הידרוקסיד deprotonate את שני המימן המחוברים nitrogens ב luminol. גז חמצן מחמצן את האלומיניום למצבו המעורר. כאשר הוא נרגע בחזרה למצב הקרקע, הוא משחרר אור כחול-לבן.
<h4>הפניות</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2015). כימיה ותגובתיות כימית. בלמונט, קליפורניה: Brooks/Cole, Cengage Learning.</li>
</ol>

תגובות כימיות יכולות להיות מסווגות כאנדותרמיות או אקסותרמיות, תלוי אם התגובות סופגות או משחררות חום כאשר הן משתנות למוצרים. ...

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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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thin-layer chromatography

TLC Separation by Polarity

Thin-layer chromatography (TLC) separates compounds in a mixture by polarity. It uses the same basic parts as other chromatography methods: a stationary phase, a mobile phase, and the solute. In TLC, the stationary phase is a thin coating on a plate instead of material packed into a column. The stationary phase in most TLC systems is polar silica gel, a form of silicon dioxide. Its surface has OH groups that can form hydrogen bonds. The sample is first spotted on a pencil start line with a...

column chromatography

Separating Mixtures by Polarity

Column chromatography separates compounds in an organic chemistry lab by using differences in physical properties. It is especially useful for microscale separations when less than 10 grams of material is available for analysis. The method can separate a mixture based on solubility, polarity, hydrophobicity, size, and charge. Any chromatography system has three main parts: the stationary phase, the mobile phase, and the sample mixture. In column chromatography, the stationary phase is usually...

hydrolysis of an ester

Soap Formation from Triglycerides

Triglycerides are lipids made of a glycerol backbone linked to three fatty acids. They are common in vegetable oils and animal fats. Plant lipids are the basis of vegetable oils, while animal lipids are used as lard or general sources of fats. A fatty acid has a long carbon chain, usually between 12 and 20 carbons, with a weak organic acid at one end. Some fatty acids contain double bonds. These are unsaturated fats. The double bond usually gives the chain a bent, cis shape. As a result,...

synthesis of luminol

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...

esterification

Ester Formation and Everyday Fragrances

Esters are organic compounds with a carbonyl group attached to an oxygen that connects to another alkyl or aryl group. Their general formula is RCOOR'. They are often made by reacting a carboxylic acid with an alcohol in an esterification reaction. In this process, the hydroxyl group of the acid is replaced by an alkyl or aryl group. Many simple esters have familiar fruit and flower smells because their structures are small and less complex. Formic acid and methanol can combine to form methyl...

identification of unknown aldehydes and ketones

identification-of-unknown-aldehydes-and-ketones

Carbonyl Tests for Aldehydes and Ketones

Aldehydes and ketones are carbonyl compounds, which means they contain a C=O functional group. A ketone has two alkyl or aryl groups attached to the carbonyl carbon. Acetone is the simplest ketone, with two methyl groups. An aldehyde is similar, but it has at least one hydrogen attached to the carbonyl carbon. Formaldehyde is the simplest aldehyde, and acetaldehyde has one hydrogen and one methyl group. The carbonyl carbon in both aldehydes and ketones is electrophilic because the oxygen atom...

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

Comprehensive, curriculum-focused videos for both instructors and students in introductory chemistry lab courses.

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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.

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Luminol Chemiluminescence in Forensics

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