Name: hückel rule and aromatic ring criteria
Uploaded: 2023-04-30T10:31:08.000Z
Duration: 1 min 20 s
Channel: JoVE

בנזן, ציקלובוטאדיאן וציקלואוקטטראן מיוצגים כהכלאה של שני מבנים שקולים.
עם זאת, בניגוד לבנזן, cyclobutadiene ו cyclooctatetraene אינם יציבים ולא ארומטיים.
כדי להסביר הבדלים אלה, אריך היקל התווה קבוצה של תכונות מבניות העומדות בבסיס הארומטיות, הידועה בשם חוק היקל או כלל 4n + 2.
חוק היקל קובע כי תרכובת ארומטית חייבת להיות מחזורית ומישורית, עם אורביטל p בלתי היברידי על כל פחמן.
אורביטלי p חייבים לחפוף כדי ליצור לולאה רציפה של (4n + 2) π אלקטרונים, כאשר n הוא מספר שלם חיובי.
בנזן עונה על כל הקריטריונים לשלטונו של הוקל, ומבסס את ארומטיותו.
לעומת זאת, ציקלואוקטטראן מניח קונפורמציה לא מישורית של גיגית ללא חפיפה בין אורביטלי p סמוכים. בנוסף, חסרים לו (4n + 2) אלקטרונים π ולכן אינו ארומטי.
באופן דומה, ציקלובוטאדיאן הוא מחזורי, מישורי, ויש לו חפיפה רציפה של אורביטלי p. עם זאת, הוא חסר (4n + 2) אלקטרונים π ומאבד ארומטיות.

﻿קריטריונים לארומטיות ולחוק hückel 4n + 2

Criteria for Aromaticity and the H&uuml;ckel 4n + 2 Rule

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 	 כמו בנזן, ציקלובוטדיאן וציקלוקטטטראן הם תרכובות טבעתיות עם קשרים בודדים וכפולים לסירוגין. עם זאת, ההתנהגות הכימית שלהם שונה מבנזן, מכיוון שהם לא יציבים ולא ארומטיים. אז מה הם המאפיינים המבניים של תרכובות בלתי רוויות מסווגות כארומטיות? 

 	 בפעם הראשונה, אריק הוקל, פיזיקאי כימי גרמני, הסיק קבוצה של מאפיינים מבניים לתרכובת שתסווג כארומטית. זה ידוע כעת בתור הכלל של Hückel או הכלל 4n + 2. לפי כלל זה, תרכובת ארומטית חייבת להיות טבעתית, מישורית, מצומדת ברציפות, בעלת מספר אי זוגי של זוגות π אלקטרונים או 4n + 2  π אלקטרונים, כאשר n הוא מספר שלם (n = 0, 1, 2...). לכן, לתרכובת ארומטית צריך להיות 2 (n = 0), 6 (n = 1), 10 (n = 2), 14 (n = 3) וכו', מספר π אלקטרונים או 1, 3, 5, 7 וכו', זוגות של אלקטרונים π. 

 לדוגמה, ל-1,3,5-הקסטריאן ולבנזן יש 6 π אלקטרונים; גם אז, 1,3,5-הקסטריאן אינו ארומטי מכיוון שאורביטלי ה-p בשני אטומי הפחמן הטרמינלי אינם חופפים מכיוון שקשר אינו מחבר את הפחמנים הללו. לכן, תרכובות חייבות להיות טבעתיות כדי להיות ארומטיות. 

 <img alt="Figure1" src="/files/ftp_upload/12453/12453_PT-Figure_1.svg" style="height: 150px;"> 

 למרות שמדובר בתרכובת טבעתית בעלת שמונה איברים עם ארבעה קשרי π רציפים, cyclooctatetraene אינו יציב ואינו ארומטי. הסיבה לכך היא ש-cyclooctatetraene אינו מישורי; הוא מאמץ קונפורמציית סירה. כתוצאה מכך, חפיפה יעילה של אורביטלי p ליצירת לולאה סגורה של ענן האלקטרונים π אינה אפשרית. יש לציין, שלא כמו בנזן, הוא עובר תגובות סיפוח. 

 כדי שתרכובת תהיה מצומדת באופן רציף, היא חייבת לכלול את כל הפחמנים של sp^2 מוכלא עם אורביטל p לא מוכלא מלא למחצה בכל אחד מהם. לדוגמה, ל-1,3-cyclohexadiene ו-1,3,5-cycloheptatriene יש פחמנים מוכלאים sp^3 שאין להם אורביטל p לא מוכלא, כך שחפיפה רציפה אינה ניתנת להשגה. לכן, שניהם אינם ארומטיים. 

 <img alt="Figure2" src="/files/ftp_upload/12453/12453_PT-Figure_2.svg" style="height: 85px;"> 

 מצד שני, ציקלובוטדיאן, תרכובת טבעתית מישורית עם צימוד מתמשך, אינו יציב ואינו ארומטי. יש לו 2 זוגות של אלקטרונים π או 4 אלקטרונים π, שאינם עומדים בקריטריונים של 4n + 2 π אלקטרונים. לכן, הוא לא ארומטי. 
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﻿קריטריונים לארומטיות ולחוק Hückel 4n + 2

כמו בנזן, cyclobutadiene ו cyclooctatetraene הם תרכובות מחזוריות עם קשרים בודדים וכפולים לסירוגין. עם זאת, ההתנהגות הכימית שלהם שונה ...

criteria-for-aromaticity-and-the-hckel-4n-2-rule

Education

JoVE Core

Organic Chemistry

כימיה אורגנית

Unit 1

יחידה 1

Aromatic Compounds

תרכובות ארומטיות

KEY TERMS AND CONCEPTS

מונחים ומושגים מרכזיים

h&uuml;ckel rule and aromatic ring criteria

Aromatic rings follow H&amp;uuml;ckel’s rule, also called the 4n + 2 rule. This rule helps identify which unsaturated cyclic compounds are aromatic and why some ring systems are unstable instead. Benzene is the classic example, while cyclobutadiene and cyclooctatetraene show different behavior and are not aromatic.

Eric H&amp;uuml;ckel, a German chemical physicist, first described the structural features needed for aromaticity. An aromatic compound must be cyclic, planar, and continuously conjugated. It must also contain an odd number of pairs of pi electrons, which means it has 4n + 2 pi electrons, where n is an integer.

That electron pattern gives possible counts of 2, 6, 10, 14, and so on pi electrons. These are the same as 1, 3, 5, 7, and so on pairs of pi electrons. Benzene fits this pattern, and it is aromatic. Cyclobutadiene does not, even though it is cyclic and planar with continuous conjugation. It has 4 pi electrons, or 2 pairs of pi electrons, so it fails the 4n + 2 rule and is not aromatic.

A ring must also stay closed to count as aromatic. 1,3,5-Hexatriene has 6 pi electrons, like benzene, but it is not aromatic because it is not cyclic. The p orbitals on the two terminal carbon atoms do not overlap, since no bond connects those carbons.

Planarity is another key requirement. Cyclooctatetraene has four pi bonds in an eight-membered ring, but it is nonplanar and adopts a tub shape. Because of that shape, its p orbitals cannot overlap well enough to form a closed loop of pi electrons. It is unstable, not aromatic, and unlike benzene it undergoes addition reactions.

Continuous conjugation also matters. Every carbon in the ring must be sp2 hybridized and have a half-filled unhybridized p orbital. In 1,3-cyclohexadiene and 1,3,5-cycloheptatriene, some carbons are sp3 hybridized, so they do not have the needed p orbital. As a result, continuous overlap cannot happen, and both compounds are not aromatic.

H&uuml;ckel Rule and Aromatic Ring Criteria

aromatic compounds: overview

Aromatic Compounds in Plants and Fuels

Aromatic compounds are linked to both pleasant smells and important chemical structures. The word aromatic first came from fragrant natural sources such as fresh flowers and spices. Early organic chemists isolated benzene derivatives from oils of plants, including vanillin from vanilla, methyl salicylate from wintergreen, and cinnamaldehyde from cinnamon. The history of benzene also helped shape the idea of aromatic compounds. In 1825, Faraday isolated benzene from compressed illuminating gas,...

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Naming Benzene Derivatives with One Group

Benzene derivatives with one substituent are named by attaching the substituent name to benzene. Benzene is the simplest aromatic hydrocarbon, also called an arene. For simple monosubstituted benzene compounds, IUPAC naming uses the group name as a prefix. Halogen-substituted benzene compounds follow this pattern. The halogen prefix can be fluoro, chloro, bromo, or iodo. When an alkyl group is attached to the benzene ring, the name depends on the chain length of that group. If the alkyl chain...

nomenclature of aromatic compounds with multiple substituents

nomenclature-of-aromatic-compounds-with-multiple-substituents

Naming Benzene Rings with Multiple Groups

Naming benzene rings with multiple groups depends on how many substituents are attached. For benzene derivatives with two groups, three constitutional isomers are possible. The position of each group is described with IUPAC naming rules. For disubstituted benzene compounds, the relative positions can be shown with ortho, meta, and para. These terms are also written as o, m, and p. They can also be written with numbers, such as 1,2- for ortho, 1,3- for meta, and 1,4- for para. Dimethyl benzene,...

structure of benzene: kekul&eacute; model

Kekule’s Benzene Model and Its Limits

Kekule’s benzene model describes benzene as a six-carbon ring with alternating pi bonds. August Kekule proposed this structure in 1865 from the formula C6H6, the idea that all benzene hydrogens are equivalent, and the rule that each carbon has four bonds because of tetravalency. In this model, each carbon is attached to one hydrogen atom. It also predicts two isomers for 1,2-disubstituted benzene derivatives. In one form, the substituted carbons are linked by a double bond, and in the other...

structure of benzene: molecular orbital model

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Benzene π Orbitals and Electron Delocalization

Benzene has a planar ring structure with delocalized π electrons. Its six carbon atoms form a regular hexagon, and each carbon is sp2 hybridized. The C–C–C and H–C–C bond angles are 120°, which matches the flat shape of the ring. Each carbon is bonded to three other atoms. The C–H bond length is 109 pm, and the C–C bond length is 139 pm. That C–C distance is midway between the single bond length of sp3 hybridized carbons, 154 pm, and the sp2 value, 133 pm. Beneath and above the ring plane,...

criteria for aromaticity and the h&uuml;ckel 4n + 2 rule

Aromatic rings follow H&amp;uuml;ckel’s rule, also called the 4n + 2 rule. This rule helps identify which unsaturated cyclic compounds are aromatic and why some ring systems are unstable instead. Benzene is the classic example, while cyclobutadiene and cyclooctatetraene show different behavior and are not aromatic. Eric H&amp;uuml;ckel, a German chemical physicist, first described the structural features needed for aromaticity. An aromatic compound must be cyclic, planar, and continuously...

h&uuml;ckel's rule diagram of &pi; mos: frost circle

Frost Circle for Pi Orbital Energies

The Frost circle is a graphical method for showing the relative energies of pi molecular orbitals in planar, fully conjugated, monocyclic compounds. It is also called the inscribed polygon method. A. A. Frost and Boris Musulin first described this method in 1953. To build a Frost circle, draw a polygon with the same number of edges as the number of carbons in the cyclic system. Place one vertex pointing downward. Then draw a circle around the polygon so that every vertex touches the circle.

frost circles for different conjugated systems

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Using Frost Circles to Test Aromaticity

Frost circles help test whether a cyclic compound is aromatic by using the inscribed polygon method and Hückel’s 4n + 2 rule. A compound is stable and aromatic when every bonding molecular orbital (MO) is completely filled with a pair of electrons. If electrons occupy non-bonding or antibonding orbitals, the compound is unstable and not aromatic. Frost circle diagrams for cycloalkenes with 4 to 8 carbons show a clear pattern. The number of bonding MOs is always odd. The electrons needed to...

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Cyclopentadienyl Anion and Aromaticity

Cyclopentadiene is a neutral hydrocarbon that is not aromatic. It contains an odd number of carbon atoms and one CH2 group in the ring. With only 4 pi electrons, it does not follow the 4n + 2 pi electron rule. The CH2 carbon is sp3 hybridized, so it has no vacant p orbital to keep p orbitals overlapped around the ring. That break in overlap stops pi electrons from spreading across the entire ring. As a result, cyclopentadiene does not meet the usual aromaticity criteria. If one hydrogen is...

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Cycloheptatrienyl Cation and Aromaticity

Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon with seven carbon atoms. One carbon in the ring is sp3 hybridized, which breaks the continuous overlap of p orbitals. Even though the ring has three double bonds and 6 π electrons, cycloheptatriene is not aromatic because the p orbitals do not form one uninterrupted loop. Removing one hydrogen from the CH2 group changes the ring. If that hydrogen is removed with both, one, or no electrons, the products are the cycloheptatrienyl...

five-membered heterocyclic aromatic compounds: overview

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Five-Membered Aromatic Rings with Heteroatoms

Five-membered aromatic rings with heteroatoms are cyclic compounds that contain one or more atoms other than carbon in the ring. They are aromatic and can have either five or six atoms in the ring. Common examples include pyrrole, furan, thiophene, and imidazole. These rings differ by the heteroatom present. Pyrrole has one nitrogen atom with one lone pair of electrons. Furan has one oxygen atom, and thiophene has one sulfur atom. Both furan and thiophene have two lone pairs of electrons.

nmr spectroscopy of aromatic compounds

Aromatic Ring Shifts in NMR Spectra

Aromatic ring shifts in NMR spectra help identify aromatic compounds using proton NMR and carbon-13 NMR. Aromatic hydrogens are often strongly deshielded by the aromatic ring current, so they usually appear in the 6.5–8.0 ppm range in proton NMR. In benzene, the hydrogens directly bonded to the ring absorb at 7.3 ppm. Larger aromatic rings can place hydrogens outside the usual range. In [18]annulene, the 12 outer hydrogens are highly deshielded by the ring current and absorb downfield at 9.3...

key terms and concepts

Learn the H&uuml;ckel 4n + 2 rule and the key features of aromatic rings, including cyclic shape, planarity, continuous conjugation, and pi electrons.

H&uuml;ckel Rule and Aromatic Ring Criteria | JoVE K12

organic chemistry

JoVE Core Organic Chemistry explains basic concepts of stereochemistry, reactions, and mechanisms using concise and easy-to-understand animated video lessons. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

Hückel Rule and Aromatic Ring Criteria

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