Name: kekule’s benzene model and its limits
Uploaded: 2023-04-30T10:30:54.000Z
Duration: 1 min 7 s
Channel: JoVE

בשנת 1865 הציע אוגוסט קקולה את המבנה המוגדר הראשון לבנזן.
הוא הציע שלבנזן יש מבנה מחזורי של שישה אטומי פחמן עם קשרים בודדים וכפולים לסירוגין. כל אטום פחמן מחובר לאטום מימן אחד.
מודל זה סיפק את הטטרוולנטיות של פחמן, מה שהופך את כל המימנים לשווי ערך.
המודל של קקולה חזה שתי צורות של 1,2 -dibromobenzene, אשר נבדלו במיקומים של הקשרים הכפולים. אבל, רק צורה אחת של 1,2-dibromobenzene בודדה.
קקולה הגן עליה על ידי הצעת שיווי משקל מהיר בין שתי צורות בלתי נפרדות של בנזן, הידועות כיום כמבני קקולה. עם זאת, כיום ידוע כי לא קיים שיווי משקל כזה.
יתר על כן, הוא לא הצליח להסביר את ההתנהגות הכימית יוצאת הדופן של בנזן, שעובר תגובת החלפה עם ברום, בניגוד לאלקנים, שעוברים תגובת תוספת.

﻿מבנה הבנזן: דגם kekulé

Structure of Benzene: Kekul&eacute; Model

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 	 בשנת 1865, אוגוסט קקולה הציע את המבנה של בנזן לפי התיאוריה המבנית של כימיה אורגנית בהתבסס על שלוש הקביעות - הנוסחה של בנזן היא C_6H_6, כל המימנים של בנזן שווים, ולכל פחמן חייבים להיות ארבעה קשרים בשל tetravalency שלו. 

 	 הוא הציע שלבנזן יש מבנה טבעתי של שישה אטומי פחמן המחוברים לאטום מימן אחד כל אחד, עם שלושה קשרי pi לסירוגין. 

 <img alt="Figure1" src="/files/ftp_upload/12450/12450_PT_Figure1.svg" style="height: 100px;"> 

 על פי המודל שלו, חייבים להיות שני איזומרים לנגזרות הבנזן המופרעות ב-1,2, כפי שמוצג באיור 2, שבו במבנה אחד, שני הפחמנים המותמרים מחוברים בקשר כפול אך בקשר יחיד באחר. 

 <img alt="Figure2" src="/files/ftp_upload/12450/12450_PT_Figure2.svg" style="height: 100px;"> 

 מושג זה של Kekule הוכח כשגוי מכיוון שנמצא רק מבנה אחד עבור בנזן 1,2-לא-מותמר. כדי להסביר זאת, הוא קבע ששתי צורות הבנזן נמצאות בשיווי משקל מהיר ושלא ניתן להפריד בין שתי התרכובות. 

 <img alt="Figure3" src="/files/ftp_upload/12450/12450_PT_Figure3.svg" style="height: 100px;"> 

 עם זאת, לא קיים שיווי משקל כזה, ונמצא רק מבנה אחד לנגזרות בנזן 1,2-בלתי-מותמרות. 

 <img alt="Figure4" src="/files/ftp_upload/12450/12450_PT_Figure4.svg" style="height: 80px;"> 

 לבנזן יש ארבע דרגות של אי רוויון, המהוות טבעת אחת ושלושה קשרי pi. עם זאת, בנזן אינו עובר תגובות סיפוח, בניגוד לפחמימנים בלתי רוויים אחרים כמו אלקנים ואלקינים. לדוגמה, אתילן עובר תגובת סיפוח עם ברום ליצירת אתילן דיברומיד, אבל בנזן לא. 

 <img alt="Figure5" src="/files/ftp_upload/12450/12450_PT_Figure5.svg" style="height: 120px;"> 

 <img alt="Figure6" src="/files/ftp_upload/12450/12450_PT_Figure6.svg" style="height: 60px;"> 

 בנזן אכן מגיב עם ברום ליצירת ברומובנזן באמצעות תגובת התמרה בנוכחות חומצת לואיס ברזל ברומיד. 

 <img alt="Figure7" src="/files/ftp_upload/12450/12450_PT_Figure7.svg" style="height: 90px;"> 

 החיסרון העיקרי של המודל של Kekule הוא שהוא לא הצליח להסביר את התגובתיות החריגה הזו של בנזן בהשוואה לפחמימנים בלתי רוויים אחרים כמו אלקנים ואלקינים. 
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﻿מבנה הבנזן: דגם Kekulé

בשנת 1865, אוגוסט Kekule הציע את המבנה של בנזן על פי התאוריה המבנית של כימיה אורגנית המבוססת על שלושת ...

Education

JoVE Core

Organic Chemistry

כימיה אורגנית

Unit 1

יחידה 1

Aromatic Compounds

תרכובות ארומטיות

KEY TERMS AND CONCEPTS

מונחים ומושגים מרכזיים

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 they are linked by a single bond.

That prediction did not match experimental results. Only one structure is found for 1,2-disubstituted benzene derivatives. Kekule tried to explain this by saying the two forms of benzene stay in rapid equilibrium and cannot be separated, but no such equilibrium exists.

Benzene has four degrees of unsaturation, which means one ring and three pi bonds. Even so, it does not undergo addition reactions the way alkenes and alkynes do. Ethylene, for example, adds bromine to form ethylene dibromide, but benzene does not.

Instead, benzene reacts with bromine by substitution when ferric bromide, a Lewis acid, is present. The main weakness of Kekule’s model is that it does not explain this unusual reactivity compared with other unsaturated hydrocarbons.

Kekule’s Benzene Model and Its Limits

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...

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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 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,...

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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...

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 how Kekule proposed benzene’s ring structure, why it predicted two isomers, and why the model could not explain benzene’s unusual reactivity.

Kekule’s Benzene Model and Its Limits | 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.

Kekule’s Benzene Model and Its Limits

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