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

Il benzene, il ciclobutadiene e il cicloottatetraene sono rappresentati come un ibrido di due strutture equivalenti.
Tuttavia, a differenza del benzene, il ciclobutadiene e il cicloottatetraene sono instabili e non aromatici.
Per spiegare queste differenze, Erich Hückel ha delineato una serie di caratteristiche strutturali alla base dell'aromaticità, nota come regola di Hückel o regola del 4n + 2.
La regola di Hückel afferma che un composto aromatico deve essere ciclico e planare, con un orbitale p non ibridato su ciascun carbonio.
Gli orbitali p devono sovrapporsi per formare un ciclo continuo di (4n + 2) π elettroni, dove n è un numero intero positivo.
Il benzene soddisfa tutti i criteri per la regola di Hückel, stabilendo la sua aromaticità.
Al contrario, il cicloottatetraene assume una conformazione a vasca non planare senza sovrapposizione tra orbitali p adiacenti. Inoltre, manca (4n + 2) di elettroni π e quindi non è aromatico.
Allo stesso modo, il ciclobutadiene è ciclico, planare e ha una sovrapposizione continua di orbitali p. Tuttavia, manca (4n + 2) π elettroni e perde aromaticità.

criteri di aromaticità e regola di hückel 4n+2

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

Il ciclobutadiene e il cicloottatetraene sono composti ciclici con legami singoli e doppi alternati, come il benzene. Tuttavia, il loro comportamento chimico differisce da quello del benzene, poiché sono instabili e non aromatici. Quindi, quali sono le caratteristiche strutturali dei composti insaturi classificati come aromatici?

Per la prima volta Eric Hückel, un fisico chimico tedesco, derivò una serie di caratteristiche strutturali affinché un composto potesse essere classificato come aromatico. Questa è ora conosciuta come regola di Hückel o regola 4n + 2. Secondo questa regola, un composto aromatico deve essere ciclico, planare, continuamente coniugato, avente un numero dispari di coppie di elettroni π oppure 4n + 2 elettroni π, dove n è un numero intero (n = 0, 1, 2…). Pertanto, un composto aromatico dovrebbe avere 2 (n = 0), 6 (n = 1), 10 (n = 2), 14 (n = 3), ecc…, elettroni π oppure 1, 3, 5, 7 , ecc…, coppie di elettroni π.

Ad esempio, l'1,3,5-esatriene e il benzene hanno 6 elettroni π; anche in questo caso, l'1,3,5-esatriene non è aromatico poiché gli orbitali p sui due atomi di carbonio terminali non si sovrappongono poiché un legame non collega questi carboni. Pertanto, i composti devono essere ciclici per essere aromatici.

<img alt="Figure1" src="/files/ftp_upload/12453/12453_PT-Figure_1.svg" style="text-align: center; height: 150px;">

Sebbene sia un composto ciclico a otto membri con quattro legami π continui, il cicloottatetraene è instabile e non aromatico. Questo perché il cicloottatetraene non è planare; adotta una conformazione a vasca. Di conseguenza, una sovrapposizione efficace degli orbitali p per formare un anello chiuso della nuvola di elettroni π non è fattibile. In particolare il cicloottatetraene, a differenza del benzene, subisce reazioni di addizione.

Affinché un composto possa essere coniugato continuamente, deve avere tutti gli atomi di carbonio dell'ibridazione sp^2 con un orbitale p non ibridato riempito a metà su ciascuno. Ad esempio, l'1,3-cicloesadiene e l'1,3,5-cicloeptatriene hanno carboni ibridati sp^3 che non hanno un orbitale p non ibridato, quindi la sovrapposizione continua è impossibile. Entrambi non sono, quindi, aromatici.

<img alt="Figure2" src="/files/ftp_upload/12453/12453_PT-Figure_2.svg" style="text-align: center; height: 85px;">

D'altra parte, il ciclobutadiene, un composto ciclico e planare con coniugazione continua, è instabile e non aromatico. Avendo 2 coppie di elettroni π o 4 elettroni π, il criterio di avere 4n + 2 elettroni π non è soddisfatto. Pertanto non è aromatico.

Criteri di Aromaticità e Regola di Hückel 4n+2

Come il benzene, il ciclobutadiene e il cicloottatetraene sono composti ciclici con legami singoli e doppi alternati. Tuttavia, il loro comportamento chimico differisce ...

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

Education

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JoVE Core

Organic Chemistry

Chimica Organica

Unit 1

Unità 1

Aromatic Compounds

Composti aromatici

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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

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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 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...

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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...

aromatic hydrocarbon anions: structural overview

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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...

aromatic hydrocarbon cations: structural overview

aromatic-hydrocarbon-cations-structural-overview

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

five-membered-heterocyclic-aromatic-compounds-overview

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

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Hückel Rule and Aromatic Ring Criteria

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