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

Le benzène, le cyclobutadiène et le cyclooctatétraène sont représentés sous la forme d’un hybride de deux structures équivalentes.
Cependant, contrairement au benzène, le cyclobutadiène et le cyclooctatétraène sont instables et non aromatiques.
Pour expliquer ces différences, Erich Hückel a décrit un ensemble de caractéristiques structurelles sous-jacentes à l'aromaticité, connues sous le nom de règle de Hückel ou règle 4n + 2.
La règle de Hückel stipule qu'un composé aromatique doit être cyclique et planaire, avec une orbitale p non hybridée sur chaque carbone.
Les orbitales p doivent se chevaucher pour former une boucle continue de (4n + 2) π électrons, où n est un entier positif.
Le benzène remplit tous les critères de la règle de Hückel, établissant son aromaticité.
En revanche, le cyclooctatétraène suppose une conformation de cuve non plane sans chevauchement entre les orbitales p adjacentes. De plus, il lui manque (4n + 2) π électrons et n’est donc pas aromatique.
De même, le cyclobutadiène est cyclique, planaire et a un chevauchement continu des orbitales p. Cependant, il lui manque (4n + 2) π électrons et perd de son aromaticité.

critères d'aromaticité et règle de hückel 4n + 2

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

Comme le benzène, le cyclobutadiène et le cyclooctatétraène sont des composés cycliques avec des liaisons simples et doubles alternées. Cependant, leur comportement chimique diffère de celui du benzène, car ils sont instables et non aromatiques. Alors, quelles sont les caractéristiques structurelles des composés insaturés classés comme aromatiques ?

Pour la première fois, Eric Hückel, un physicien chimiste allemand, a dérivé un ensemble de caractéristiques structurelles pour qu'un composé soit classé comme aromatique. C’est ce qu’on appelle désormais la règle de Hückel ou la règle 4n + 2. Selon cette règle, un composé aromatique doit être cyclique, plan, conjugué de manière continue, possédant un nombre impair de paires d'électrons π ou 4n + 2 électrons π, où n est un nombre entier (n = 0, 1, 2…). Par conséquent, un composé aromatique doit avoir 2 (n = 0), 6 (n = 1), 10 (n = 2), 14 (n = 3), etc., nombre d'électrons π ou 1, 3, 5, 7 , etc., des paires d'électrons π.

Par exemple, le 1,3,5-hexatriène et le benzène ont 6 électrons π ; même dans ce cas, le 1,3,5-hexatriène n'est pas aromatique car les orbitales p sur les deux atomes de carbone terminaux ne se chevauchent pas car aucune liaison ne relie ces carbones. Les composés doivent donc être cycliques pour être aromatiques.

<img alt="Figure1" src="/files/ftp_upload/12453/12453_PT-Figure_1.svg" style="text-align: center; height: 150px;">

Bien qu'il s'agisse d'un composé cyclique à huit chaînons avec quatre liaisons π continues, le cyclooctatétraène est instable et non aromatique. En effet, le cyclooctatétraène n'est pas plan ; il adopte une conformation en baignoire. Par conséquent, un chevauchement efficace des orbitales p pour former une boucle fermée du nuage d’électrons π n’est pas réalisable. Notamment, contrairement au benzène, il subit des réactions d'addition.

Pour qu’un composé soit conjugué en continu, il doit avoir tous les carbones de l’hybridation sp2 avec une orbitale p non hybridée à moitié remplie sur chacun. Par exemple, le 1,3-cyclohexadiène et le 1,3,5-cycloheptatriène ont des carbones hybridés sp3 qui n'ont pas d'orbitale p non hybridée, un chevauchement continu est donc impossible. Par conséquent, ils ne sont pas tous deux aromatiques.

<img alt="Figure2" src="/files/ftp_upload/12453/12453_PT-Figure_2.svg" style="text-align: center; height: 85px;">

D'autre part, le cyclobutadiène, un composé planaire cyclique à conjugaison continue, est instable et non aromatique. Il possède 2 paires d'électrons π ou 4 électrons π, ce qui ne satisfait pas au critère d'avoir 4n + 2 électrons π. Il n’est donc pas aromatique.

Critères d'aromaticité et règle de Hückel 4n + 2

Comme le benzène, le cyclobutadiène et le cyclooctatétraène sont des composés cycliques avec une alternance de liaisons simples et doubles. Cependant, leur comportement chimique diffère...

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

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Organic Chemistry

Chimie organique

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Aromatic Compounds

Composés aromatiques

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

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

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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...

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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.

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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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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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