Name: using frost circles to test aromaticity
Uploaded: 2025-05-22T09:05:43.000Z
Duration: 1 min 18 s
Channel: JoVE

Selon la règle de Huckel, le benzène est stable et aromatique, tandis que le cyclobutadiène et le cyclooctatétraène sont instables et non aromatiques.
L’aromaticité de ces composés peut être comprise à partir de leurs diagrammes circulaires de givre.
Le cercle de Frost pour le benzène a six orbitales moléculaires π. Toutes les orbitales liantes de basse énergie sont entièrement occupées par six électrons π, tandis que les orbitales antiliantes de haute énergie sont vides. Par conséquent, le benzène est stable et aromatique.
Les cercles de Frost pour le cyclobutadiène et le cyclooctatétraène ont respectivement quatre et huit orbitales moléculaires π.
Les orbitales de liaison sont entièrement occupées pour les deux composés, tandis que les orbitales non liantes sont occupées individuellement. Les orbitales antiliantes sont vides.
Comme il est évident, la présence de π électrons dans les orbitales non liantes de haute énergie rend les deux composés instables et non aromatiques.
Dans l’ensemble, une molécule donnée est considérée comme aromatique lorsque tous les électrons π sont appariés dans des orbitales moléculaires liantes. Les composés avec des électrons dans des orbitales autres que la liaison ne sont pas aromatiques.

cercles de givre pour différents systèmes conjugués

Frost Circles for Different Conjugated Systems

La méthode des polygones inscrits est cohérente avec la règle 4n + 2 de Hückel et permet de savoir si le composé cyclique donné est aromatique ou non. Le composé est stable et aromatique si chaque orbitale moléculaire de liaison (MO) est complètement remplie d’une paire d’électrons. Cependant, si les orbitales non liantes ou antiliantes sont remplies d’électrons, le composé est instable et non aromatique. Considérez les diagrammes du cercle de Frost pour les cycloalcènes contenant 4 à 8 carbones.

<img alt="Figure1" src="/files/ftp_upload/12866/12866_Figure_1.svg" style="text-align: center; height: 75px;">

En regardant les cercles de Frost, on observe que le nombre de MO de liaison est toujours impair et que le nombre requis d'électrons π dans le MO de liaison est soit 2, soit 6. Le nombre d'électrons satisfait parfaitement à la règle d'aromaticité 4n + 2. . Par conséquent, les composés avec une liaison MO complètement remplie sont aromatiques.

Dans le cas du cyclobutadiène à quatre chaînons avec des électrons 4π et du cyclooctatétraène à huit chaînons avec des électrons 8π, les MO de liaison sont complètement remplis et chacun des MO non liants est occupé individuellement. La présence d'électrons dans les MO non liants à haute énergie les rend instables et non aromatiques. Cependant, l'anion cyclopentadiényle à cinq chaînons, le benzène à six chaînons et le cation cycloheptatrienyle à sept chaînons ont des électrons 6π, et les MO de liaison correspondants sont entièrement occupés et stables, ils sont donc aromatiques.

Dans l’ensemble, les molécules ayant des orbitales moléculaires de liaison complètement remplies sont considérées comme aromatiques, alors que les composés avec des électrons dans des orbitales autres que la liaison ne sont pas aromatiques.

Cercles de givre pour différents systèmes conjugués

La méthode des polygones inscrits est cohérente avec la règle 4n + 2 de Hückel et aide à savoir si le composé cyclique donné est aromatique ou non.

frost-circles-for-different-conjugated-systems

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Organic Chemistry

Chimie organique

Unit 1

Unité 1

Aromatic Compounds

Composés aromatiques

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

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 fill only the bonding MOs are usually 2 or 6, and that matches the 4n + 2 rule for aromaticity. When those bonding orbitals are completely filled, the compound is aromatic.

Some examples do not fit this stable pattern. Cyclobutadiene has 4π electrons, and cyclooctatetraene has 8π electrons. In both cases, the bonding MOs are filled, but each non-bonding MO is singly occupied. Electrons in these higher-energy non-bonding orbitals make the compounds unstable and not aromatic.

Other examples do satisfy the aromatic pattern. The cyclopentadienyl anion, benzene, and the cycloheptatrienyl cation each have 6π electrons. Their bonding MOs are fully occupied, so they are stable and aromatic. In general, molecules with completely filled bonding molecular orbitals are aromatic, while molecules with electrons in orbitals other than bonding are not.

Using Frost Circles to Test Aromaticity

aromatic compounds: overview

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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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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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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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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

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

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Cycloheptatrienyl Cation and Aromaticity

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key terms and concepts

Learn how Frost circles and the inscribed polygon method use Hückel’s 4n + 2 rule to identify aromatic cyclic compounds and unstable non-aromatic cases.

Using Frost Circles to Test Aromaticity | JoVE K12

organic chemistry

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Using Frost Circles to Test Aromaticity

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