Name: benzene π orbitals and electron delocalization
Uploaded: 2025-05-22T09:05:38.000Z
Duration: 1 min 18 s
Channel: JoVE

Le modèle orbital moléculaire décrit le benzène comme un hexagone plan régulier avec six carbones hybrides sp2.
Les atomes de carbone adjacents forment des liaisons σ via le chevauchement orbital sp2-sp 2. Chaque atome de carbone forme également une liaison σ avec l’hydrogène par chevauchementorbital sp 2–1. Tous les angles de liaison sont de 120°.
De plus, chaque carbone a une orbitale atomique 2p non hybridée semi-remplie perpendiculaire au plan de l’anneau.
Ces six orbitales 2p se combinent pour former trois orbitales moléculaires liantes et trois orbitales moléculaires antiliantes.
Les orbitales liantes sont complètement remplies par les six électrons π, tandis que les orbitales antiliantes sont vides, ce qui donne au benzène une configuration à couche fermée qui confère une stabilité.
Par conséquent, la densité électronique π est délocalisée sous la forme de régions en forme de beignet au-dessus et au-dessous du plan de l’anneau.
La délocalisation explique également pourquoi une seule longueur de liaison carbone-carbone est observée pour le benzène, avec une valeur comprise entre les longueurs typiques de liaison simple et double carbone-carbone.

structure du benzène : modèle orbital moléculaire

Structure of Benzene: Molecular Orbital Model

Selon le modèle d'orbitale moléculaire (MO), le benzène a une structure planaire avec un hexagone régulier de six carbones hybridés sp2. Comme le montre la figure 1, chaque carbone est lié à trois autres atomes avec des angles de liaison C-C-C et H-C-C de 120°. La longueur de la liaison C – H est de 109 pm et la longueur de la liaison C – C est de 139 pm, ce qui est à mi-chemin entre la longueur de la liaison simple des carbones hybridés sp3 (154 pm) et des carbones hybridés sp2 (133 pm).

<img alt="Figure1" src="/files/ftp_upload/12451/12451_PT_Figure_1.svg" style="text-align: center; height: 190px;">

Les atomes de carbone ont également une orbitale atomique 2p non hybridée avec un électron, perpendiculaire au plan de l'anneau, qui chevauche les orbitales p des carbones voisins, formant une boucle continue d'électrons π au-dessus et au-dessous du plan de l'anneau. Selon la théorie des orbitales moléculaires (MO), ces six orbitales 2p se combinent pour former six orbitales moléculaires cycliques (MO) π cyclic molecular orbitals (MOs) ψ1, ψ2, ψ3, ψ4, ψ5, et ψ6, comme le montre la figure ci-dessous. Parmi eux, ψ1, ψ2 et ψ3 sont des liaisons, tandis que ψ4, ψ5 et ψ6 sont des MO anti-liantes. Généralement, l'énergie des MO et le nombre de nœuds augmentent de ψ1 à ψ6,  alors que l'interaction de liaison diminue. Cependant, ces MO de systèmes cycliques diffèrent des systèmes linéaires tels que le 1,3-butadiène en ayant deux MO dégénérées.

<img alt="Figure2" src="/files/ftp_upload/12451/12451_PT_Figure_2.svg" style="text-align: center; height: 250px;">

La liaison MO de plus basse énergie, ψ1, n'a pas de nœuds où toutes les orbitales sont en phase. Le MO le plus bas suivant a un nœud et peut être représenté de deux manières, où le plan nodal passe par une liaison ou un atome. Ces deux MO de liaison sont dégénérés et représentés par ψ2 et ψ3. De même, les deux plans nodaux peuvent traverser des liaisons ou des atomes, fournissant deux MO antiliantes dégénérées de ψ4 et ψ5. Le MO final, ψ6,, a l'énergie la plus élevée, avec trois plans nodaux représentant la combinaison déphasée de toutes les orbitales p.

Les six électrons π qui forment une couche fermée de densité électronique π délocalisée au-dessus et au-dessous du plan de l'anneau occupent les trois MO de liaison, ψ1, ψ2 et ψ3, dont les énergies sont inférieures à celle de l'orbitale p isolée, conduisant ainsi à stabilité inhabituelle du benzène.

Structure du benzène : modèle orbital moléculaire

Selon le modèle de l’orbitale moléculaire (MO), le benzène a une structure plane avec un hexagone régulier de six carbones hybridés sp2. Comme le montre la figure 1, ...

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, each carbon also has one unhybridized 2p atomic orbital with one electron. These p orbitals stand perpendicular to the ring and overlap with neighboring p orbitals. Together, they form a continuous loop of π electron density above and below the ring.

In molecular orbital theory, the six 2p orbitals combine to make six cyclic π molecular orbitals: ψ1, ψ2, ψ3, ψ4, ψ5, and ψ6. The first three, ψ1 through ψ3, are bonding molecular orbitals. The last three, ψ4 through ψ6, are antibonding molecular orbitals.

The energy of the molecular orbitals rises from ψ1 to ψ6, and the number of nodes also increases. As the nodes increase, the bonding interaction decreases. Cyclic systems differ from linear systems such as 1,3-butadiene because they have two degenerate molecular orbitals, or orbitals with the same energy.

The lowest-energy bonding orbital, ψ1, has no nodes and places all orbitals in phase. The next bonding level has one node and can be drawn in two ways, with the nodal plane passing through a bond or through an atom. These two orbitals are degenerate and are labeled ψ2 and ψ3.

The same idea applies to the antibonding set. Two nodal planes can pass through bonds or atoms, giving two degenerate antibonding orbitals, ψ4 and ψ5. The final orbital, ψ6, has the highest energy and three nodal planes. It represents the out-of-phase combination of all p orbitals.

Benzene has six π electrons, and they occupy the three bonding orbitals, ψ1, ψ2, and ψ3. This creates a closed shell of delocalized π electron density above and below the ring. Because these electrons are in orbitals lower in energy than an isolated p orbital, benzene shows unusual stability.

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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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key terms and concepts

Explore benzene’s planar ring, sp2 carbons, and six π molecular orbitals. Learn how delocalized electrons and bonding orbitals give benzene its stability.

Benzene π Orbitals and Electron Delocalization | JoVE K12

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Benzene π Orbitals and Electron Delocalization

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