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

Model orbitalu molekularnego opisuje benzen jako regularny płaski sześciokąt z sześcioma zhybrydyzowanymi atomami węgla sp2.
Sąsiednie atomy węgla tworzą wiązania σ poprzez nakładanie się orbit sp2–sp2. Każdy atom węgla tworzy również wiązanie σ z wodorem poprzez nakładanie się orbit sp2-1 s. Wszystkie kąty wiązania wynoszą 120°.
Ponadto każdy węgiel ma do połowy wypełniony niezhybrydyzowany orbital atomowy 2p prostopadły do płaszczyzny pierścienia.
Te sześć orbitali 2p łączy się, tworząc trzy orbitale molekularne wiążące i trzy antywiążące.
Orbitale wiążące są całkowicie wypełnione przez sześć elektronów π, podczas gdy orbitale antywiążące są puste, co nadaje benzenowi konfigurację zamkniętej powłoki, która zapewnia stabilność.
W związku z tym gęstość elektronów π jest zdelokalizowana w postaci obszarów w kształcie pączka powyżej i poniżej płaszczyzny pierścienia.
Delokalizacja wyjaśnia również, dlaczego dla benzenu obserwuje się tylko jedną długość wiązania węgiel-węgiel, z wartością między typowymi pojedynczymi i podwójnymi długościami wiązań węgiel-węgiel.

struktura benzenu: model orbitali molekularnych

Structure of Benzene: Molecular Orbital Model

Zgodnie z modelem orbitali molekularnych (MO), benzen ma strukturę planarną z regularnym sześciokątem złożonym z sześciu zhybrydyzowanych węgli sp^2.

Jak pokazano na rysunku 1, każdy węgiel jest związany z trzema innymi atomami przy kącie wiązania C – C – C i H – C – C wynoszącym 120°. Długość wiązania C – H wynosi 109 µm, a długość wiązania C – C wynosi 139 µm, czyli jest w połowie odległości pomiędzy długością pojedynczego wiązania węgli zhybrydyzowanych sp^3 (154 µm) i węgli zhybrydyzowanych sp^2 (133 µm).

<img alt="Figure1" src="/files/ftp_upload/12451/12451_PT_Figure_1.svg" style="text-align: center; height: 190px;">

Atomy węgla mają również niezhybrydyzowany orbital atomowy 2p z jednym elektronem, prostopadły do płaszczyzny pierścienia, który pokrywa się z orbitalami p sąsiednich węgli, tworząc ciągłą pętlę elektronów π powyżej i poniżej płaszczyzny pierścienia. Zgodnie z teorią orbitali molekularnych (MO) te sześć orbitali 2p łączy się, tworząc sześć cyklicznych orbitali molekularnych π (MO) ψ_1, ψ_2, ψ_3, ψ_4, ψ_5 i ψ_6, jak pokazano na poniższym rysunku. Wśród nich ψ_1, ψ_2 i ψ_3 wiążą się, podczas gdy ψ_4, ψ_5 i ψ_6 są antywiążącymi MO. Ogólnie rzecz biorąc, energia MO i liczba węzłów wzrasta od ψ_1 do ψ_6, podczas gdy oddziaływanie wiązania maleje. Jednakże te MO układów cyklicznych różnią się od układów liniowych, takich jak 1,3-butadien, tym, że mają dwa zdegenerowane MO.

<img alt="Figure2" src="/files/ftp_upload/12451/12451_PT_Figure_2.svg" style="text-align: center; height: 250px;">

Wiązanie MO o najniższej energii, ψ_1, nie ma węzłów, w których wszystkie orbitale są w fazie. Następny najniższy MO ma jeden węzeł i można go przedstawić na dwa sposoby, gdzie płaszczyzna węzłowa przechodzi przez wiązanie lub atom. Te dwa MO wiążące są zdegenerowane i reprezentowane jako ψ_2 i ψ_3.Podobnie, dwie płaszczyzny węzłowe mogą przechodzić przez wiązania lub atomy, zapewniając dwa zdegenerowane anty-wiążące MO ψ_4 i ψ_5. Końcowy MO, ψ_6, ma najwyższą energię, a trzy płaszczyzny węzłowe reprezentują kombinację poza fazą wszystkich orbitali p.

Sześć elektronów π tworzących zamkniętą powłokę o zdelokalizowanej gęstości elektronów π powyżej i poniżej płaszczyzny pierścienia zajmuje trzy wiążące MO, ψ_1, ψ_2 i ψ_3, których energie są niższe niż izolowany orbital p, co prowadzi do niezwykła stabilność benzenu.

Struktura benzenu: model orbitali molekularnych

Zgodnie z modelem orbitalu molekularnego (MO), benzen ma płaską strukturę z regularnym sześciokątem sześciu zhybrydyzowanych atomów węgla sp2. Jak pokazano na rysunku 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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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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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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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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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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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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