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

Das Molekülorbitalmodell beschreibt Benzol als ein regelmäßiges planares Sechseck mit sechs sp2-hybridisierten Kohlenstoffen.
Benachbarte Kohlenstoffatome bilden σ Bindungen über die Orbitalüberlappung sp2–sp2. Jedes Kohlenstoffatom bildet auch eine σ Bindung mit Wasserstoff durch die Orbitalüberlappung von sp2–1s. Alle Bindungswinkel betragen 120°.
Darüber hinaus weist jeder Kohlenstoff ein halbgefülltes, nicht hybridisiertes 2 p-Atomorbital senkrecht zur Ebene des Rings auf.
Diese sechs2-p-Orbitale verbinden sich zu drei Bindungs- und drei Antibindungsmolekülorbitalen.
Die Bindungsorbitale sind vollständig mit den sechs π Elektronen gefüllt, während die Antibindungsorbitale leer sind, was dem Benzol eine geschlossene Schalenkonfiguration verleiht, die Stabilität verleiht.
Folglich wird die π Elektronendichte in Form von ringförmigen Bereichen oberhalb und unterhalb der Ebene des Rings delokalisiert.
Die Delokalisierung erklärt auch, warum für Benzol nur eine Kohlenstoff-Kohlenstoff-Bindungslänge beobachtet wird, mit einem Wert zwischen typischen Kohlenstoff-Kohlenstoff-Einfach- und Doppelbindungslängen.

struktur von benzol: molekülorbitalmodell

Structure of Benzene: Molecular Orbital Model

Gemäß dem Molekülorbitalmodell (MO-Modell) hat Benzol eine planare Struktur mit einem regelmäßigen Sechseck aus sechs sp^2-hybridisierten Kohlensubstanzatomen. Wie in Abbildung 1 dargestellt, ist jeder Kohlensubstanz mit C-C-C- und H-C-C-Bindungswinkeln von 120° an drei andere Atome gebunden. Die C-H-Bindungslänge beträgt 109 pm und die C-C-Bindungslänge 139 pm, was in der Mitte zwischen der Einzelbindungslänge von sp^3-hybridisierten Kohlensubstanzen (154 pm) und sp^2-hybridisierten Kohlensubstanzen (133 pm) liegt.

<img alt="Figure1" src="/files/ftp_upload/12451/12451_PT_Figure_1.svg" style="text-align: center; height: 190px;">

Die Kohlensubstanzatome haben außerdem ein unhybridisiertes 2p-Atomorbital mit einem Elektron senkrecht zur Ringebene, das mit den p-Orbitalen benachbarter Kohlensubstanze überlappt und eine kontinuierliche Schleife aus π-Elektronen oberhalb und unterhalb der Ringebene bildet. Gemäß der Molekülorbitaltheorie (MO) verbinden sich diese sechs 2p-Orbitale zu sechs π-zyklischen Molekülorbitalen (MOs) ψ_1, ψ_2, ψ_3, ψ_4, ψ_5 und ψ_6, wie in der Abbildung unten dargestellt. Unter ihnen sind ψ_1, ψ_2 und ψ_3 bindende MOs, während ψ_4, ψ_5 und ψ_6 antibindende MOs sind. Im Allgemeinen nimmt die Energie der MOs und die Anzahl der Knoten von ψ_1 auf ψ_6 zu, während die Bindungswechselwirkung abnimmt. Allerdings unterscheiden sich diese MOs zyklischer Systeme von den linearen Systemen wie 1,3-Butadien dadurch, dass sie zwei entartete MOs aufweisen.

<img alt="Figure2" src="/files/ftp_upload/12451/12451_PT_Figure_2.svg" style="text-align: center; height: 250px;">

Das Bindungs-MO mit der niedrigsten Energie, ψ_1, hat keine Knotenpunkte, an denen alle Orbitale in Phase sind. Das nächstniedrigere MO hat einen Knoten und kann auf zwei Arten dargestellt werden, wobei die Knotenebene durch eine Bindung oder ein Atom verläuft. Diese beiden Bindungs-MOs sind entartet und werden als ψ_2 und ψ_3 dargestellt. In ähnlicher Weise können die beiden Knotenebenen durch Bindungen oder Atome verlaufen, wodurch zwei entartete antibindende MOs von ψ_4 und ψ_5 entstehen. Das endgültige MO, ψ_6, hat die höchste Energie, wobei drei Knotenebenen die phasenverschobene Kombination aller p-Orbitale darstellen.

Die sechs π-Elektronen, die oberhalb und unterhalb der Ringebene eine geschlossene Hülle aus delokalisierter π-Elektronendichte bilden, besetzen die drei Bindungs-MOs ψ_1, ψ_2 und ψ_3, deren Energien niedriger sind als die des isolierten p-Orbitals, was zu einer ungewöhnlichen Stabilität von Benzol führt.

Struktur von Benzol: Molekülorbitalmodell

Nach dem Molekülorbitalmodell (MO) hat Benzol eine planare Struktur mit einem regelmäßigen Sechseck aus sechs sp2-hybridisierten Kohlenstoffen. Wie in Abbildung 1 dargestellt, ...

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.

Benzene π Orbitals and Electron Delocalization

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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...

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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,...

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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...

structure of benzene: molecular orbital model

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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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.

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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...

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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...

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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...

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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.

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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

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

organic chemistry

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Benzene π Orbitals and Electron Delocalization

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