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

El modelo de orbitales moleculares describe el benceno como un hexágono plano regular con seis carbonos hibridados sp2.
Los átomos de carbono adyacentes forman enlaces σ a través de la superposición orbital sp2–sp2. Cada átomo de carbono también forma un enlace σ con el hidrógeno por superposición orbitalsp 2-1 s. Todos los ángulos de unión son de 120°.
Además, cada carbono tiene un orbital atómico de 2p no hibridado medio lleno perpendicular al plano del anillo.
Estos seis orbitales2 p se combinan para formar tres orbitales moleculares de enlace y tres de antienlace.
Los orbitales de enlace están completamente llenos por los seis electrones π, mientras que los orbitales de enlace están vacíos, lo que le da al benceno una configuración de capa cerrada que confiere estabilidad.
En consecuencia, la densidad de electrones π se deslocaliza en forma de regiones en forma de rosquilla por encima y por debajo del plano del anillo.
La deslocalización también explica por qué solo se observa una longitud de enlace carbono-carbono para el benceno, con un valor entre las longitudes típicas de enlace simple y doble carbono-carbono.

estructura del benceno: modelo de orbital molecular

Structure of Benzene: Molecular Orbital Model

Según el modelo de orbital molecular (MO), el benceno tiene una estructura plana con un hexágono regular de seis carbonos con hibridación sp^2. Como se muestra en la Figura 1, cada carbono está unido a otros tres átomos con ángulos de enlace C – C – C y H – C – C de 120 °. La longitud del enlace C – H es 109 pm y la longitud del enlace C – C es 139 pm, que está a medio camino entre la longitud del enlace simple de los carbonos con hibridación sp^3 (154 pm) y los carbonos con hibridación sp^2 (133 pm).

<img alt="Figure1" src="/files/ftp_upload/12451/12451_PT_Figure_1.svg" style="text-align: center; height: 190px;">

 Los átomos de carbono también tienen un orbital atómico 2p no hibridado con un electrón, perpendicular al plano del anillo, que se superpone con los orbitales p de los carbonos vecinos, formando un bucle continuo de electrones π por encima y por debajo del plano del anillo. Según la teoría de los orbitales moleculares (MO), estos seis orbitales 2p se combinan para formar seis orbitales moleculares (MO) cíclicos π ψ_1, ψ_2, ψ_3, ψ_4, ψ_5 y ψ_6, como se muestra en la siguiente figura. Entre ellos, ψ_1, ψ_2 y ψ_3 son MO enlazantes, mientras que ψ_4, ψ_5 y ψ_6 son MO antienlazantes. Generalmente, la energía de los OM y el número de nodos aumenta de ψ_1 a ψ_6, mientras que la interacción de enlace disminuye. Sin embargo, estos MO de sistemas cíclicos se diferencian de los sistemas lineales como el 1,3-butadieno por tener dos MO degenerados.

<img alt="Figure2" src="/files/ftp_upload/12451/12451_PT_Figure_2.svg" style="text-align: center; height: 250px;">

 El MO de enlace de menor energía, ψ_1, no tiene nodos donde todos los orbitales estén en fase. El siguiente MO más bajo tiene un nodo y se puede representar de dos maneras, donde el plano nodal pasa a través de un enlace o un átomo. Estos dos OM enlazantes son degenerados y se representan como ψ_2 y ψ_3. De manera similar, los dos planos nodales pueden pasar a través de enlaces o átomos, proporcionando dos MO antienlazantes degenerados de ψ_4 y ψ_5. El MO final, ψ_6, tiene la energía más alta, con tres planos nodales que representan la combinación desfasada de todos los orbitales p.


 Los seis electrones π que forman una capa cerrada de densidad de electrones π deslocalizados por encima y por debajo del plano del anillo ocupan los tres OM de enlace, ψ_1, ψ_2 y ψ_3, cuyas energías son menores que las del orbital p aislado, lo que conduce a Estabilidad inusual del benceno.

Estructura del benceno: modelo de orbital molecular

De acuerdo con el modelo de orbitales moleculares (MO), el benceno tiene una estructura plana con un hexágono regular de seis carbonos hibridados sp2. Como se muestra en la Figura 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.

Benzene π Orbitals and Electron Delocalization

benzene-pi-orbitals-electron-delocalization

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

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 and Its Limits

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.

frost circles for different conjugated systems

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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.

nmr spectroscopy of aromatic compounds

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

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Benzene π Orbitals and Electron Delocalization

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