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

O modelo orbital molecular descreve o benzeno como um hexágono planar regular com seis carbonos hibridizados sp2.
Os átomos de carbono adjacentes formam ligações σ via sobreposição orbital sp2–sp2. Cada átomo de carbono também forma uma ligação σ com o hidrogênio por sobreposição orbital sp2–1s. Todos os ângulos de ligação são de 120°.
Além disso, cada carbono tem um orbital atômico 2p não hibridizado semipreenchido perpendicular ao plano do anel.
Esses seis orbitais 2p se combinam para formar três orbitais moleculares ligantes e três antiligantes.
Os orbitais de ligação são completamente preenchidos pelos seis elétrons π, enquanto os orbitais antiligantes estão vazios, dando ao benzeno uma configuração de camada fechada que confere estabilidade.
Consequentemente, a densidade eletrônica π é deslocalizada na forma de regiões em forma de rosquinha acima e abaixo do plano do anel.
A deslocalização também explica por que apenas um comprimento de ligação carbono-carbono é observado para o benzeno, com um valor entre os comprimentos típicos de ligação simples e dupla carbono-carbono.

estrutura do benzeno: modelo de orbital molecular

Structure of Benzene: Molecular Orbital Model

De acordo com o modelo de orbital molecular (MO), o benzeno possui uma estrutura plana com um hexágono regular de seis carbonos hibridizados sp^2. Conforme mostrado na Figura 1, cada carbono está ligado a três outros átomos com ângulos de ligação C–C–C e H–C–C de 120°. O comprimento da ligação C–H é 109 pm, e o comprimento da ligação C–C é 139 pm, que está a meio caminho entre o comprimento da ligação simples de carbonos hibridizados sp^3 (154 pm) e carbonos hibridizados sp^2 (133 pm).

<img alt="Figure1" src="/files/ftp_upload/12451/12451_PT_Figure_1.svg" style="text-align: center; height: 190px;">

Os átomos de carbono também possuem um orbital atômico 2p não hibridizado com um elétron, perpendicular ao plano do anel, que se sobrepõe aos orbitais p dos carbonos vizinhos, formando um loop contínuo de elétrons π acima e abaixo do plano do anel. De acordo com a teoria dos orbitais moleculares (MO), esses seis orbitais 2p se combinam para formar seis orbitais moleculares (MOs) π cíclicos ψ_1, ψ_2, ψ_3, ψ_4, ψ_5 e ψ_6, conforme mostrado na figura abaixo. Entre eles, ψ_1, ψ_2 e ψ_3 são ligantes, enquanto ψ_4, ψ_5 e ψ_6 são MOs antiligantes. Geralmente, a energia dos MOs e o número de nós aumentam de ψ_1 para ψ_6, enquanto a interação de ligação diminui. No entanto, esses MOs de sistemas cíclicos diferem dos sistemas lineares como o 1,3-butadieno por terem dois MOs degenerados.

<img alt="Figure2" src="/files/ftp_upload/12451/12451_PT_Figure_2.svg" style="text-align: center; height: 250px;">

O MO ligante de menor energia, ψ_1, não possui nós onde todos os orbitais estejam em fase. O próximo MO de menor energia tem um nó e pode ser representado de duas maneiras, onde o plano nodal passa por uma ligação ou átomo. Esses dois MOs ligantes são degenerados e representados como ψ_2 e ψ_3. Da mesma forma, os dois planos nodais podem passar por ligações ou átomos, fornecendo dois MOs antiligantes degenerados de ψ_4 e ψ_5. O MO final, ψ_6, tem a energia mais alta, com três planos nodais representando a combinação fora de fase de todos os orbitais p.

Os seis elétrons π que formam uma camada fechada de densidade eletrônica π deslocalizada acima e abaixo do plano do anel ocupam os três MOs ligantes, ψ_1, ψ_2 e ψ_3, cujas energias são inferiores às do orbital p isolado, levando assim à estabilidade incomum do benzeno.

Estrutura do Benzeno: Modelo de Orbital Molecular

De acordo com o modelo de orbital molecular (MO), o benzeno tem uma estrutura plana com um hexágono regular de seis carbonos hibridizados sp2. Conforme mostrado na 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.

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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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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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