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

分子軌道モデルは、ベンゼンを6つのsp2混成炭素を持つ正則平面六角形として記述しています。
隣接する炭素原子は、sp2–sp2軌道の重なりを介してσ結合を形成します。各炭素原子は、sp2–1s軌道の重なりによって水素とσ結合も形成します。すべてのボンド角度は120°です。
さらに、各炭素は、リングの平面に垂直な半分に充填された非ハイブリダイゼーション2p原子軌道を有する。
これらの6つの2p軌道が組み合わさって、3つの結合性分子軌道と3つの反結合性分子軌道を形成します。
結合性軌道は6つのπ電子によって完全に満たされていますが、反結合性軌道は空であるため、ベンゼンは安定性をもたらす閉殻構造になっています。
その結果、π電子密度は、リングの平面の上下にドーナツ状の領域の形で非局在化されます。
また、ベンゼンでは炭素-炭素結合長が1つだけ観察され、典型的な炭素-炭素結合長と二重結合長の間の値である理由も非局在化で説明できます。

ベンゼンの構造: 分子軌道モデル

Structure of Benzene: Molecular Orbital Model

分子軌道 (MO) モデルによると、ベンゼンは 6 つの sp^2 混成炭素からなる正六角形の平面構造を持っています。 図 1 に示すように、各炭素は 120°の C-C-C および H-C-C 結合角で他の 3 つの原子に結合しています。 C-H 結合の長さは 109 pm、C-C 結合の長さは 139 pm で、sp^3 混成炭素の単結合長 (154 pm) と sp^2 混成炭素の単結合長 (133 pm) の中間に位置します。

<img alt="Figure1" src="/files/ftp_upload/12451/12451_PT_Figure_1.svg" style="text-align: center; height: 190px;">

炭素原子はまた、環の平面に垂直な、1 つの電子を含む混成していない 2p 原子軌道を持ち、これは隣接する炭素の p 軌道と重なり、環の平面の上下に π 電子の連続ループを形成します。 分子軌道 (MO) 理論によれば、以下の図に示すように、これら 6 つの 2p 軌道が結合して 6 つの π 環状分子軌道 (MO) ψ_1、ψ_2、ψ_3、ψ_4、ψ_5、ψ_6 を形成します。 このうち、ψ_1、ψ_2、ψ_3 は結合性ですが、ψ_4、ψ_5、ψ_6 は反結合性 MO です。 一般に、MO のエネルギーとノードの数は ψ_1 から ψ_6 に増加しますが、結合相互作用は減少します。 ただし、これらの環状系の MO は、2 つの縮退した MO を持つ点で 1,3-ブタジエンなどの線形系とは異なります。

<img alt="Figure2" src="/files/ftp_upload/12451/12451_PT_Figure_2.svg" style="text-align: center; height: 250px;">

最もエネルギーの低い結合 MO ψ_1 には、すべての軌道が同位相であるノードがありません。 次に低い MO には 1 つのノードがあり、ノード面が結合または原子を通過する 2 つの方法で表すことができます。 これら 2 つの結合 MO は縮退しており、ψ_2 および ψ_3 として表されます。 同様に、2 つの節面は結合または原子を通過することができ、ψ_4 および ψ_5 の 2 つの縮退反結合 MO が提供されます。 最後の MO ψ_6 は最も高いエネルギーを持ち、3 つの節面はすべての p 軌道の位相がずれた組み合わせを表します。

リング面の上下で非局在化したπ電子密度の閉殻を形成する 6 つの π 電子は、孤立した p 軌道のエネルギーよりもエネルギーが低い 3 つの結合 MO ψ_1、ψ_2、ψ_3 を占有し、したがって ベンゼンの異常な安定性をもたらします。

分子軌道(MO)モデルによれば、ベンゼンは6つのsp2混成炭素の正六角形を持つ平面構造をしています。図1に示すように、...

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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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

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