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

분자 궤도 모델은 벤젠을 6 개의 sp2 혼성화 된 탄소가있는 규칙적인 평면 육각형으로 설명합니다.
인접한 탄소 원자는 sp2-sp 2 궤도 겹침을 통해 σ 결합을 형성합니다. 각 탄소 원자는 또한 sp2-1 s 궤도 겹침에 의해 수소와 σ 결합을 형성합니다. 모든 접착 각도는 120°입니다.
또한, 각 탄소는 고리의 평면에 수직인 반쯤 채워진 혼성화되지 않은2p 원자 궤도를 가지고 있습니다.
이 6개의2p 오비탈이 결합되어 3개의 결합 분자 오비탈과 3개의 반결합 분자 오비탈을 형성합니다.
결합 오비탈은 6개의 π 전자로 완전히 채워지는 반면, 반결합 오비탈은 비어 있어 벤젠에 안정성을 부여하는 폐쇄 쉘 구성을 제공합니다.
결과적으로, π 전자 밀도는 고리 평면의 위와 아래에 도넛 모양의 영역 형태로 비편재화됩니다.
비편재화(delocalization)는 또한 벤젠에 대해 탄소-탄소 결합 길이가 하나만 관찰되는 이유를 설명하며, 일반적인 탄소-탄소 단일 결합 길이와 이중 결합 길이 사이의 값을 갖습니다.

벤젠의 구조: 분자 궤도 모델

Structure of Benzene: Molecular Orbital Model

분자 궤도(MO) 모델에 따르면 벤젠은 6개의 sp^2 혼성 탄소로 구성된 정육각형의 평면 구조를 가지고 있습니다. 그림 1에서 볼 수 있듯이, 각 탄소는 120°의 C-C-C 및 H-C-C 결합각으로 3개의 다른 원자와 결합되어 있습니다. C-H 결합 길이는 109pm이고 C-C 결합 길이는 139pm이며 이는 sp^3 혼성화 탄소의 단일 결합 길이(154pm)와 sp^2 혼성화 탄소(133pm)의 중간입니다.

<img alt="Figure1" src="/files/ftp_upload/12451/12451_PT_Figure_1.svg" style="text-align: center; height: 190px;">

탄소 원자는 또한 고리 평면에 수직인 하나의 전자를 갖는 혼성화되지 않은 2p 원자 궤도를 가지며, 이는 이웃 탄소의 p 궤도와 겹쳐 고리 평면 위와 아래에 π 전자의 연속 루프를 형성합니다. 분자 궤도(MO) 이론에 따르면, 이 6개의 2p 궤도는 결합하여 아래 그림과 같이 6개의 π 순환 분자 궤도(MO) ψ_1, ψ_2, ψ_3, ψ_4, ψ_5 및 ψ_6을 형성합니다. 그 중 ψ_1, ψ_2, ψ_3은 결합이고, ψ_4, ψ_5, ψ_6은 반결합 MO이다. 일반적으로 MO의 에너지와 노드 수는 ψ_1에서 ψ_6으로 증가하는 반면 결합 상호 작용은 감소합니다. 그러나 이러한 순환 시스템의 MO는 두 개의 축퇴 MO를 갖는다는 점에서 1,3-부타디엔과 같은 선형 시스템과 다릅니다.

<img alt="Figure2" src="/files/ftp_upload/12451/12451_PT_Figure_2.svg" style="text-align: center; height: 250px;">

가장 낮은 에너지 결합 MO인 ψ_1에는 모든 궤도가 같은 위상에 있는 노드가 없습니다. 다음으로 낮은 MO는 하나의 노드를 가지며 노드 평면이 결합 또는 원자를 통과하는 두 가지 방식으로 표현될 수 있습니다. 이 두 결합 MO는 축퇴되어 ψ_2 및 ψ_3으로 표시됩니다. 마찬가지로 두 개의 절점 평면은 결합이나 원자를 통과하여 ψ_4 및 ψ_5의 두 개의 축퇴 방지 결합 MO를 제공할 수 있습니다. 최종 MO인 ψ_6은 가장 높은 에너지를 가지며, 모든 p 오비탈의 위상차 조합을 나타내는 3개의 노드 평면이 있습니다.

링 평면 위와 아래에 비편재화된 π 전자 밀도의 닫힌 껍질을 형성하는 6개의 π 전자는 3개의 결합 MO, ψ_1, ψ_2 및 ψ_3를 차지하며, 그 에너지는 고립된 p 오비탈의 에너지보다 낮아서 벤젠의 특이한 안정성을 이끌어 냅니다.

분자 궤도(MO) 모델에 따르면 벤젠은 6개의 sp2 혼성화 탄소로 구성된 정육각형을 가진 평면 구조를 가지고 있습니다. 그림 1에서 볼 수 있듯이 ...

structure-of-benzene-molecular-orbital-model

Education

교육

JoVE Core

Organic Chemistry

유기 화학

Unit 1

제1단원

Aromatic Compounds

방향족 화합물

KEY TERMS AND CONCEPTS

주요 용어 및 개념

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,...

nomenclature of aromatic compounds with a single substituent

nomenclature-of-aromatic-compounds-with-a-single-substituent

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...

nomenclature of aromatic compounds with multiple substituents

nomenclature-of-aromatic-compounds-with-multiple-substituents

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,...

structure of benzene: kekul&eacute; model

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,...

criteria for aromaticity and the h&uuml;ckel 4n + 2 rule

criteria-for-aromaticity-and-the-hckel-4n-2-rule

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

frost-circles-for-different-conjugated-systems

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...

aromatic hydrocarbon anions: structural overview

aromatic-hydrocarbon-anions-structural-overview

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...

aromatic hydrocarbon cations: structural overview

aromatic-hydrocarbon-cations-structural-overview

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...

five-membered heterocyclic aromatic compounds: overview

five-membered-heterocyclic-aromatic-compounds-overview

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

organic chemistry

JoVE Core Organic Chemistry explains basic concepts of stereochemistry, reactions, and mechanisms using concise and easy-to-understand animated video lessons. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

Benzene π Orbitals and Electron Delocalization

Description

Transcript

Tags