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

Il modello degli orbitali molecolari descrive il benzene come un esagono planare regolare con sei atomi di carbonio ibridizzati sp2.
Gli atomi di carbonio adiacenti formano legami σ attraverso la sovrapposizione orbitale sp2-sp 2. Ogni atomo di carbonio forma anche un legame σ con l'idrogeno per sovrapposizione orbitale sp2–1s. Tutti gli angoli di incollaggio sono di 120°.
Inoltre, ogni carbonio ha un orbitale atomico 2p non ibridato riempito a metà perpendicolare al piano dell'anello.
Questi sei orbitali 2p si combinano per formare tre orbitali molecolari di legame e tre orbitali molecolari di antilegame.
Gli orbitali di legame sono completamente riempiti dai sei elettroni π, mentre gli orbitali di antilegame sono vuoti, conferendo al benzene una configurazione a guscio chiuso che conferisce stabilità.
Di conseguenza, la densità elettronica π viene delocalizzata sotto forma di regioni a forma di ciambella sopra e sotto il piano dell'anello.
La delocalizzazione spiega anche perché per il benzene si osserva una sola lunghezza di legame carbonio-carbonio, con un valore compreso tra le tipiche lunghezze di legame singolo e doppio carbonio-carbonio.

struttura del benzene: modello orbitale molecolare

Structure of Benzene: Molecular Orbital Model

Secondo il modello dell'orbitale molecolare (MO), il benzene ha una struttura planare con un esagono regolare di sei carboni ibridati sp^2. Come mostrato in Figura 1, ciascun carbonio è legato ad altri tre atomi con angoli di legame C–C–C e H–C–C di 120°. La lunghezza del legame C–H è 109 µm e la lunghezza del legame C–C è 139 µm, che è a metà strada tra la lunghezza del legame singolo dei carboni ibridati sp^3 (154 µm) e dei carboni ibridati sp^2 (133 µm).

<img alt="Figure1" src="/files/ftp_upload/12451/12451_PT_Figure_1.svg" style="text-align: center; height: 190px;">

Gli atomi di carbonio hanno anche un orbitale atomico 2p non ibridato con un elettrone, perpendicolare al piano dell'anello, che si sovrappone agli orbitali p degli atomi di carbonio vicini, formando un anello continuo di elettroni π, sopra e sotto il piano dell'anello. Secondo la teoria degli orbitali molecolari (MO), questi sei orbitali 2p si combinano per formare sei orbitali molecolari ciclici (MO) π ψ_1, ψ_2, ψ_3, ψ_4, ψ_5 e ψ_6, come mostrato nella figura seguente. Tra questi, ψ_1, ψ_2 e ψ_3 sono MO leganti, mentre ψ_4, ψ_5 e ψ_6 sono MO antileganti. Generalmente, l'energia degli MO e il numero di nodi aumenta da ψ_1 a ψ_6, mentre l'interazione di legame diminuisce. Tuttavia, questi MO dei sistemi ciclici differiscono dai sistemi lineari come l'1,3-butadiene poiché hanno due MO degeneri.

<img alt="Figure2" src="/files/ftp_upload/12451/12451_PT_Figure_2.svg" style="text-align: center; height: 250px;">

Il legame MO a più bassa energia, ψ_1, non ha nodi in cui tutti gli orbitali sono in fase. Il successivo MO più basso ha un nodo e può essere rappresentato in due modi, dove il piano nodale passa attraverso un legame o un atomo. Questi due MO di legame sono degenerati e rappresentati come ψ_2 e ψ_3. Allo stesso modo, i due piani nodali possono passare attraverso legami o atomi, fornendo due MO antileganti degeneri di ψ_4 e ψ_5. L'MO finale, ψ_6, ha l'energia più alta, con tre piani nodali che rappresentano la combinazione fuori fase di tutti gli orbitali p.

I sei elettroni π che formano un guscio chiuso di densità elettronica π delocalizzata sopra e sotto il piano dell'anello occupano i tre MO leganti, ψ_1, ψ_2 e ψ_3, le cui energie sono inferiori a quella dell'orbitale p isolato, portando così ad una stabilità insolita del benzene.

Struttura del benzene: modello orbitale molecolare

Secondo il modello degli orbitali molecolari (MO), il benzene ha una struttura planare con un esagono regolare di sei atomi di carbonio ibridati sp2. Come mostrato nella 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,...

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

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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

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

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Benzene π Orbitals and Electron Delocalization

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