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

يصف النموذج المداري الجزيئي البنزين بأنه مسدس مستوي منتظم مع ستة كربون مهجن sp2.
تشكل ذرات الكربون المجاورة روابط σ عبر التداخل المداري sp2-sp 2. تشكل كل ذرة كربون أيضا رابطة σ مع الهيدروجين من خلال التداخل المداري sp2-1 s. جميع زوايا الرابطة هي 120 درجة.
بالإضافة إلى ذلك ، يحتوي كل كربون على مدار ذري نصف مملوء غير مهجن 2ص عموديا على مستوى الحلقة.
تتحد هذه المدارات الستة 2p لتشكيل ثلاثة مدارات جزيئية مترابطة وثلاثة مدارات جزيئية مضادة للترابط.
تمتلئ مدارات الترابط بالكامل بإلكترونات π الستة ، في حين أن المدارات المضادة للترابط فارغة ، مما يمنح البنزين تكوينا مغلقا يمنح الاستقرار.
وبالتالي ، يتم إلغاء تحديد كثافة الإلكترون π في شكل مناطق على شكل دونات أعلى وأسفل مستوى الحلقة.
يفسر التوطين أيضا سبب ملاحظة طول واحد فقط لرابطة الكربون والكربون للبنزين ، مع قيمة بين أطوال الرابطة المفردة والمزدوجة النموذجية بين الكربون والكربون.

هيكل البنزين: النموذج المداري الجزيئي

Structure of Benzene: Molecular Orbital Model

<div style="direction:rtl">وفقًا للنموذج المداري الجزيئي (MO)، يمتلك البنزين بنية مستوية ذات شكل سداسي منتظم يتكون من ستة ذرات كربون مهجنة من نوع sp2. كما هو موضح في الشكل 1، يرتبط كل كربون بثلاث ذرات أخرى بزوايا رابطة C–C–C وH–C–C تبلغ 120 درجة. يبلغ طول الرابطة C – H 109 pm، ويبلغ طول الرابطة C – C 139 pm وهو في منتصف الطريق بين طول الرابطة الفردية للكربون المهجن sp3 (154 pm ) والكربون المهجن sp2 (133 pm ). <img alt="Figure1" src="/files/ftp_upload/12451/12451_PT_Figure_1.svg" style="text-align: center; height: 190px;"> تحتوي ذرات الكربون أيضًا على مدار ذري غير مهجن 2p مع إلكترون واحد، متعامد على مستوى الحلقة، والذي يتداخل مع مدارات p للكربونات المجاورة، مما يشكل حلقة مستمرة من إلكترونات π أعلى وأسفل مستوى الحلقة. وفقًا لنظرية المدار الجزيئي (MO)، تتحد هذه المدارات الستة 2p لتشكل ستة مدارات جزيئية حلقية (MOs) ψ1 وψ2 وψ3 وψ4 وψ5 وψ6، كما هو موضح في الشكل أدناه. من بينها، ψ1 وψ2 وψ3 عبارة عن روابط، في حين أن ψ4 وψ5 وψ6 هي مدارات جزيئية مضادة للترابط. وبشكل عام، تزداد طاقة المدارات الجزيئية وعدد العقد من ψ1 إلى ψ6، بينما يتناقص تفاعل الترابط. ومع ذلك، فإن هذه المدارات الجزيئية للأنظمة الحلقية تختلف عن الأنظمة الخطية مثل 1،3-بوتادين من خلال وجود مدارين جزيئيين متحللين. <img alt="Figure2" src="/files/ftp_upload/12451/12451_PT_Figure_2.svg" style="text-align: center; height: 250px;"> أدنى رابطة للطاقة MO، ψ1، لا تحتوي على عقد حيث تكون جميع المدارات في الطور. يحتوي MO الأقل طاقة التالي على عقدة واحدة ويمكن تمثيله بطريقتين، حيث يمر المستوى العقدي عبر رابطة أو ذرة. يتم تحليل هذين المدارين الجزيئيين الرابطين على أنهما ψ2 وψ3. وبالمثل، يمكن تمر مستويي العقدتين عبر روابط أو ذرات، مما يوفر اثنين من MOs المضادين المتحللين لـ ψ4 وψ5. يحتوي المدار الجزيئي الأخير، ψ6، أعلى طاقة، مع ثلاث مستويات عقدية تمثل مزيجًا خارج الطور لجميع p المدارات. تشغل إلكترونات π الستة التي تشكل غلافًا مغلقًا من كثافة إلكترون π غير المتمركزة أعلى وأسفل مستوى الحلقة، المدارات الجزيئية الرابطة الثلاثة، ψ1 وψ2 وψ3، والتي تكون طاقاتها أقل من طاقات المدار p المعزول، مما يؤدي إلى استقرار غير عادي للبنزين.</div>

وفقا لنموذج المدار الجزيئي (MO) ، يحتوي البنزين على بنية مستوية ذات شكل مسدس منتظم من ستة كربون مهجن sp2. كما هو موضح في الشكل 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

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aromatic compounds: overview

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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 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.

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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...

aromatic hydrocarbon cations: structural overview

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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...

five-membered heterocyclic aromatic compounds: overview

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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.

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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

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Benzene π Orbitals and Electron Delocalization

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