Name: kekule’s benzene model and its limits
Uploaded: 2023-04-30T10:30:54.000Z
Duration: 1 min 7 s
Channel: JoVE

في عام 1865 ، اقترح أغسطس كيكولي أول هيكل محدد للبنزين.
واقترح أن البنزين له بنية دورية من ست ذرات كربون مع روابط مفردة ومزدوجة بالتناوب. ترتبط كل ذرة كربون بذرة هيدروجين واحدة.
استوفى هذا النموذج رباعي التكافؤ للكربون ، مما يجعل جميع الهيدروجينات مكافئة.
تنبأ نموذج Kekulé بشكلين من 1،2 -dibromobenzene ، والتي تختلف في مواضع الروابط المزدوجة. ولكن ، تم عزل شكل واحد فقط من 1،2-ثنائي بروموبنزين.
دافع كيكولي عنها من خلال اقتراح توازن سريع بين الشكلين اللذين لا ينفصلان عن البنزين ، والمعروفين الآن باسم هياكل كيكولي. ومع ذلك ، فمن المعروف الآن أنه لا يوجد مثل هذا التوازن.
علاوة على ذلك ، فشل في تفسير السلوك الكيميائي غير العادي للبنزين ، الذي يخضع لتفاعل استبدال مع البروم ، على عكس الألكينات ، التي تخضع لتفاعل إضافي.

هيكل البنزين: نموذج كيكولي

Structure of Benzene: Kekul&eacute; Model

<div style="direction:rtl">في عام 1865، اقترح أوغست كيكولي بنية البنزين وفقًا للنظرية البنيوية للكيمياء العضوية استنادًا إلى ثلاثة افتراضات - صيغة البنزين هي C6H6، وجميع ذرات الهيدروجين في البنزين متكافئة، ويجب أن يحتوي كل كربون على أربع روابط بسبب رباعي التكافؤ. واقترح أن البنزين له بنية دائرية مكونة من ست ذرات كربون مرتبطة بكل منها بذرة هيدروجين واحدة، مع ثلاث روابط باي متناوبة. <img alt="Figure1" src="/files/ftp_upload/12450/12450_PT_Figure1.svg" style="text-align: center; height: 100px;"> وفقا لنموذجه، يجب أن يكون هناك أيزومرين لمشتقات البنزين 1،2، كما هو مبين في الشكل 2، حيث في بنية واحدة، يتم ربط ذرتي الكربون المستبدلتين بواسطة رابطة مزدوجة ولكن بواسطة رابطة واحدة في بنية أخرى. <img alt="Figure2" src="/files/ftp_upload/12450/12450_PT_Figure2.svg" style="text-align: center; height: 100px;"> ثبت خطأ فكرة كيكولي هذه حيث تم العثور على بنية واحدة فقط للبنزين ثنائي الاستبدال 1،2. ولمراعاة ذلك ذكر أن شكلي البنزين في حالة توازن سريع وأنه لا يمكن فصل المركبين. <img alt="Figure3" src="/files/ftp_upload/12450/12450_PT_Figure3.svg" style="text-align: center; height: 100px;"> ومع ذلك، لا يوجد مثل هذا التوازن، وتم العثور على بنية واحدة فقط لمشتقات البنزين ثنائية الاستبدال 1،2. <img alt="Figure4" src="/files/ftp_upload/12450/12450_PT_Figure4.svg" style="text-align: center; height: 80px;"> للبنزين أربع درجات من عدم التشبع، مكونة حلقة واحدة وثلاث روابط باي. ومع ذلك، لا يخضع البنزين لتفاعلات الإضافة، على عكس الهيدروكربونات غير المشبعة الأخرى مثل الألكينات والألكينات. على سبيل المثال، يخضع الإيثيلين لتفاعل إضافة مع البروم لتكوين ثنائي بروميد الإيثيلين، لكن البنزين لا يتفاعل. <img alt="Figure5" src="/files/ftp_upload/12450/12450_PT_Figure5.svg" style="text-align: center; height: 120px;"> <img alt="Figure6" src="/files/ftp_upload/12450/12450_PT_Figure6.svg" style="text-align: center; height: 60px;"> يتفاعل البنزين مع البروم لتكوين البروموبنزين عن طريق تفاعل الاستبدال في وجود بروميد الحديديك حمض لويس. <img alt="Figure7" src="/files/ftp_upload/12450/12450_PT_Figure7.svg" style="text-align: center; height: 90px;"> العيب الرئيسي في نموذج كيكولي هو أنه فشل في تفسير هذه التفاعلية غير المعتادة للبنزين مقارنة بالهيدروكربونات غير المشبعة الأخرى مثل الألكينات والألكاينات.</div>

في عام 1865 ، اقترح أغسطس كيكولي بنية البنزين وفقا للنظرية الهيكلية للكيمياء العضوية بناء على ثلاثة ...

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Organic Chemistry

الكيمياء العضوية

Unit 1

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Aromatic Compounds

المركبات العطرية

KEY TERMS AND CONCEPTS

المصطلحات والمفاهيم الأساسية

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 they are linked by a single bond.

That prediction did not match experimental results. Only one structure is found for 1,2-disubstituted benzene derivatives. Kekule tried to explain this by saying the two forms of benzene stay in rapid equilibrium and cannot be separated, but no such equilibrium exists.

Benzene has four degrees of unsaturation, which means one ring and three pi bonds. Even so, it does not undergo addition reactions the way alkenes and alkynes do. Ethylene, for example, adds bromine to form ethylene dibromide, but benzene does not.

Instead, benzene reacts with bromine by substitution when ferric bromide, a Lewis acid, is present. The main weakness of Kekule’s model is that it does not explain this unusual reactivity compared with other unsaturated hydrocarbons.

Kekule’s Benzene Model and Its Limits

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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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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 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 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...

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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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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...

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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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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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Cycloheptatrienyl Cation and Aromaticity

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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

Learn how Kekule proposed benzene’s ring structure, why it predicted two isomers, and why the model could not explain benzene’s unusual reactivity.

Kekule’s Benzene Model and Its Limits | 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.

Kekule’s Benzene Model and Its Limits

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