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

1865年，奥古斯特·凯库勒提出了苯的第一个明确结构。
他提出苯具有由六个碳原子组成的环状结构，碳原子之间交替连接着单键和双键。每个碳原子连接一个氢原子。
该模型满足了碳的四价性，使得所有氢原子均等价。
凯库勒模型预测了邻二溴苯存在两种异构体，二者双键位置不同。然而，实际仅分离得到一种邻二溴苯。
凯库勒通过提出苯的两种不可分离结构之间存在快速平衡来为其辩护，这两种结构现在被称为凯库勒式。然而，现在已知并不存在这样的平衡。
此外，它无法解释苯的异常化学行为：苯与溴发生取代反应，而烯烃则发生加成反应。

苯的结构:凯库勒模型

Structure of Benzene: Kekul&eacute; Model

1865年，奥古斯特·凯库勒根据有机化学结构理论，基于苯的分子式为C6H6、苯的所有氢等价以及每个碳是四价而必须有四个键等三个断言，提出了苯的结构。


 他提出苯是具有六个碳原子且每个碳原子与一个氢原子相连的环状结构，具有三个交替的π键。

 <img alt="Figure1" src="/files/ftp_upload/12450/12450_PT_Figure1.svg" style="text-align: center; height: 100px;">

 根据他的模型，1,2-二取代苯衍生物必定存在两种异构体，如图2所示，在其中一种结构中，两个取代的碳通过双键连接，而在另一种结构中，两个取代的碳通过单键连接。

 <img alt="Figure2" src="/files/ftp_upload/12450/12450_PT_Figure2.svg" style="text-align: center; height: 100px;">

 凯库勒的这一概念被证明是错误的，因为仅发现了 1,2-二取代苯的一种结构。 为了解释这一点，他表示苯的两种形式处于快速平衡状态，并且不可能分离这两种化合物。

 <img alt="Figure3" src="/files/ftp_upload/12450/12450_PT_Figure3.svg" style="text-align: center; height: 100px;">

 然而，不存在这种平衡，并且仅发现了 1,2-二取代苯衍生物的一种结构。

 <img alt="Figure4" src="/files/ftp_upload/12450/12450_PT_Figure4.svg" style="text-align: center; height: 80px;">

 苯有四个不饱和度，构成一个环和三个π键。 然而，与烯烃和炔烃等其他不饱和烃不同，苯不会发生加成反应。 例如，乙烯与溴发生加成反应，形成二溴乙烷，但苯则不会。

 <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;">

 在路易斯酸溴化铁存在下，苯与溴通过取代反应生成溴苯。

 <img alt="Figure7" src="/files/ftp_upload/12450/12450_PT_Figure7.svg" style="text-align: center; height: 90px;">

 凯库勒模型的主要缺点是它无法解释苯与其他不饱和烃（例如烯烃和炔烃）相比的这种不寻常的反应性。

1865年，奥古斯特·凯库勒根据有机化学的结构理论，结合当时的三个……提出了苯的结构。

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JoVE Core

Organic Chemistry

有机化学

Unit 1

第一单元

Aromatic Compounds

芳香族化合物

KEY TERMS AND CONCEPTS

关键术语与概念

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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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

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JoVE Core 有机化学（Organic Chemistry）使用简明易懂的动画视频课程解释立体化学、反应和机制的基本概念。 此外，科学家在行动视频展示了相关概念在当今世界各地实验室进行的经典和原创研究实验中的应用。

Kekule’s Benzene Model and Its Limits

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