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

En 1865, August Kekulé a proposé la première structure définie pour le benzène.
Il a suggéré que le benzène a une structure cyclique de six atomes de carbone avec une alternance de liaisons simples et doubles. Chaque atome de carbone est attaché à un atome d’hydrogène.
Ce modèle satisfaisait la tétravalence du carbone, rendant tous les hydrogènes équivalents.
Le modèle de Kekulé a prédit deux formes de 1,2-dibromobenzène, qui différaient dans les positions des doubles liaisons. Cependant, une seule forme de 1,2-dibromobenzène a été isolée.
Kekulé l’a défendu en proposant un équilibre rapide entre les deux formes indissociables du benzène, aujourd’hui connues sous le nom de structures de Kekulé. Cependant, on sait maintenant qu’un tel équilibre n’existe pas.
De plus, il n’a pas réussi à expliquer le comportement chimique inhabituel du benzène, qui subit une réaction de substitution avec le brome, contrairement aux alcènes, qui subissent une réaction d’addition.

structure du benzène : modèle kekulé

Structure of Benzene: Kekul&eacute; Model

En 1865, August Kekule a suggéré la structure du benzène selon la théorie structurale de la chimie organique basée sur trois affirmations : la formule du benzène est C6H6, tous les hydrogènes du benzène sont équivalents et chaque carbone doit avoir quatre liaisons en raison de sa tétravalence.

Il a proposé que le benzène ait une structure cyclique de six atomes de carbone attachés à un atome d'hydrogène chacun, avec trois liaisons pi alternées.

<img alt="Figure1" src="/files/ftp_upload/12450/12450_PT_Figure1.svg" style="text-align: center; height: 100px;">

Selon son modèle, il doit y avoir deux isomères pour les dérivés du benzène 1,2-disubstitués, comme le montre la figure 2, dans laquelle dans une structure, les deux carbones substitués sont reliés par une double liaison mais par une simple liaison dans une autre.

<img alt="Figure2" src="/files/ftp_upload/12450/12450_PT_Figure2.svg" style="text-align: center; height: 100px;">

Cette notion de Kekule s'est avérée fausse car une seule structure est trouvée pour le benzène 1,2-disubstitué. Pour expliquer cela, il a déclaré que les deux formes de benzène sont en équilibre rapide et qu’il n’est pas possible de séparer les deux composés.

<img alt="Figure3" src="/files/ftp_upload/12450/12450_PT_Figure3.svg" style="text-align: center; height: 100px;">

Cependant, un tel équilibre n’existe pas et une seule structure est trouvée pour les dérivés du benzène 1,2-disubstitués.

<img alt="Figure4" src="/files/ftp_upload/12450/12450_PT_Figure4.svg" style="text-align: center; height: 80px;">

Le benzène a quatre degrés d'insaturation, constituant un cycle et trois liaisons pi. Cependant, le benzène ne subit pas de réactions d’addition, contrairement à d’autres hydrocarbures insaturés comme les alcènes et les alcynes. Par exemple, l’éthylène subit une réaction d’addition avec le brome pour former du dibromure d’éthylène, mais pas le benzène.

<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;">

Le benzène réagit avec le brome pour former du bromobenzène via une réaction de substitution en présence de bromure ferrique d'acide de Lewis.

<img alt="Figure7" src="/files/ftp_upload/12450/12450_PT_Figure7.svg" style="text-align: center; height: 90px;">

Le principal inconvénient du modèle de Kekule est qu’il n’a pas réussi à expliquer cette réactivité inhabituelle du benzène par rapport à d’autres hydrocarbures insaturés tels que les alcènes et les alcynes.

Structure du Benzène : Modèle Kekulé

En 1865, August Kekule a suggéré la structure du benzène selon la théorie structurale de la chimie organique basée sur les trois ...

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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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Kekule’s Benzene Model and Its Limits

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