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

Em 1865, August Kekulé propôs a primeira estrutura definitiva para o benzeno.
Ele sugeriu que o benzeno tem uma estrutura cíclica de seis átomos de carbono com ligações simples e duplas alternadas. Cada átomo de carbono está ligado a um átomo de hidrogênio.
Este modelo satisfez a tetravalência do carbono, tornando todos os hidrogênios equivalentes.
O modelo de Kekulé previu duas formas de 1,2-dibromobenzeno, que diferiam nas posições das ligações duplas. Mas, apenas uma forma de 1,2-dibromobenzeno foi isolada.
Kekulé defendeu-o propondo um equilíbrio rápido entre as duas formas inseparáveis de benzeno, agora conhecidas como estruturas de Kekulé. No entanto, sabe-se agora que esse equilíbrio não existe.
Além disso, não conseguiu explicar o comportamento químico incomum do benzeno, que sofre uma reação de substituição com o bromo, ao contrário dos alcenos, que sofrem uma reação de adição.

estrutura do benzeno: modelo de kekulé

Structure of Benzene: Kekul&eacute; Model

Em 1865, August Kekulé sugeriu a estrutura do benzeno de acordo com a teoria estrutural da química orgânica baseada em três afirmações - a fórmula do benzeno é C_6H_6, todos os hidrogênios do benzeno são equivalentes, e cada carbono deve ter quatro ligações devido à sua tetravalência.

Ele propôs que o benzeno tem uma estrutura cíclica de seis átomos de carbono ligados a um átomo de hidrogênio cada, com três ligações pi alternadas.

<img alt="Figure1" src="/files/ftp_upload/12450/12450_PT_Figure1.svg" style="text-align: center; height: 100px;">

De acordo com seu modelo, deve haver dois isômeros para os derivados de benzeno 1,2-dissubstituídos, como mostrado na Figura 2, em que em uma estrutura, os dois carbonos substituídos estão conectados por uma ligação dupla, mas por uma ligação simples em outra.

<img alt="Figure2" src="/files/ftp_upload/12450/12450_PT_Figure2.svg" style="text-align: center; height: 100px;">

Essa noção de Kekulé foi provada errada, pois apenas uma estrutura foi encontrada para o benzeno 1,2-dissubstituído. Para explicar isso, ele afirmou que as duas formas de benzeno estão em rápido equilíbrio e que não é possível separar os dois compostos.

<img alt="Figure3" src="/files/ftp_upload/12450/12450_PT_Figure3.svg" style="text-align: center; height: 100px;">

No entanto, tal equilíbrio não existe e apenas uma estrutura para derivados de benzeno 1,2-dissubstituídos é encontrada.

<img alt="Figure4" src="/files/ftp_upload/12450/12450_PT_Figure4.svg" style="text-align: center; height: 80px;">

O benzeno possui quatro graus de insaturação, constituindo um anel e três ligações pi. Contudo, o benzeno não sofre reações de adição, ao contrário de outros hidrocarbonetos insaturados, como alcenos e alcinos. Por exemplo, o etileno sofre uma reação de adição com o bromo para formar dibrometo de etileno, mas o benzeno não.

<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;">

O benzeno reage com o bromo para formar bromobenzeno por meio de reação de substituição na presença de brometo férrico do ácido de Lewis.

<img alt="Figure7" src="/files/ftp_upload/12450/12450_PT_Figure7.svg" style="text-align: center; height: 90px;">

A principal desvantagem do modelo de Kekulé é que ele não conseguiu explicar essa reatividade incomum do benzeno em comparação com outros hidrocarbonetos insaturados, como alcenos e alcinos.

Estrutura do Benzeno: Modelo de Kekulé

Em 1865, August Kekule sugeriu a estrutura do benzeno de acordo com a teoria estrutural da química orgânica baseada nos três ...

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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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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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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

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Kekule’s Benzene Model and Its Limits

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