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

Nel 1865, August Kekulé propose la prima struttura definita per il benzene.
Ha suggerito che il benzene ha una struttura ciclica di sei atomi di carbonio con legami singoli e doppi alternati. Ogni atomo di carbonio è attaccato a un atomo di idrogeno.
Questo modello soddisfaceva la tetravalenza del carbonio, rendendo equivalenti tutti gli idrogeni.
Il modello di Kekulé prevedeva due forme di 1,2-dibromobenzene, che differivano nelle posizioni dei doppi legami. Ma è stata isolata solo una forma di 1,2-dibrobobenzene.
Kekulé lo difese proponendo un rapido equilibrio tra le due forme inseparabili di benzene, ora note come strutture di Kekulé. Tuttavia, è ormai noto che tale equilibrio non esiste.
Inoltre, non è riuscito a spiegare l'insolito comportamento chimico del benzene, che subisce una reazione di sostituzione con il bromo, al contrario degli alcheni, che subiscono una reazione di addizione.

struttura del benzene: modello di kekulé

Structure of Benzene: Kekul&eacute; Model

Nel 1865, August Kekulé suggerì la struttura del benzene secondo la teoria strutturale della chimica organica basata su tre asserzioni: la formula del benzene è C_6H_6, tutti gli idrogeni del benzene sono equivalenti e ciascun carbonio deve avere quattro legami a causa della sua tetravalenza.

Propose che il benzene avesse una struttura ciclica di sei atomi di carbonio attaccati ciascuno a un atomo di idrogeno, con tre legami pi alternati.

<img alt="Figure1" src="/files/ftp_upload/12450/12450_PT_Figure1.svg" style="text-align: center; height: 100px;">

Secondo il suo modello, devono esserci due isomeri per i derivati del benzene 1,2-disostituiti, come mostrato in Figura 2, in cui in una struttura i due carboni sostituiti sono collegati da un doppio legame, mentre in un'altra da un legame singolo.

<img alt="Figure2" src="/files/ftp_upload/12450/12450_PT_Figure2.svg" style="text-align: center; height: 100px;">

Questa nozione di Kekulé è stata smentita poiché viene trovata una sola struttura per il benzene 1,2-disostituito. Per spiegare ciò affermò che le due forme di benzene sono in rapido equilibrio e che non è possibile separare i due composti.

<img alt="Figure3" src="/files/ftp_upload/12450/12450_PT_Figure3.svg" style="text-align: center; height: 100px;">

Tuttavia, non esiste un tale equilibrio e viene trovata solo una struttura per i derivati del benzene 1,2-disostituiti.

<img alt="Figure4" src="/files/ftp_upload/12450/12450_PT_Figure4.svg" style="text-align: center; height: 80px;">

Il benzene ha quattro gradi di insaturazione, costituendo un anello e tre legami π. Tuttavia, il benzene non subisce reazioni di addizione, a differenza di altri idrocarburi insaturi come gli alcheni e gli alchini. Ad esempio, l'etilene subisce una reazione di addizione con il bromo per formare dibromuro di etilene, ma il benzene no.

<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;">

Il benzene reagisce con il bromo per formare bromobenzene tramite reazione di sostituzione in presenza di bromuro ferrico dell'acido di Lewis.

<img alt="Figure7" src="/files/ftp_upload/12450/12450_PT_Figure7.svg" style="text-align: center; height: 90px;">

Lo svantaggio principale del modello di Kekulé è che non sia riuscito a spiegare questa insolita reattività del benzene rispetto ad altri idrocarburi insaturi come alcheni e alchini.

Struttura del benzene: modello di Kekulé

Nel 1865, August Kekule suggerì la struttura del benzene secondo la teoria strutturale della chimica organica basata sui tre ...

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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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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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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

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Kekule’s Benzene Model and Its Limits

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