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

In 1865, August Kekul&#233; proposed the first definite structure for benzene.
He suggested that benzene has a cyclic structure of six carbon atoms with alternating single and double bonds. Each carbon atom is attached to one hydrogen atom.
This model satisfied the tetravalency of carbon, making all the hydrogens equivalent.
Kekul&#233;&#39;s model predicted two forms of 1,2 -dibromobenzene, which differed in the positions of the double bonds. But, only one form of 1,2-dibromobenzene was isolated.
Kekul&#233; defended it by proposing a rapid equilibrium between the two inseparable forms of benzene, now known as Kekul&#233; structures. However, it is now known that no such equilibrium exists.
Moreover, it failed to explain the unusual chemical behavior of benzene, which undergoes a substitution reaction with bromine, as opposed to alkenes, which undergo an addition reaction.

structuur van benzeen: kekulé-model

Structure of Benzene: Kekul&eacute; Model

In 1865 suggereerde August Kekule de structuur van benzeen volgens de structuurtheorie van de organische chemie, gebaseerd op de drie beweringen: de formule van benzeen is C6H6, alle waterstofatomen van benzeen zijn gelijkwaardig en elke koolstof moet vier bindingen hebben vanwege zijn tetravalentie.


 Hij stelde voor dat benzeen een cyclische structuur heeft van zes koolstofatomen die elk aan één waterstofatoom zijn gebonden, met drie afwisselende pi-bindingen.


 <img alt="Figure1" src="/files/ftp_upload/12450/12450_PT_Figure1.svg" style="text-align: center; height: 100px;" />


 Volgens zijn model moeten er twee isomeren zijn voor de 1,2-digesubstitueerde benzeenderivaten, zoals weergegeven in figuur 2, waarbij in de ene structuur de twee gesubstitueerde koolstofatomen zijn verbonden door een dubbele binding, maar door een enkele binding in een andere.


 <img alt="Figure2" src="/files/ftp_upload/12450/12450_PT_Figure2.svg" style="text-align: center; height: 100px;" />


 Dit idee van Kekule bleek onjuist, aangezien er slechts één structuur wordt gevonden voor 1,2-digesubstitueerd benzeen. Om dit te verklaren stelde hij dat de twee vormen van benzeen snel in evenwicht zijn en dat het niet mogelijk is de twee verbindingen te scheiden.


 <img alt="Figure3" src="/files/ftp_upload/12450/12450_PT_Figure3.svg" style="text-align: center; height: 100px;" />


 Echter bestaat zo een evenwicht niet en wordt er slechts één structuur voor 1,2-digesubstitueerde benzeenderivaten gevonden.


 <img alt="Figure4" src="/files/ftp_upload/12450/12450_PT_Figure4.svg" style="text-align: center; height: 80px;" />


 Benzeen heeft vier graden van onverzadiging, die één ring en drie pi-bindingen vormen. Benzeen ondergaat echter geen additiereacties, in tegenstelling tot andere onverzadigde koolwaterstoffen zoals alkenen en alkynen. Ethyleen ondergaat bijvoorbeeld een additiereactie met broom om ethyleendibromide te vormen, maar benzeen niet.


 <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;" />


 Benzeen reageert met broom en vormt broombenzeen via een substitutiereactie in aanwezigheid van Lewis-zuur ijzerbromide.


 <img alt="Figure7" src="/files/ftp_upload/12450/12450_PT_Figure7.svg" style="text-align: center; height: 90px;" />


 Het belangrijkste nadeel van het model van Kekule is dat het deze ongebruikelijke reactiviteit van benzeen in vergelijking met andere onverzadigde koolwaterstoffen zoals alkenen en alkynen niet kon verklaren.

Structuur van benzeen: Kekulé-model

In 1865, August Kekule suggested the structure of benzene according to the structural theory of organic chemistry based on the three ...

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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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