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.

structure of benzene: kekul&eacute; model

Structure of Benzene: Kekul&eacute; Model

In 1865, August Kekule suggested the structure of benzene according to the structural theory of organic chemistry based on the three assertions&#8212;formula of benzene is C6H6, all the hydrogens of benzene are equivalent, and each carbon must have four bonds due to its tetravalency.
He proposed that benzene has a cyclic structure of six carbon atoms attached to one hydrogen atom each, with three alternating pi bonds.
<img alt="Benzene molecular structure, hexagonal ring, chemical bonding diagram, aromatic compound." src="/files/ftp_upload/12450/12450_PT_Figure1.svg" style="text-align: center; height: 100px;" data-altupdatedat="1761830439000" data-alt="Figure1" />
According to his model, there must be two isomers for the 1,2-disubstituted benzene derivatives, as shown in Figure 2, wherein in one structure, the two substituted carbons are connected by a double bond but by a single bond in another.
<img alt="Isomer diagram; 1,2-disubstituted benzene derivatives; chemical structure; organic chemistry study." src="/files/ftp_upload/12450/12450_PT_Figure2.svg" style="text-align: center; height: 100px;" data-altupdatedat="1761830439000" data-alt="Figure2" />
This notion of Kekule was proven wrong as only one structure is found for 1,2-disubstituted benzene. To account for this, he stated that the two forms of benzene are in rapid equilibrium and that it is not possible to separate the two compounds.
<img alt="Equilibrium of 1,2-disubstituted benzene derivatives, chemical reaction diagram, isomerization." src="/files/ftp_upload/12450/12450_PT_Figure3.svg" style="text-align: center; height: 100px;" data-altupdatedat="1761830439000" data-alt="Figure3" />
However, no such equilibrium exists, and only one structure for 1,2-disubstituted benzene derivatives is found.
<img alt="Keto-enol tautomerism mechanism diagram with aromatic compound structures, illustrating chemical equilibrium." src="/files/ftp_upload/12450/12450_PT_Figure4.svg" style="text-align: center; height: 80px;" data-altupdatedat="1761830439000" data-alt="Figure4" />
Benzene has four degrees of unsaturation, constituting one ring and three pi bonds. However, benzene does not undergo addition reactions, unlike other unsaturated hydrocarbons such as alkenes and alkynes. For example, ethylene undergoes an addition reaction with bromine to form ethylene dibromide, but benzene does not.
<img alt="Ethylene addition reaction with bromine, chemical equation diagram showing ethylene dibromide formation." src="/files/ftp_upload/12450/12450_PT_Figure5.svg" style="text-align: center; height: 120px;" data-altupdatedat="1761830439000" data-alt="Figure5" />
<img alt="Benzene non-reactivity with Br2; chemical equation diagram; demonstrates lack of addition reaction." src="/files/ftp_upload/12450/12450_PT_Figure6.svg" style="text-align: center; height: 60px;" data-altupdatedat="1761830439000" data-alt="Figure6" />
Benzene does react with bromine to form bromobenzene via substitution reaction in the presence of Lewis acid ferric bromide.
<img alt="Substitution reaction diagram of benzene with bromine using FeBr3 catalyst, chemical equation." src="/files/ftp_upload/12450/12450_PT_Figure7.svg" style="text-align: center; height: 90px;" data-altupdatedat="1761830439000" data-alt="Figure7" />
The main drawback of Kekule&#8217;s model is that it failed to explain this unusual reactivity of benzene compared with other unsaturated hydrocarbons such as alkenes and alkynes.

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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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

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Kekule’s Benzene Model and Its Limits

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