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

W 1865 roku August Kekulé zaproponował pierwszą określoną strukturę benzenu.
Zasugerował, że benzen ma cykliczną strukturę sześciu atomów węgla z naprzemiennymi wiązaniami pojedynczymi i podwójnymi. Każdy atom węgla jest przyłączony do jednego atomu wodoru.
Model ten spełniał wymóg czterowartościowości węgla, dzięki czemu wszystkie wodory były równoważne.
Model Kekulégo przewidywał dwie formy 1,2-dibromobenzenu, które różniły się położeniem wiązań podwójnych. Ale wyizolowano tylko jedną formę 1,2-dibromobenzenu.
Kekulé bronił jej, proponując szybką równowagę między dwiema nierozłącznymi formami benzenu, znanymi obecnie jako struktury Kekulé. Obecnie wiadomo jednak, że taka równowaga nie istnieje.
Co więcej, nie udało się wyjaśnić niezwykłego zachowania chemicznego benzenu, który ulega reakcji substytucji bromem, w przeciwieństwie do alkenów, które ulegają reakcji addycji.

struktura benzenu: model kekulé’a

Structure of Benzene: Kekul&eacute; Model

W 1865 roku August Kekulé’a zasugerował strukturę benzenu zgodnie ze strukturalną teorią chemii organicznej opartą na trzech twierdzeniach - wzór benzenu to C_6H_6, wszystkie wodory benzenu są równoważne, a każdy węgiel musi mieć cztery wiązania ze względu na jego tetrawalencję.

Zaproponował, że benzen ma cykliczną strukturę składającą się z sześciu atomów węgla przyłączonych do jednego atomu wodoru każdy, z trzema naprzemiennymi wiązaniami pi.

<img alt="Figure1" src="/files/ftp_upload/12450/12450_PT_Figure1.svg" style="text-align: center; height: 100px;">

Według jego modelu muszą istnieć dwa izomery dla 1,2-dipodstawionych pochodnych benzenu, jak pokazano na rysunku 2, gdzie w jednej strukturze dwa podstawione węgle są połączone wiązaniem podwójnym, ale wiązaniem pojedynczym w drugiej.

<img alt="Figure2" src="/files/ftp_upload/12450/12450_PT_Figure2.svg" style="text-align: center; height: 100px;">

Udowodniono, że ta koncepcja Kekulé’a jest błędna, ponieważ dla 1,2-dipodstawionego benzenu znaleziono tylko jedną strukturę. Aby to wyjaśnić, stwierdził, że dwie formy benzenu znajdują się w szybkiej równowadze i że nie jest możliwe rozdzielenie tych dwóch związków.

<img alt="Figure3" src="/files/ftp_upload/12450/12450_PT_Figure3.svg" style="text-align: center; height: 100px;">

Jednak taka równowaga nie istnieje i znaleziono tylko jedną strukturę dla 1,2-dipodstawionych pochodnych benzenu.

<img alt="Figure4" src="/files/ftp_upload/12450/12450_PT_Figure4.svg" style="text-align: center; height: 80px;">

Benzen ma cztery stopnie nienasycenia, co stanowi jeden pierścień i trzy wiązania pi. Jednak benzen nie ulega reakcjom addycji, w przeciwieństwie do innych nienasyconych węglowodorów, takich jak alkeny i alkiny. Na przykład etylen ulega reakcji addycji z bromem, tworząc dibromek etylenu, ale benzen nie.

<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;">

Benzen reaguje z bromem, tworząc bromobenzen poprzez reakcję podstawienia w obecności bromku żelaza kwasu Lewisa.

<img alt="Figure7" src="/files/ftp_upload/12450/12450_PT_Figure7.svg" style="text-align: center; height: 90px;">

Główną wadą modelu Kekulé’a jest to, że nie udało mu się wyjaśnić tej niezwykłej reaktywności benzenu w porównaniu z innymi nienasyconymi węglowodorami, takimi jak alkeny i alkiny.

Struktura benzenu: model Kekulé’a

W 1865 roku August Kekule zasugerował strukturę benzenu zgodnie z teorią strukturalną chemii organicznej opartą na trzech ...

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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

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Kekule’s Benzene Model and Its Limits

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