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

В 1865 году Август Кекуле предложил первую определенную структуру бензола.
Он предположил, что бензол имеет циклическую структуру из шести атомов углерода с чередующимися одинарными и двойными связями. Каждый атом углерода присоединен к одному атому водорода.
Эта модель удовлетворяла четырехвалентности углерода, делая все водороды эквивалентными.
Модель Кекуле предсказывала две формы 1,2-дибромбензола, которые различались по положениям двойных связей. Но, была выделена только одна форма 1,2-дибромбензола.
Кекуле защищал его, предполагая быстрое равновесие между двумя неразделимыми формами бензола, ныне известными как структуры Кекуле. Однако в настоящее время известно, что такого равновесия не существует.
Более того, она не смогла объяснить необычное химическое поведение бензола, который вступает в реакцию замещения с бромом, в отличие от алкенов, которые подвергаются реакции присоединения.

Структура бензола: модель Кекуле

Structure of Benzene: Kekul&eacute; Model

В 1865 году Август Кекуле предложил структуру бензола в соответствии со структурной теорией органической химии, основанной на трех утверждениях: формула бензола — C_6H_6, все атомы водорода бензола эквивалентны, а каждый углерод должен иметь четыре связи из-за своей четырехвалентности.

Он предположил, что бензол имеет циклическую структуру из шести атомов углерода, присоединенных к одному атому водорода каждый, с тремя чередующимися пи-связями.

<img alt="Figure1" src="/files/ftp_upload/12450/12450_PT_Figure1.svg" style="text-align: center; height: 100px;">

Согласно его модели, у 1,2-дизамещенных производных бензола должно быть два изомера, как показано на рисунке 2, где в одной структуре два замещенных углерода соединены двойной связью, а в другой - одинарной.

<img alt="Figure2" src="/files/ftp_upload/12450/12450_PT_Figure2.svg" style="text-align: center; height: 100px;">

Это представление Кекуле оказалось ошибочным, поскольку для 1,2-дизамещенного бензола обнаружена только одна структура. Чтобы объяснить это, он заявил, что две формы бензола находятся в быстром равновесии и что разделить два соединения невозможно.

<img alt="Figure3" src="/files/ftp_upload/12450/12450_PT_Figure3.svg" style="text-align: center; height: 100px;">

Однако такого равновесия не существует, и обнаружена только одна структура для 1,2-дизамещенных производных бензола.

<img alt="Figure4" src="/files/ftp_upload/12450/12450_PT_Figure4.svg" style="text-align: center; height: 80px;">

Бензол имеет четыре степени ненасыщенности, составляющие одно кольцо и три пи-связи. Однако бензол не вступает в реакции присоединения, в отличие от других ненасыщенных углеводородов, таких как алкены и алкины. Например, этилен подвергается реакции присоединения с бромом с образованием дибромида этилена, а бензол — нет.

<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;">

Бензол действительно реагирует с бромом с образованием бромбензола посредством реакции замещения в присутствии бромида железа кислоты Льюиса.

<img alt="Figure7" src="/files/ftp_upload/12450/12450_PT_Figure7.svg" style="text-align: center; height: 90px;">

Главный недостаток модели Кекуле заключается в том, что она не смогла объяснить необычную реакционную способность бензола по сравнению с другими ненасыщенными углеводородами, такими как алкены и алкины.

В 1865 году Август Кекуле предложил структуру бензола в соответствии со структурной теорией органической химии, основанной на трех ...

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Organic Chemistry

Органическая химия

Unit 1

Раздел 1

Aromatic Compounds

Ароматические соединения

KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

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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