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

1865年、アウグスト・ケクレはベンゼンの最初の明確な構造を提案しました。
彼は、ベンゼンが6つの炭素原子が交互に単結合と二重結合を持つ環状構造を持っていることを示唆しました。各炭素原子は1つの水素原子に結合しています。
このモデルは炭素の四価性を満たし、すべての水素を同等にしました。
ケクレのモデルは、二重結合の位置が異なる1,2-ジブロモベンゼンの2つの形態を予測しました。しかし、1,2-ジブロモベンゼンの1つの形態のみが単離されました。
ケクレは、現在ケクレ構造として知られている2つの不可分な形態のベンゼンの間の急速な平衡を提案することによってそれを擁護しました。しかし、現在ではそのような均衡は存在しないことがわかっています。
さらに、ベンゼンは臭素と置換反応を起こし、アルケンは付加反応を起こすのとは対照的に、ベンゼンの異常な化学的挙動を説明することができませんでした。

ベンゼンの構造: ケクレモデル

Structure of Benzene: Kekul&eacute; Model

1865 年、アウグストケクレは、ベンゼンの式は C_6H_6、ベンゼンのすべての水素は同等、四価であるため各炭素には 4 つの結合がなければならないという 3 つの主張に基づいて、有機化学の構造理論に従ってベンゼンの構造を提案しました。

彼は、ベンゼンは 6 つの炭素原子がそれぞれ 1 つの水素原子に結合し、3 つのパイ結合が交互に存在する環状構造を持っていると提案しました。

<img alt="Figure1" src="/files/ftp_upload/12450/12450_PT_Figure1.svg" style="text-align: center; height: 100px;">

彼のモデルによれば、図 2 に示すように、1,2-二置換ベンゼン誘導体には 2 つの異性体が存在するはずです。ある構造では 2 つの置換炭素が二重結合で接続されていますが、別の構造では単結合で接続されています。

<img alt="Figure2" src="/files/ftp_upload/12450/12450_PT_Figure2.svg" style="text-align: center; height: 100px;">

1,2-二置換ベンゼンには構造が1つしか見つからないため、このケクレの概念は間違っていることが証明されました。 これを説明するために、彼は、ベンゼンの 2 つの形態は急速に平衡状態にあり、2 つの化合物を分離することは不可能であると述べました。

<img alt="Figure3" src="/files/ftp_upload/12450/12450_PT_Figure3.svg" style="text-align: center; height: 100px;">

しかし、そのような平衡は存在せず、1,2-二置換ベンゼン誘導体の構造は 1 つだけ見つかっています。

<img alt="Figure4" src="/files/ftp_upload/12450/12450_PT_Figure4.svg" style="text-align: center; height: 80px;">

ベンゼンには 4 つの不飽和度があり、1 つの環と 3 つのパイ結合を構成しています。 ただし、アルケンやアルキンなどの他の不飽和炭化水素とは異なり、ベンゼンは付加反応を起こしません。 たとえば、エチレンは臭素と付加反応して二臭化エチレンを形成しますが、ベンゼンは反応しません。

<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;">

ベンゼンは、ルイス酸臭化第二鉄の存在下での置換反応により臭素と反応してブロモベンゼンを形成します。

<img alt="Figure7" src="/files/ftp_upload/12450/12450_PT_Figure7.svg" style="text-align: center; height: 90px;">

ケクレのモデルの主な欠点は、アルケンやアルキンなどの他の不飽和炭化水素と比較したベンゼンのこの異常な反応性を説明できなかったことです。

1865年に、アウグストケクレは、3つの...

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Organic Chemistry

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Unit 1

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Aromatic Compounds

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KEY TERMS AND CONCEPTS

主要用語と概念

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

organic chemistry

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Kekule’s Benzene Model and Its Limits

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