Name: resonance in nitrite and carbonate ions
Uploaded: 2025-05-22T09:05:33.000Z
Duration: 2 min 16 s
Channel: JoVE

ほとんどの分子とイオンは、単一のルイス構造で表されます。しかし、特定の化合物では、一部の電子は特定の結合や原子に局在するのではなく、複数の結合や原子に非局在化しています。これらの化合物は、複数のルイス構造で正確に表すことができます。
三酸化硫黄のルイス構造を考えてみましょう。各酸素と中央の硫黄原子との間の単結合は、酸素原子のオクテットを満たします。ただし、硫黄の完全なオクテットに達するには、硫黄と酸素原子の1つとの間に追加の結合を形成する必要があります。
これらの三酸化硫黄の複数のルイス構造は、共鳴構造または寄与構造と呼ばれます。すべての構成原子の空間位置は同じままです。ただし、価電子は異なる分布をしています。
これらの両方向矢印はコンマと考えることができ、共振構造が平衡状態にあることと混同しないでください。これらの分子では、実際の構造は加重平均またはその共鳴構造のハイブリッドです。
共鳴構造は、「電子プッシュ」、つまり曲がった矢印で示されているように、孤立電子対を結合に、またはその逆に変換することによって識別されます。これにより、非ローカライズされた領域がマップされます。
このような非局在化は、共鳴安定化、つまり、理論上の非非局在化構造よりも低いポテンシャルエネルギーを持つ分子をもたらします。
寄与する構造が他の構造よりもエネルギーが低い場合、それは実際の分子構造により近くなります。特定の選好は、さまざまな寄与構造の相対エネルギーを推定するために使用されます。
まず、すべての原子が価電子殻を満たしている構造は、より安定しています。第二に、共有結合の数が多い構造はより安定しています。第三に、形式的な電荷を最小化する構造は、より安定しています。
最後に、電気陰性度の高い原子に負の電荷を帯びた構造と、電気陰性度の低い原子に正の電荷を持つ構造は、より安定しています。

共振とハイブリッド構造

Resonance and Hybrid Structures

共鳴理論によれば、同じ原子配置を持つ 2 つ以上のルイス構造式を 1 つの分子、イオン、またはラジカルに対して記述することができる場合、実際の電子の分布は、さまざまなルイス構造によって示される分布の平均になります。

共鳴構造と共鳴ハイブリッド

亜硝酸アニオン (NO2-) のルイス構造は、実際には 2 つの異なる方法で描画され、N-O 結合と N=O 結合の位置によって区別されます。

<img alt="Figure1" src="/files/ftp_upload/11684/11684%20PT%20Figure%201.svg" style="text-align: center; height: 50px;">

亜硝酸イオンに単結合と二重結合が含まれる場合、2 つの結合長は異なることが予想されます。 2 つの原子間の二重結合は、同じ 2 つの原子間の単結合よりも短く (そして強い) なります。 ただし、実験では、NO2- の両方の N-O 結合が同じ強度と長さを持ち、他のすべての特性が同一であることが示されています。 窒素がオクテットを持ち、両方の結合が同等である NO2- について単一のルイス構造を書くことは不可能です。

代わりに、共鳴の概念が使用されます。NO2- の各窒素 - 酸素結合における電子の実際の分布は、二重結合と単結合の平均です。

個々のルイス構造は共鳴構造と呼ばれます。 分子の実際の電子構造 (共鳴形態の平均) は、個々の共鳴形態の共鳴ハイブリッドと呼ばれます。 ルイス構造間の両矢印は、それらが共鳴形態であることを示します。

<img alt="Figure2" src="/files/ftp_upload/11684/11684%20PT%20Figure%202.svg" style="text-align: center; height: 50px;">

炭酸アニオン CO3 は、共鳴の 2 番目の例を提供します。

<img alt="Figure3" src="/files/ftp_upload/11684/11684%20PT%20Figure%203.svg" style="text-align: center; height: 75px;">

<ul style="list-style-type: disc; padding-left: 35px; margin: 0;">
	<li>中心原子のオクテットを完成するには、1 つの酸素原子が炭素との二重結合を持っている必要があります。</li>
	<li>ただし、すべての酸素原子は同等であり、二重結合は 3 つの原子のいずれかから形成される可能性があります。 これにより、炭酸イオンの 3 つの共鳴形態が生じます。</li>
	<li>3 つの同一の共鳴構造を記述することができるため、炭酸イオン内の電子の実際の配置は 3 つの構造の平均であることがわかります。</li>
	<li>繰り返しになりますが、実験により、3 つの C-O 結合はすべてまったく同じであることがわかりました。</li>
</ul>

共鳴ハイブリッドとして記述される分子は、いずれかの共鳴形式で記述される電子構造を決して持ちません。 共鳴形式間で変動しません。 むしろ、実際の電子構造は常に、すべての共鳴形態によって示されるものの平均です。

このテキストは、Openstax、Chemistry 2e、第 7.4 章 形式電荷と共鳴から改変されたものです。

共鳴の理論によれば、分子、イオン、またはラジカルに対して、同じ原子の配置を持つ2つ以上のルイス構造を書くことができる場合、...

Education

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Organic Chemistry

有機化学

Unit 1

ユニット1

Covalent Bonding and Structure

共有結合と構造

KEY TERMS AND CONCEPTS

主要用語と概念

resonance in nitrite and carbonate ions

Resonance explains why some Lewis structures do not match the measured properties of a molecule or ion. When two or more Lewis structures have the same atom arrangement, the real electron distribution is an average of the structures shown.

The nitrite anion, NO2−, is one example. It can be drawn in two ways, with the N–O and N=O bonds in different places. If nitrite had one single bond and one double bond, the two bond lengths should be different, because a double bond is shorter and stronger than a single bond.

Experiments show that both N–O bonds in NO2− have the same length and strength. They are identical in all other properties as well. A single Lewis structure cannot show this and still give nitrogen an octet, so resonance is used to describe the ion.

The separate Lewis structures are called resonance structures. The true electronic structure is called a resonance hybrid, which is the average of the resonance forms. A double-headed arrow between Lewis structures shows that they are resonance forms. Nitrite does not switch back and forth between them. Its electron distribution is always the average of the forms.

The carbonate anion, CO3 2−, gives a second example. One oxygen must form a double bond to carbon to complete the octet on the central atom, but each oxygen atom is equivalent. That means the double bond can be placed with any one of the three oxygens, creating three resonance forms.

Because the three resonance structures are identical, the electron arrangement in carbonate is the average of all three. Experiments show that all three C–O bonds are exactly the same. A resonance hybrid never has the exact electronic structure of just one resonance form.

Resonance in Nitrite and Carbonate Ions

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Explore how resonance explains equal bond lengths in nitrite and carbonate ions, and learn the difference between resonance structures and a resonance hybrid.

Resonance in Nitrite and Carbonate Ions | JoVE K12

organic chemistry

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Resonance in Nitrite and Carbonate Ions

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