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

Большинство молекул и ионов представлены одной структурой Льюиса. Однако, в некоторых соединениях, некоторые электроны делокализованы в нескольких связях или атомах, а не локализованы в конкретной связи или атоме. Эти соединения могут быть точно представлены с помощью нескольких структур Льюиса.
Рассмотрим структуру Льюиса для триоксида серы. Одиночные связи между каждым атомом кислорода и центральным атомом серы удовлетворяют октету для атомов кислорода. Однако, чтобы достичь полного октета для серы, должна быть сформирована дополнительная связь между серой и одним из атомов кислорода.
Эти множественные структуры Льюиса триоксида серы называются резонансными структурами или вкладными структурами. Пространственное положение всех составляющих атомов остается прежним; Однако валентные электроны распределены иначе.
Эти двунаправленные стрелки можно рассматривать как запятые, и их не следует путать с резонансными структурами, находящимися в равновесии. В этих молекулах фактическая структура является средневзвешенной или гибридом ее резонансных структур.
Резонансные структуры идентифицируются «выталкиванием электронов» или превращением одиноких пар в связи и наоборот, как обозначено изогнутыми стрелками. Это наносит на карту делокализованные области.
Такая делокализация приводит к резонансной стабилизации, т. е. к молекуле с меньшей потенциальной энергией, чем у любой теоретической неделокализованной структуры.
Если одна из составляющих структур имеет меньшую энергию, чем другая, она больше похожа на реальную молекулярную структуру. Определенные предпочтения используются для оценки относительных энергий различных структур, вносящих свой вклад.
Во-первых, структуры, в которых все атомы заполнены валентными оболочками, более стабильны. Во-вторых, более устойчивы структуры с большим числом ковалентных связей. В-третьих, структуры, которые минимизируют формальные сборы, более стабильны.
Наконец, структуры, несущие отрицательные заряды на большем количестве электроотрицательных атомов и положительные заряды на менее электроотрицательных атомах, более стабильны.

Резонансные и гибридные структуры

Resonance and Hybrid Structures

Согласно теории резонанса, если для молекулы, иона или радикала можно записать две или более структуры Льюиса с одинаковым расположением атомов, фактическое распределение электронов является средним по сравнению с тем, которое показано различными структурами Льюиса.

Резонансные структуры и резонансные гибриды

Структура Льюиса нитрит-аниона (NO2-) на самом деле может быть изображена двумя разными способами, отличающимися расположением связей N–O и N=O.


<img alt="Figure1" src="/files/ftp_upload/11684/11684%20PT%20Figure%201.svg" style="text-align: center; height: 50px;">

Если ионы нитрита содержат одинарную и двойную связь, ожидается, что длины двух связей будут разными. Двойная связь между двумя атомами короче (и прочнее), чем силы Ван-дер-Ваальса между теми же двумя атомами. Однако эксперименты показывают, что обе связи N–O в NO2- имеют одинаковую прочность и длину, а по всем остальным свойствам идентичны. Невозможно написать единую структуру Льюиса для NO2-, в которой азот имеет октет и обе связи эквивалентны.


Вместо этого используется концепция резонанса: фактическое распределение электронов в каждой из связей азот–кислород в NO2- представляет собой среднее значение двойной связи и одинарной связи.


Отдельные структуры Льюиса называются резонансными структурами. Фактическая электронная структура молекулы (среднее значение резонансных форм) называется резонансным гибридом отдельных резонансных форм. Двусторонняя стрелка между структурами Льюиса указывает на то, что они являются резонансными формами.


<img alt="Figure2" src="/files/ftp_upload/11684/11684%20PT%20Figure%202.svg" style="text-align: center; height: 50px;">

Карбонат-анион CO3 представляет собой второй пример резонанса.


<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; display: flex; flex-direction: column; gap: 10px; padding: 0 0 0 40px;">
 <li> Один атом кислорода должен иметь двойную связь с углеродом, чтобы завершить октет центрального атома.</li>
 <li> Однако все атомы кислорода эквивалентны, и двойная связь может образоваться из любого из трех атомов. Это приводит к появлению трех резонансных форм карбонат-иона.</li>
 <li> Поскольку можно записать три идентичные резонансные структуры, известно, что фактическое расположение электронов в карбонат-ионе является средним из трех структур.</li>
 <li> Опять же, эксперименты показывают, что все три связи C–O совершенно одинаковы.</li>
</ul>

Молекула, описываемая как резонансный гибрид, никогда не обладает электронной структурой, описываемой какой-либо резонансной формой. Она не колеблется между резонансными формами; скорее, реальная электронная структура всегда представляет собой среднее значение всех резонансных форм.

Согласно теории резонанса, если две или более структуры Льюиса с одинаковым расположением атомов могут быть записаны для молекулы, иона или радикала, ...

Education

JoVE Core

Organic Chemistry

Органическая химия

Unit 1

Раздел 1

Covalent Bonding and Structure

Ковалентная связь и структура

KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

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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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...

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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

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Resonance in Nitrite and Carbonate Ions

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