Name: resonance forms in lewis structures
Uploaded: 2020-09-03T13:38:28.000Z
Duration: 2 min 52 s
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Chemistry

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PS.Chm.3.1

Resonance

Большинство молекул и ионов можно представить с помощью уникальных структур Льюиса. Однако некоторые соединения могут быть представлены множеством одинаково достоверных структур Льюиса. Рассмотрим структуру Льюиса для триоксида серы.Одинарные связи между каждым кислородом и центральными атомами серы удовлетворяют октету для атомов кислорода. Однако для достижения полного октета атомом серы необходима дополнительная связь между серой и одним из атомов кислорода. Поскольку любой из трех атомов кислорода может образовывать двойную связь с серой, можно выделить три различные структуры Льюиса.Эти множественные структуры Льюиса называются резонансными структурами, в которых скелетные структуры остаются неизменными, но электроны распределяются по-разному. Все три структуры являются достоверными и эквивалентными представлениями молекулы, но все они не существуют в природе. Фактическая структура не колеблется между резонансными структурами, а является гибридом или средним значением трех структур Льюиса, которые можно измерить в длинах связей.В сульфите длина одинарной связи сера-кислород составляет 1, 51 ангстрема, в то время как в триоксиде серы длина связи сера-кислород составляет 1, 42 ангстрема. Таким образом, в гибридной молекуле длина связи является промежуточной между одинарной и двойной связями. В гибридных молекулах электроны, участвующие в образовании двойных связей или входящие в неподеленные пары, часто делокализованы по нескольким связям или атомам, что означает, что они не зафиксированы на одном конкретном атоме.Делокализация снижает потенциальную энергию электронов, что приводит к стабилизации, называемой резонансной стабилизацией. Резонанс наблюдается также для ароматических соединений, таких как бензол. Бензол представляет собой шестичленное углеродное кольцо с одним водородом, связанным с каждым из атомов углерода, и чередующимися одинарными и двойными связями между атомами углерода.В зависимости от расположения двойных связей углерод-углерод бензол может иметь две резонансные структуры. Напомним, что двойные связи обычно короче одинарных. Однако все углерод-углеродные связи в бензоле имеют одинаковую длину, которая занимает промежуточное положение между углерод-углеродными одинарными и двойными связями.Бензол, следовательно, существует как резонансный гибрид и может быть представлен в виде шестиугольника с кругом внутри. Круг указывает, что бензол представляет собой смесь двух резонансных структур, и двойные связи не могут быть локализованы на каких-либо двух конкретных атомах углерода.

Резонанс

Структура Льюиса нитрит аниона (NО2) может быть начерчена двумя различными способами, отличаемыми расположением связей N-O и N=O.  
 
 <img alt=""Figure1"" src=""https://www.jove.com/files/core/CDNSource/lessons/1064/1064Image_1.svg"" style=""height: 40px;"" />
 
 Если ионы нитритов действительно содержат одну и двойную связь, то ожидается, что две длины связи будут разными. Двойная связь между двумя атомами короче (и сильнее), чем одна связь между двумя атомами. Однако эксперименты показывают, что и связи N–O в No2 имеют одинаковую прочность и длину и идентичны по всем другим свойствам. Невозможно создать единую структуру Льюиса для NО2, в которой азот имеет октет, и обе связи эквивалентны. 
 
 Вместо этого используется понятие резонанса: Если для молекулы или иона могут быть записаны две или более структуры Льюиса с одинаковым расположением атомов, фактическое распределение электронов является средним значением, показанным различными структурами Льюиса. Фактическое распределение электронов в каждом из азотно-кислородных связей NО2 - это среднее значение двойной связи и одной связи.  
 
 Отдельные структуры Льюиса называются резонансными формами. Фактическую электронную структуру молекулы (среднее от резонансных форм) называют резонансным гибридным из отдельных резонансных форм. Двуглавая стрелка между структурами Льюиса указывает на то, что они являются резонансной формой. 
 
 <img alt=""Figure2"" src=""https://www.jove.com/files/core/CDNSource/lessons/1064/1064Image_2.svg"" style=""height: 40px;"" />
 
 Карбонат анион, CO32-, является вторым примером резонанса.  
 
 <img alt=""Figure3"" src=""https://www.jove.com/files/core/CDNSource/lessons/1064/1064Image_3.svg"" style=""height: 80px;"" />
 
 <ul>
 <li>Один атом кислорода должен иметь двойную связь с углеродом, чтобы завершить октет на центральном атоме.  </li>
 <li>Однако все атомы кислорода эквивалентны, и двойная связь может образоваться от любого из трех атомов. Это приводит к появлению трех резонансных форм карбонатного иона.  </li>
 <li>Поскольку можно записать три идентичные резонансные структуры, фактическое расположение электронов в карбонатномионе, как известно, является средним из трех структур.  </li>
 <li>Опять же, эксперименты показывают, что все три связи с-о абсолютно одинаковы. </li>
 </ul>
 
 Всегда помните, что молекула, описанная как резонансный гибрид, никогда не обладает электронной структурой, описанной ни по резонансной форме. Он не колеблется между резонансной формой; скорее, фактическая электронная структура всегда является средним значением, показанным всеми резонансными формами.  
 
 Джордж Веланд, один из пионеров теории резонанса, использовал историческую аналогию для описания взаимосвязи между резонансными формами и резонансными гибридами. Средневековый путешественник, никогда ранее не видел носорога, описал его как гибрид дракона и единорога, потому что он имел много общих с ними свойств. Так же как носорог не является ни драконом, ни единорогом в другое время, резонансный гибрид не является ни его резонансной формой в данный момент времени. 
 
 Как и носорог, это реальный объект, существование которого показали экспериментальные данные. У него есть некоторые общие характеристики с его резонансными формами, но сами резонансные формы представляют собой удобные воображаемые изображения (например, единорог и дракон). 
 
 Этот текст адаптирован к <a href=""https://openstax.org/books/chemistry-2e/pages/7-4-formal-charges-and-resonance"">Openstax, Химия 2е изд., раздел 7.4: Формальные сборы и резонанс.</a>

Структура Льюиса нитритного аниона (NO2−) на самом деле может быть построена двумя различными способами, различающимися по расположению N-O и N=O ...

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resonance forms in lewis structures

Resonance forms in Lewis structures help explain molecules and ions that cannot be shown correctly with just one drawing. In nitrite ion, NO2−, the Lewis structure can be drawn in two ways. The N-O and N=O bonds switch positions in the two drawings, but the atoms stay in the same arrangement.

If nitrite really had one single bond and one double bond, the two bond lengths should be different. A double bond is shorter and stronger than a single bond between the same atoms. Experiments show that both N-O bonds in NO2− have the same length, strength, and other properties. That means the real structure is not one Lewis form alone.

Resonance is the idea used when two or more Lewis structures with the same atom arrangement can be written for one species. The actual electron distribution is an average of the electrons shown in the different forms. In NO2−, each N-O bond has an average of a single bond and a double bond. The separate Lewis structures are called resonance forms, and the real structure is called a resonance hybrid.

A double-headed arrow is written between resonance forms to show this relationship. Carbonate ion, CO3 2−, is another clear example. One oxygen must form a double bond to carbon to complete the carbon octet, but any of the three oxygen atoms can make that bond. This gives three resonance forms, and experiments show that all three C-O bonds are exactly the same.

A resonance hybrid never has the electronic structure of just one resonance form. It does not switch back and forth between forms. Instead, it always exists as the average of all the resonance forms. George Wheland used a historical comparison to explain this idea. A medieval traveler who had never seen a rhinoceros described it as a hybrid of a dragon and a unicorn, but a rhino is neither one at different times. In the same way, resonance forms are useful imaginary pictures, while the resonance hybrid is the real entity shown by evidence.

Resonance Forms in Lewis Structures

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Learn how resonance forms explain equal bond lengths in nitrite and carbonate ions, and how a resonance hybrid represents the real electron distribution.

Resonance Forms in Lewis Structures | JoVE K12

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Resonance Forms in Lewis Structures

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