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

Die meisten Moleküle und Ionen werden durch eine einzige Lewis-Struktur dargestellt. Bei bestimmten Verbindungen sind jedoch einige Elektronen über mehrere Bindungen oder Atome delokalisiert, anstatt an eine bestimmte Bindung oder ein bestimmtes Atom zu lokalisieren. Diese Verbindungen können mit mehreren Lewis-Strukturen genau dargestellt werden.
Betrachten Sie die Lewis-Struktur für Schwefeltrioxid. Die Einfachbindungen zwischen jedem Sauerstoff- und den zentralen Schwefelatomen erfüllen das Oktett für die Sauerstoffatome. Um jedoch ein volles Oktett für den Schwefel zu erreichen, muss eine zusätzliche Bindung zwischen Schwefel und einem der Sauerstoffatome gebildet werden.
Diese multiplen Lewis-Strukturen aus Schwefeltrioxid werden als Resonanzstrukturen oder beitragende Strukturen bezeichnet. Die räumlichen Positionen aller Komponentenatome bleiben gleich; Die Valenzelektronen sind jedoch unterschiedlich verteilt.
Diese doppelköpfigen Pfeile können als Kommas betrachtet werden und sollten nicht mit Resonanzstrukturen verwechselt werden, die sich im Gleichgewicht befinden. Bei diesen Molekülen ist die eigentliche Struktur der gewichtete Durchschnitt oder ein Hybrid ihrer Resonanzstrukturen.
Resonanzstrukturen werden durch "Elektronenschieben" oder die Umwandlung von Einzelpaaren in Bindungen und umgekehrt gekennzeichnet, wie durch gekrümmte Pfeile gekennzeichnet. Dadurch werden die delokalisierten Gebiete kartiert.
Eine solche Delokalisierung führt zu einer Resonanzstabilisierung, d. h. zu einem Molekül mit einer geringeren potentiellen Energie als die einer theoretischen nicht-delokalisierten Struktur.
Wenn eine beitragende Struktur einen niedrigeren Energieverbrauch hat als eine andere, ähnelt sie eher der tatsächlichen Molekülstruktur. Bestimmte Präferenzen werden verwendet, um die relativen Energien verschiedener beitragender Strukturen abzuschätzen.
Erstens sind Strukturen, in denen alle Atome gefüllte Valenzschalen haben, stabiler. Zweitens sind Strukturen mit einer größeren Anzahl kovalenter Bindungen stabiler. Drittens sind Strukturen, die formale Gebühren minimieren, stabiler.
Schließlich sind Strukturen, die negative Ladungen auf mehr elektronegativen Atomen und positive Ladungen auf weniger elektronegativen Atomen tragen, stabiler.

resonanz- und hybridstrukturen

Resonance and Hybrid Structures

Wenn nach der Resonanztheorie zwei oder mehr Lewis-Strukturen mit der gleichen Atomanordnung für ein Molekül, Ion oder Radikal geschrieben werden können, ist die tatsächliche Elektronenverteilung ein Durchschnitt der verschiedenen Lewis-Strukturen.

Resonanzstrukturen und Resonanzhybride

Die Lewis-Struktur eines Nitritanions (NO2-) kann tatsächlich auf zwei verschiedene Arten dargestellt werden, die sich durch die Lage der N-O- und N=O-Bindungen unterscheiden.

<img alt="Figure1" src="/files/ftp_upload/11684/11684%20PT%20Figure%201.svg" style="text-align: center; height: 50px;">

Wenn Nitritionen eine Einfach- und eine Doppelbindung enthalten, ist zu erwarten, dass die beiden Bindungslängen unterschiedlich sind. Eine Doppelbindung zwischen zwei Atomen ist kürzer (und stärker) als eine Einfachbindung zwischen denselben beiden Atomen. Experimente zeigen jedoch, dass beide N-O-Bindungen in NO2- die gleiche Stärke und Länge haben und in allen anderen Eigenschaften identisch sind. Es ist nicht möglich, eine einzige Lewis-Struktur für NO2- zu schreiben, in der Stickstoff ein Oktett aufweist und beide Bindungen äquivalent sind.

Stattdessen wird das Konzept der Resonanz verwendet: Die tatsächliche Verteilung der Elektronen in jeder der Stickstoff-Sauerstoff-Bindungen in NO2- ist der Durchschnitt einer Doppelbindung und einer Einfachbindung.

Die einzelnen Lewis-Strukturen werden Resonanzstrukturen genannt. Die tatsächliche elektronische Struktur des Moleküls (der Durchschnitt der Resonanzformen) wird als Resonanzhybrid der einzelnen Resonanzformen bezeichnet. Ein Doppelpfeil zwischen Lewis-Strukturen zeigt an, dass es sich um Resonanzformen handelt.

<img alt="Figure2" src="/files/ftp_upload/11684/11684%20PT%20Figure%202.svg" style="text-align: center; height: 50px;">

Das Carbonatanion CO32- ist ein zweites Beispiel für Resonanz.

<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; margin-left: 35px;">
	<li>Ein Sauerstoffatom muss eine Doppelbindung zum Kohlenstoff haben, um das Oktett am Zentralatom zu vervollständigen.</li>
	<li>Alle Sauerstoffatome sind jedoch gleichwertig und die Doppelbindung könnte aus jedem der drei Atome entstehen. Dadurch entstehen drei Resonanzformen des Carbonations.</li>
	<li>Da drei identische Resonanzstrukturen geschrieben werden können, ist die tatsächliche Anordnung der Elektronen im Carbonation bekanntermaßen der Durchschnitt der drei Strukturen.</li>
	<li>Auch hier zeigen Experimente, dass alle drei CO-Bindungen genau gleich sind.</li>
</ul>

Ein als Resonanzhybrid beschriebenes Molekül besitzt niemals eine elektronische Struktur, die durch eine der beiden Resonanzformen beschrieben werden kann. Es schwankt nicht zwischen Resonanzformen; Vielmehr ist die tatsächliche elektronische Struktur immer der Durchschnitt aller Resonanzformen.

Dieser Text ist übernommen aus  <a href="https://openstax.org/books/chemistry-2e/pages/7-4-formal-charges-and-resonance">Openstax, Chemistry 2e, Chapter 7.4 Formal Charges and Resonance</a>.

Resonanz- und Hybridstrukturen

Wenn nach der Resonanztheorie zwei oder mehr Lewis-Strukturen mit der gleichen Anordnung von Atomen für ein Molekül, Ion oder Radikal geschrieben werden können, ...

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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

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Resonance in Nitrite and Carbonate Ions

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