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Resonance

In chemistry, the term resonance is used to describe instances where the actual electronic distribution in a molecule is a combination of several contributing structures. The contributing structures are known as resonance structures or resonance forms.

Examples of common resonance structures

  • A carboxylate anion is written with a double bond between carbon and one oxygen atom and a single bond between carbon and the other oxygen atom, which bears a negative charge. But the other resonance structure is equally correct. The real structure of a carboxylate ion is an average of the two.

    There are two equivalent forms of the carboxylate anion where the negative charge and double bond to the carbon interchanges between both oxygen atoms in the ion. The resonance form is indicated with a double ended arrow in between both structures

  • A benzene ring (or any aromatic compound) can be written as two different resonance structures, placing the double bonds in the ring in two different ways. In reality, the structure of benzene is an average of the two structures, with each bond being in between a single and a double bond.

    Six membered benzene ring with a double ended arrow and it's equivalent resonance form which has shifted all three double bonds around the ring

  • A protonated carbonyl compound is written with a double bond between carbon and oxygen and a formal charge on oxygen, but the other resonance structure, placing the formal charge on carbon and having only a single bond between carbon and oxygen is equally correct. In reality, the real structure is somewhere in between.

    The oxygen in the C O double bond is protonated so now has a positive charge a double ended arrow points to the equivalent resonance form where the double bond between carbon and oxygen is broken and the carbon atom now bears the positive charge

Referred from:

Article
Electrophiles