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Resonance

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structure of benzene

Resonance or Mesomerism

Let us begin this section with an example of benzene :

The formula of benzene is C6H6 and its structure has alternating C−C single and C=C double bonds. Benzene can be represented by the following two structures :

Resonance structures of Benzene

Resonance example : benzene

The actual bond lengths of C-C single and C=C double bonds are 1.54 Å and 1.34 Å respectively. However, all the six carbon-carbon bond lengths in benzene are equal (1.39 Å). This implies that any two adjacent carbon atoms in benzene are neither joined by a pure single bond nor by a pure double bond. This leads to the following conclusion :

The behaviour of some molecules cannot be explained by a single Lewis structure. In such cases the molecule can be represented by two or more electronic structures one of which can explain most of the properties but none of them can explain all the properties of the molecule. The real structure of the molecule is an approximate intermediate between all these electronic structures. Such molecules are said to exhibit resonance or mesomerism. The various Lewis structures of these molecules are known as resonance structures or canonical structures and the real structure is called resonance hybrid.

Resonance hybrid of benzene

Resonance hybrid of benzene

Actual molecule of benzene is usually represented by this structure.

Note : A molecule that exhibits resonance property does not oscillate from one structure to another. The molecule has a single structure which is the resonance hybrid of the canonical forms.


Types of Resonance Effect

There are two types of resonance effects :

  1. Positive resonance effect (+R Effect or +M Effect)
  2. Negative resonance effect (-R Effect or -M Effect)

Positive Resonance Effect

Positive Resonance example : Aniline

In positive resonance, the transfer of electrons is away from an atom or substituent group attached to the conjugated system ( conjugated systems contain alternate σ and π bonds ).

Positive resonance example : aniline

Positive Resonance example : Aniline Positive Resonance example : Aniline Positive Resonance example : Aniline Positive Resonance example : Aniline Positive Resonance example : Aniline Positive Resonance example : Aniline Positive Resonance example : Aniline

Positive resonance example : aniline

Positive Resonance example : Aniline
Positive Resonance example : Aniline
Positive Resonance example : Aniline
Positive Resonance example : Aniline
Positive Resonance example : Aniline
Positive Resonance example : Aniline
Positive Resonance example : Aniline

Negative Resonance Effect

In negative resonance, the transfer of electrons is towards the atom or substituent group attached to the conjugated system.

Example : CH2CHCN

Negative Resonance example : CH2CHCN

Transfer of electron towards CN

Negative Resonance example : CH2CHCN Negative Resonance example : CH2CHCN Negative Resonance example : CH2CHCN

Conditions For Resonance

Major and Minor Contributors in Resonance

Of all resonance structures, the structure which is more stable resembles the actual molecule more than the others. Stability of resonance structures can be understood with the help of the following points :

true/false

The following pair of structures is an example of resonance :

resonance structures examples
Examples of resonance

true/false

The following pair of structures is an example of resonance :

resonance structures examples
Examples of resonance

Which one of the following resonance structures is the most stable and which one is the least?

Resonance example : benzene
Resonance example : benzene
Resonance example : benzene

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