Organic Chemistry

Resonance Effect Or Mesomeric effect

Resonance Effect Or Mesomeric effect:-

  1. The chemical structure can be written in two or more different form.
  2. These different structures are called Resonance structure or canonical form.
  3. Resonance structure is made by shifting of π (pi) electrons.
  4. All resonance or canonical structure is an imaginary structure.
  5. None of them can explain all the properties of molecules.
  6. All resonance or canonical structure can be mixed to form a Hydride structure.
  7. Hybrid structure is call resonance hybrid (real structure).
  8. All the properties of molecules can be explained from resonance hybrid structure (real structure).

All C-C bond length is equal and it is near about 139 pedometers.

In the below diagram, It is a Berzen structure.

There are some more molecules which exist resonance effect.

– CO2

– R-NO2


Show resonance the molecule must have π (pi) Electrons

Condition for π (pi) Electrons :

(Conjugation system)

π electrons are alternative electrons

π electrons have the positive charge

π electrons have the negative charge

π electrons have lone pairs

π electrons have radical partials

Types of Resonance effect :

+R Effect: – Lets example with phenol suppose that benzene part of phenol is a system and OH part is the group. In the group, there is two lone pair available in oxygen. Oxygen gives electrons to benzene. Therefore we can say the shifting of electrons is group into the system.

Let’s example with chlorine. There is three lone pair available in Cl. Cl will give their electron to Bond. Therefore you can say Groups which donate electrons to the double bond or conjugate system or aromatic system.

Some example of + R Effect:-

– OH                                       – NH2

– OR                                       – CH3

– CH2CH3                               – SH

– Ph                                        – CH2 = CH2

Halogens: -F, -Cl, -Br, -I

-R Effect: – Let me explain with a carboxylic acid.

Let’s suppose COOH is a group and benzene part is a system. Oxygen has more negative charges.  The same thing happens with cyanide. System (conjugation system) gives electrons to the group.

Groups which withdraw electrons from double bond or conjugate system or aromatic system.

Some example of -R Effect:-

– CONH2                               – CHO

– CO-                                      – COOH

– COOR                                 – CN

– NO2

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Resonance Effect Or Mesomeric effect