Resonance/Mesomeric Effect:

 

Resonance/Mesomeric Effect:

The resonance or mesomeric effect refers to the polarity produced in a molecule as a result of interaction two π-bonds or between a π-bond and a lone pair of electrons. This effect operates through π-electron delocalization and is transmitted along a chain of conjugated bonds.

Or

This is the movement of electrons in a molecule where the electrons can shift between atoms or bonds. This shift of electrons creates resonance structures, leading to stabilization.

 

Examples of resonance effects:

      Aniline is weaker base than ammonia due to the delocalization of its lone pair into the ring.

      Phenols are more acidic than alcohols due to resonance stabilization of their conjugate bases.

Resonance Effect

Types of Resonance effect

The resonance effect is of two types:

• +R effect
• -R effect

1.      Positive resonance effect (+R)

·         Electron donating Groups like -OH, -OR, -NH₂, -Cl, -Br

·         These donate electrons towards the benzene ring, creating an increase in electron density.

·         Example: OH group donates electrons to the ring, which is called +R.

2.      Negative resonance effect (-R)

·         Electron withdrawing Groups like -NO₂, -CN, -COOH, -COOR, -CHO, -COR,

·         These withdraw electrons away from the ring, leading to a decrease in electron density.

·         These atoms or groups have an M-effect (mesomeric effect).

Key Points

The presence of a lone pair or a negative charge on an atom or group tends to donate electron density through resonance, resulting in a positive mesomeric effect (+M effect).

"LP (Lone Pair) or negative charge → +M effect"

There is an electronegativity difference between atoms, it tends to withdraw electron density through resonance, leading to a negative mesomeric effect (-M effect).

"Electronegativity difference → -M effect"