What is the resonance effect?

• The decrease in electron density at one position accompanied by a corresponding increase at another position by the movement of pi electrons is called the resonance effect.
• The resonance effect is also known as a mesomeric effect.
• The distribution of electron density depends on the resonance in a system. The distribution of electron density is different in resonant systems than the non-resonant systems.
• For example, Ammonia has a lone pair of electrons on the nitrogen atom, but does not have resonance.
•  But if one of the hydrogen atoms of the ammonia molecule is replaced by a benzene ring, then the electron pairs on the nitrogen atom show delocalization on the ring, as a result the electron density is decreased on the nitrogen atom and the electron density is increased on the benzene ring as shown below:
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Thus, it can be said that the -NH2 group in aniline gives electrons to the ring by the resonance effect.

• In a similar fashion, the –NO2 group in nitrobenzene withdraws electrons from the ring by a similar effect.

Types of resonance effect:

There are two types of resonance effects which are given below:

1. Positive resonance effect
2. Negative resonance effect

Positive resonance effect:

• The positive resonance effect takes place when electrons or pi electrons are donated from a specific group to a conjugate system, which increases the system’s electron density.
• This occurs when the groups are delocalized and donate electrons to the other molecules.
• A group which contains either a single pair of electrons or a negative charge , shows the positive resonance effect..
• It is also called as positive mesomeric effect.
• It is denoted by +R and +M.
• The positive effect can be seen in the following groups in this specific order:

Negative resonance effect:

• The negative resonance effect takes place when the groups withdraw electrons from other molecules by the process of delocalization.
• It decreases the molecular electron density.
• A group which contains either a positive charge or vacant orbital ,shows the negative resonance effect..
• It is also known as the negative mesomeric effect.
• It is denoted by -R and -M.
• The groups which are given below exhibits the negative resonance effect in this order:

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