Mesomeric effects, also known as resonance effects, refer to the electron-donating or electron-withdrawing effects produced by the delocalization of π-electrons in a molecule. These effects arise due to the presence of conjugated systems or the presence of electronegative or electron-donating groups in a molecule. Mesomeric effects have a significant impact on the reactivity and stability of organic compounds.

There are two types of mesomeric effects:

1. +M effect (Electron-donating effect):
The +M effect involves the donation of electrons from a group or atom to the rest of the molecule. It results in the stabilization of positive charges or the destabilization of negative charges. Key points regarding the +M effect include:

- Electronegative atoms (such as oxygen, nitrogen, or halogens) with lone pairs of electrons can donate electron density through resonance.
- Resonance forms are used to represent the movement of electrons in a molecule. For example, in the resonance form of the phenoxide ion (C6H5O-), the lone pair of electrons on oxygen can delocalize into the benzene ring, stabilizing the negative charge.
- The +M effect increases the electron density at the atom or group donating electrons, resulting in increased nucleophilicity and basicity.
- Examples of groups showing the +M effect include amino (-NH2), hydroxyl (-OH), and alkoxy (-OR) groups.

2. -M effect (Electron-withdrawing effect):
The -M effect involves the withdrawal of electrons from a group or atom towards the rest of the molecule. It results in the destabilization of positive charges or the stabilization of negative charges. Key points regarding the -M effect include:

- Electronegative atoms or groups (such as carbonyl, cyano, or nitro groups) can withdraw electron density through resonance.
- Resonance forms can be used to represent the movement of electrons in a molecule. For example, in the resonance form of the nitrobenzene molecule, the electrons from the nitrogen atom are delocalized into the benzene ring, destabilizing the positive charge on nitrogen.
- The -M effect decreases the electron density at the atom or group withdrawing electrons, resulting in decreased nucleophilicity and basicity.
- Examples of groups showing the -M effect include carbonyl (-C=O), cyano (-CN), and nitro (-NO2) groups.

In summary, mesomeric effects involve the delocalization of π-electrons in a molecule, resulting in electron-donating or electron-withdrawing effects. The +M effect donates electrons, while the -M effect withdraws electrons. These effects have a significant impact on the reactivity and stability of organic compounds.