Mesomeric Effect

Molecules possessing sigma bonds and pi-bonds alternatively exhibit the mesomeric effect. The effect is exhibited due to the permanent delocalisation of π-bonds. This increases the number of resonating structures which makes the molecules of organic chemistry more stable. Such kind of a system, where there are alternative sigma and pie bonds, is called conjugated.

Types of Mesomeric Effect

• Positive mesomeric effect
• Negative mesomeric effect:

Positive Mesomeric Effect

This effect is exhibited when the direction of the delocalisation of electrons is away from the position where the group is attached. Normally, groups having a lone pair of electrons attached to a conjugated system push electrons into the conjugated system, that is, away from them.

Groups in organic chemistry showing positive mesomeric effect (+M) effect are -OH, -OR, -NH₂, -SH, -X, etc.

Negative Mesomeric Effect

This effect is exhibited when the direction of the delocalisation of electrons is towards the position where the group is attached. These are generally electron-withdrawing groups of organic chemistry.