Mesomerism (resonance) is the delocalization of electrons, particularly π electrons, within a molecule, and is a type of electronic effect.

π electrons generally shift toward more electronegative atoms, stabilizing the molecule through resonance structures.

Resonance can be represented by drawing multiple contributing structures, showing the movement of electrons (e.g., in carbonyl and nitrile groups).

Extension of mesomerism occurs through adjacent double bonds, positive or negative charges, lone pairs, or odd electrons, leading to conjugation.

Conjugated molecules have alternating single and multiple bonds, allowing π electrons to be delocalized over several atoms.

Positive charges can be stabilized by conjugation with π bonds, as shown in allylic and benzylic carbocations.

Lone pairs on atoms adjacent to π bonds can also participate in conjugation, stabilizing the molecule (e.g., in enamine systems).

Essential conditions for conjugation include planarity and sp2 or sp hybridization, which allow for effective overlap of p orbitals.

All atoms in a conjugated system must be able to participate in π bonding, typically requiring sp2 or sp hybridization.

Examples:

• CH=CH-CH=CH-CH=CH-CH2 is conjugated because all carbons are sp2 hybridized.

• CH2=CH-CH=CH: sp2 and sp hybridization are required for conjugation.