Mesomerism

Mesomerism, also known as the mesomeric effect, is a term in chemistry describing the redistribution of electron density within a molecule through the interaction of pi electrons and lone pairs. This interaction gives rise to several contributing structures whose real electronic structure is a resonance hybrid. The concept arose to explain how electrons can be delocalized in conjugated systems and in functional groups with lone pairs adjacent to pi bonds.

Mesomeric effects are classified as +M (or +R) when a substituent donates electron density through the pi system, and -M (or -R) when it withdraws it. The +M effect increases electron density at certain positions of a conjugated ring or chain, often activating it toward electrophilic attack at ortho and para positions; the -M effect reduces density and can deactivate the system or direct substitution to other positions.

Examples: amino (-NH2) and methoxy (-OCH3) groups exhibit +M, donating via their lone pairs into the ring. The nitro (-NO2) and carbonyl-containing groups show -M, withdrawing electrons by resonance. Fluorine typically shows a -M effect, despite a small +I effect, reflecting the strength of its lone-pair withdrawal through resonance. In carbonyl compounds, the oxygen lone pair can donate to the carbonyl pi system, stabilizing certain intermediates.

Mesomerism remains a foundational concept for understanding substituent effects and reaction directing tendencies in organic chemistry, though it is closely related to the broader idea of resonance and electron delocalization. In modern usage, emphasis is placed on resonance hybrids and orbital interactions, with mesomeric terminology retained for describing directional effects.