mesomeriska

Mesomeriska is a Swedish adjective that refers to properties, effects, or phenomena related to mesomerism, a term commonly used in chemistry to describe the delocalization of electrons through resonance structures. The concept originates from the Latin word “mos” meaning “habit” and was adopted into German and then into Swedish in the early 20th century during the development of organic chemistry theory. In Swedish academic texts the term is frequently used interchangeably with “resonans” (resonance).

In organic chemistry, mesomeriska effekter (mesomeric effects) describe how substituents on a conjugated system influence the electron density and reactivity of the ring or chain. Two basic types are denoted by the Greek letters σ⁺ (mesomeric inductive or electron‑donating) and σ⁻ (electron‑withdrawing). A classic example is the nitro group in nitrobenzene, which exerts a strong σ⁻ effect, withdrawing electron density from the aromatic ring and increasing its acidity. Conversely, a methoxy group on anisole donates electron density via a σ⁺ effect, making the ring more nucleophilic.

The quantitative analysis of mesomeriska contributions is often performed through computational methods such as natural bond orbital analysis or by measuring Hammett constants (ρ values). In Swedish university curricula, mesomeriska effects are taught alongside inductive effects to give students a comprehensive understanding of electronic influences in aromatic and conjugated systems.

Related Swedish terminology includes “mesomerisk skevhet” (mesomeric strain), which refers to the destabilization that can result when mesomeric interactions are forced into an unfavorable geometry. The term is also used metaphorically in linguistics and psychology to describe systems where components influence each other through indirect, non‑local connections.