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Mesomerism: Definition, Effect & Relevance

Mesomerism describes the delocalization of electrons across multiple bonds in chemical molecules. It explains the stability of many organic compounds.

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Things worth knowing about "Mesomerism"

Mesomerism describes the delocalization of electrons across multiple bonds in chemical molecules. It explains the stability of many organic compounds.

What is Mesomerism?

Mesomerism (also known as resonance) is a fundamental concept in organic chemistry. It describes the phenomenon whereby the electron density in a molecule is not fixed to a single bonding structure but is instead spread (delocalized) across several atoms or bonds. The actual molecule does not correspond to any single so-called resonance structure (or limiting structure) but rather represents a superposition of all of them.

Resonance Structures and the Mesomeric Hybrid

The various theoretical structures that can be drawn for a molecule without changing the positions of atoms are called resonance structures or limiting structures. The actual molecule is referred to as the resonance hybrid, as it represents an intermediate state of all contributing structures. Resonance structures are separated by a double-headed arrow (↔) to indicate that they do not represent an equilibrium between different molecules, but rather a conceptual splitting of one real structure.

Causes and Prerequisites

Mesomerism occurs when the following conditions are met:

Presence of conjugated double bonds (alternating single and double bonds)
Lone pairs of electrons on atoms directly adjacent to a double bond system
Empty p-orbitals close to a pi-electron system
Planar or nearly planar molecular geometry that allows overlap of p-orbitals

The Mesomeric Effect (M-Effect)

The mesomeric effect (abbreviated as M-effect) describes the influence that a functional group exerts on the electron density within a molecule through the pi-electron system. Two types are distinguished:

+M-effect (positive mesomeric effect): The group donates electrons into the conjugated system, increasing the electron density. Examples: -OH, -NH₂, -OR.
-M-effect (negative mesomeric effect): The group withdraws electrons from the conjugated system, decreasing the electron density. Examples: -NO₂, -COOH, -C=O.

Stabilization Through Mesomerism

Molecules in which mesomerism occurs are generally more energetically stable than molecules without electron delocalization. This additional stabilization is referred to as resonance energy or delocalization energy. A classic example is benzene (C₆H₆), whose actual stability is significantly greater than that of a hypothetical structure with alternating single and double bonds (the Kekulé structure).

Relevance in Medicine and Pharmacy

The concept of mesomerism is of great practical importance in medicine and pharmacy:

Drug design: The stability, solubility, and reactivity of active pharmaceutical ingredients are significantly influenced by mesomeric effects. Many drugs contain aromatic ring systems whose properties are shaped by resonance.
Protein chemistry: In amino acids and peptides, the mesomerism of the peptide bond (-CO-NH-) contributes to its planar structure, which in turn affects the secondary structure of proteins.
Metabolism: Many biochemically relevant molecules such as ATP, porphyrins, and coenzymes utilize electron delocalization for stabilization.
Toxicology: The reactivity of toxic compounds and carcinogens is often explained by their mesomeric structures.

Distinction: Mesomerism vs. Tautomerism

Mesomerism and tautomerism are often confused. In mesomerism, no atoms change their positions and the molecule exists as a single unified hybrid. In contrast, tautomerism involves a true chemical equilibrium between two or more distinct compounds in which atoms (usually hydrogen atoms) actually migrate from one position to another.

References

Clayden, J.; Greeves, N.; Warren, S. - Organic Chemistry, 2nd edition, Oxford University Press (2012).
Vollhardt, K. P. C.; Schore, N. E. - Organic Chemistry: Structure and Function, 8th edition, W. H. Freeman (2018).
Stryer, L.; Berg, J. M.; Tymoczko, J. L. - Biochemistry, 8th edition, W. H. Freeman (2015).

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