Resonance (chemistry)

Lore & Background

Thiele proposed that the carbon-carbon bond in benzene is intermediate between a single and double bond. The resonance proposal also helped explain the number of isomers of benzene derivatives; for example, Kekulé's structure would predict four dibromobenzene isomers, but only three exist in reality.

Reader's Guide

Resonance is a foundational concept in chemistry, particularly within valence bond theory, for describing molecules where bonding cannot be expressed by a single Lewis structure. By averaging multiple contributing structures, the resonance hybrid provides an accurate representation of the true molecular structure, including bond lengths and partial charges that are intermediate between those of the hypothetical contributors. This approach reconciles theoretical models with experimental data, such as the equal N–O bond lengths in the nitrite anion. The resonance energy, or delocalization energy, represents the stabilization gained from electron delocalization, though it is not a directly measurable physical quantity. Resonance is distinct from isomerism; contributing structures differ only in electron assignment, not in nuclear arrangement, and the hybrid does not represent rapidly interconverting isomers. The term 'resonance' originated as a classical physics analogy for a quantum mechanical phenomenon, and some have suggested replacing it with 'delocalization' to avoid confusion. The concept remains essential for understanding the structure and stability of molecules with extended π systems, such as linear polyenes and polyaromatic compounds.

Did You Know?

• The resonance hybrid is the accurate structure for a molecule or ion; it is an average of the theoretical contributing structures.
• Resonance contributors must have the same number of valence electrons and the same spin multiplicity.
• The double-headed arrow (↔) is used to indicate contributing forms of a single chemical species, not an equilibrium.