Resonance is a fundamental concept in organic chemistry that explains how electrons are distributed in molecules. It's not about molecules flipping between different structures, but rather a way to describe the average electron arrangement that gives molecules unique properties.

Understanding resonance helps explain why some molecules are more stable than others. It's like a game of musical chairs for electrons, where they spread out to find the most comfortable arrangement, resulting in stronger bonds and lower overall energy.

Resonance

Resonance forms vs molecular structure

• Resonance forms are hypothetical structures that represent the delocalization of electrons in a molecule or ion
  • Each resonance form shows a different arrangement of electrons and bonds (benzene, carbonate ion)
  • No single resonance form accurately represents the actual structure
• The actual structure is a resonance hybrid, which is a weighted average of all the contributing resonance forms
  • The resonance hybrid has characteristics intermediate between those of the individual resonance forms (bond lengths, charge distribution)
• Resonance forms are drawn with a double-headed arrow between them to indicate that they are not interconvertible structures, but rather contribute to the overall resonance hybrid
• Resonance forms are also known as canonical structures

Characteristics of resonance hybrids

• The acetate ion, $CH_3COO^-$, has two equivalent resonance forms
  • In each form, the negative charge is localized on one of the oxygen atoms, and there is a single bond between that oxygen and the carbon
  • The other oxygen is double-bonded to the carbon in each form
• The actual structure of the acetate ion is a resonance hybrid of these two forms
  • The negative charge is delocalized over both oxygen atoms
  • The carbon-oxygen bonds are identical and have a bond order of 1.5, intermediate between a single and double bond
• The resonance hybrid is more stable than either of the individual resonance forms
  • Delocalization of the negative charge over both oxygen atoms stabilizes the ion (lower energy)
  • The identical carbon-oxygen bond lengths minimize the overall energy of the molecule
• The stability gained through resonance is called resonance energy

Bond lengths in resonance structures

• In resonance structures, the bond lengths are typically intermediate between those of single and double bonds
  • Single bonds are longer and have a lower bond order (1) compared to double bonds (C-C single bond ~1.54 Å)
  • Double bonds are shorter and have a higher bond order (2) compared to single bonds (C=C double bond ~1.34 Å)
• In the acetate ion example, the carbon-oxygen bonds in the resonance hybrid have a bond order of 1.5
  • This bond length is shorter than a typical carbon-oxygen single bond (bond order 1)
  • It is longer than a typical carbon-oxygen double bond (bond order 2)
• The intermediate bond lengths in resonance hybrids are a result of the delocalization of electrons across the contributing resonance forms
  • The actual structure is a weighted average of the resonance forms, leading to bond orders and lengths that are not whole numbers (benzene C-C bond length ~1.40 Å)

Electron delocalization and its effects

• Electron delocalization is the distribution of electrons over multiple atoms in a molecule
• It plays a crucial role in resonance and can lead to increased stability
• Aromaticity is a special case of electron delocalization in cyclic systems
• The mesomeric effect describes the electron-donating or electron-withdrawing properties of substituents through resonance