Resonance is a way chemists represent molecules whose electrons cannot be described accurately by just one Lewis structure. It is especially important in organic molecules with conjugated pi systems, such as allyl, benzyl, carboxylate, and benzene-like structures. Resonance helps explain why some bonds have intermediate lengths, why some ions are unusually stable, and why certain atoms react at specific positions.
Understanding resonance makes it easier to predict acidity, basicity, reactivity, and molecular stability.
A resonance contributor is not a separate molecule, but one possible drawing of the same molecule with the same atom positions. Only electrons move between contributors, usually pi electrons or lone pairs, and curved arrows show that electron movement. The real molecule is a resonance hybrid, with electron density spread out over several atoms instead of locked into one bond or one charge location.
Strong contributors have full octets, minimal charge separation, and negative charge on more electronegative atoms.
Key Facts
- Resonance contributors must have the same atom connectivity and differ only in electron placement.
- Only pi electrons, lone pairs, and formal charges move in resonance, not atoms or sigma bonds.
- Formal charge = valence electrons - nonbonding electrons - 1/2 bonding electrons.
- A resonance hybrid is more stable than any single resonance contributor.
- Delocalization in an allyl system spreads charge over three connected atoms: C=C-C+ can resonate with +C-C=C.
- Carboxylate ions are stabilized because the negative charge is shared equally by two oxygen atoms.
Vocabulary
- Resonance contributor
- One valid Lewis structure used to represent electron placement in a molecule that has delocalized electrons.
- Resonance hybrid
- The actual molecule described by the combined effect of all important resonance contributors.
- Delocalization
- The spreading of electron density over multiple adjacent atoms instead of keeping it between only two atoms.
- Conjugated pi system
- A connected set of overlapping p orbitals, often involving alternating single and double bonds, that allows pi electrons to delocalize.
- Curved arrow
- A symbol used in organic chemistry to show the movement of an electron pair from a bond or lone pair to a new location.
Common Mistakes to Avoid
- Moving atoms while drawing resonance is wrong because resonance changes only electron placement, not the positions or connections of nuclei.
- Breaking sigma bonds in a resonance structure is wrong because ordinary resonance moves pi electrons or lone pairs while keeping the sigma framework unchanged.
- Treating resonance contributors as rapidly switching molecules is wrong because the real molecule is a single resonance hybrid, not a mixture of separate structures.
- Ignoring formal charge is wrong because the best resonance contributors usually minimize charge separation and place negative charge on more electronegative atoms.
Practice Questions
- 1 For the allyl cation CH2=CH-CH2+, draw the second resonance contributor and identify which two carbon atoms share the positive charge in the resonance hybrid.
- 2 Calculate the formal charge on each oxygen in the resonance contributor CH3-C(=O)-O- for acetate. Use formal charge = valence electrons - nonbonding electrons - 1/2 bonding electrons.
- 3 A molecule has a lone pair next to a C=C double bond. Explain how to decide whether the lone pair can participate in resonance and what orbital alignment is required.