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.
Understanding Chemistry: Resonance in Organic Molecules
At the orbital level, resonance needs a continuous path of overlapping p orbitals. A double bond supplies p orbitals, and an adjacent atom can join the system if it has a lone pair, an empty p orbital, or an unpaired electron. The atoms in that path are usually close to flat because parallel p orbitals overlap best when they are aligned.
Rotation can break this alignment. This is why some bonds next to double bonds resist free rotation more than ordinary single bonds.
The electrons are not travelling back and forth between drawings. Their distribution is a steady feature of the molecule, spread across the aligned orbital system.
Not every possible electron movement gives an equally useful contributor. Start by checking the octet rule for second row atoms such as carbon, nitrogen, oxygen, and fluorine. A drawing that gives carbon only six electrons is usually a weak contributor unless that electron shortage is part of the species being studied.
Next, compare formal charges. Fewer charges are generally preferred, but the location of a charge matters. Oxygen holds negative charge more comfortably than carbon because oxygen attracts electrons more strongly.
Contributors that are equivalent by symmetry make equal contributions. In benzene, each carbon experiences the same average bonding, which explains why all six carbon to carbon bonds have the same length.
Resonance changes the behavior of acids and bases because it changes the stability of charged products. A phenol loses a hydrogen ion more readily than a simple alcohol because its remaining negative charge can be spread into the ring. An amide nitrogen is less basic than an amine nitrogen because its lone pair is shared with the nearby carbonyl group.
That lone pair is less available to bond to a hydrogen ion. The same idea guides reaction predictions.
Electron rich parts of a conjugated system can attract electrophiles, while electron poor parts can attract nucleophiles. Chemists use resonance to identify where charge is partly concentrated rather than relying only on the charge shown in one drawing.
A reliable method begins with a correct Lewis structure and clear formal charges. Mark atoms that have p orbitals or lone pairs next to a pi bond. Move one electron pair at a time through that connected region, then redraw every charge and every bond carefully.
Curved arrows begin at electrons, never at a positive charge, because arrows show where electrons originate. Do not move a hydrogen atom or rearrange the carbon skeleton when drawing resonance. Students often confuse resonance with equilibrium, but equilibrium involves distinct substances that can be present in different amounts.
Resonance drawings describe one substance. It helps to treat the drawings as accounting tools for electron density, bond strength, and likely reaction sites.
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.