Difficulty: Intermediate | Prerequisites: Lewis structures from general chemistry, formal charge calculation.
Resonance is one of the most tested concepts in Organic Chemistry II. It determines molecular stability, reactivity, acidity, and spectroscopic properties. Nearly every exam question on this paper involves resonance in some form, whether you are drawing contributors, choosing the most significant one, or predicting acidity based on conjugate-base stabilisation. If you cannot confidently draw Lewis structures with correct formal charges, return to that material before continuing.
Resonance contributors are different Lewis structures for the same molecule that differ only in electron placement (not atom positions). The most significant contributor has the most bonds, fewest formal charges, and negative charges on the most electronegative atoms. Structural isomers have the same molecular formula but different atom connectivity. Exam questions test whether you can distinguish resonance forms from conjugate acids, conjugate bases, and structural isomers.
Resonance contributor (resonance structure)
One of two or more valid Lewis structures for a molecule that differ only in the placement of electrons (pi bonds and lone pairs), not in the positions of atoms. Think of it as: the atoms stay put, but the electrons move.
Most significant resonance contributor
The single resonance structure that best represents the actual electron distribution. It is the one with the most covalent bonds, the fewest formal charges, and any negative formal charges on the most electronegative atoms.
Formal charge
The charge assigned to an atom in a Lewis structure using the formula: formal charge = (valence electrons) minus (lone pair electrons) minus (1/2 bonding electrons). In simple terms, it is what the atom's charge would be if all bonds were shared equally.
Closed shell
An atom that has a complete octet (or duet for hydrogen). Exam questions that specify "closed shell" mean every atom must satisfy the octet rule with no radicals.
Structural isomer (constitutional isomer)
A molecule with the same molecular formula but a different connectivity of atoms. Unlike resonance structures, the atoms themselves are connected differently.
Conjugate acid
The species formed when a molecule gains a proton (H+). It has one more H and one more positive charge (or one fewer negative charge) than the original.
Conjugate base
The species formed when a molecule loses a proton (H+). It has one fewer H and one more negative charge (or one fewer positive charge) than the original.
Delocalisation
The spreading of electron density across multiple atoms through overlapping p orbitals. Delocalisation is what resonance describes: electrons are not stuck on one atom but shared across a pi system.
Never move atoms. Only electrons (lone pairs and pi bonds) move between contributors.
All contributors must have the same net charge.
All atoms must remain closed shell (full octets, duets for H) unless the question says otherwise.
Formal charges on any single atom should be +1, 0, or -1 (exam questions often state this explicitly).
The total formal charge across all atoms must equal the molecule's overall charge.
Rank contributors using these criteria in order of importance:
Maximise covalent bonds. More bonds = more stable contributor.
Minimise formal charges. Fewer charges = more stable contributor.
Place negative charges on electronegative atoms. Oxygen bearing a negative charge is more favourable than carbon bearing one.
Avoid charge separation when possible. A contributor with no formal charges beats one with a +1 and a -1, all else being equal.
Diazomethanone has the connectivity O-C-N-N. Since it is a neutral molecule best represented by contributors that each contain formally charged atoms, you need contributors where the charges cancel to zero.
The key constraint: formal charges must be +1, 0, or -1 only, and all atoms must be closed shell. To draw all contributors, systematically vary where the double and triple bonds sit along the O-C-N-N chain. Each valid arrangement that gives every atom an octet (and charges of only +1, 0, or -1) is a contributor.
The most significant contributor is the one with the most bonds and negative charge on the most electronegative atom (oxygen).
The exam asks for two structural isomers of diazomethanone (molecular formula CN2O):
With a ring, no sp-hybridised atoms, all closed shell and uncharged: A three-membered ring containing C, N, and O (or C, N, N with O attached) where every atom is sp2 or sp3 satisfies this. Think of an oxaziridine-type ring or similar small heterocycle with all single and double bonds arranged so that no atom needs a triple bond (which would require sp hybridisation).
Without a ring, all closed shell and uncharged: An open-chain arrangement such as H-N=C=O (isocyanic acid, but this has sp carbon) would not work. You need an arrangement where every atom is sp2 or sp3 with full octets and no formal charges. Urea-related fragments or nitroso compounds can satisfy this.
This is a classification question that appears frequently. For each structure given, compare it to the reference molecule:
Resonance form: Same connectivity, same molecular formula, only electrons have moved.
Conjugate acid: Same connectivity, but one extra H and one more positive charge (or one fewer negative charge).
Conjugate base: Same connectivity, but one fewer H and one more negative charge (or one fewer positive charge).
Structural isomer: Same molecular formula, but atoms are connected differently.
The trick is systematic: first check whether the molecular formula matches. If it has an extra or missing H, it is a conjugate acid or base. If the formula matches but atoms are bonded differently, it is a structural isomer. If the formula matches and the connectivity is identical, it is a resonance form.
For bufotenin, the exam shows one resonance form and asks you to draw the best contributor. Identify where a pi bond or lone pair can shift to move charge to a more stable position (typically moving a negative charge onto oxygen or moving a positive charge away from oxygen).
For the alizarin-TiCl4 complex, the positive charge on one oxygen can be delocalised to a different oxygen through the aromatic ring system. This tells you about relative acidity: the OH group whose oxygen can stabilise positive charge through resonance is the less acidic one; the other OH is more acidic and is deprotonated first by one equivalent of CH3Li.
Resonance explains why amides (the bonds that hold proteins together) are planar and rigid: the nitrogen lone pair delocalises into the carbonyl, giving partial double-bond character. It explains why carboxylate salts dissolve in water (delocalised charge is more stable). Alizarin's use as a 3,000-year-old dye depends on its extended conjugation, which absorbs visible light and produces colour.
Students frequently confuse resonance structures with equilibrium. Resonance contributors do not interconvert. The molecule does not "flip" between them. The real structure is a weighted average of all contributors, all at once.
Students sometimes move atoms when drawing resonance structures. If atoms have moved, it is a different molecule (a structural isomer or a tautomer), not a resonance form.
A common mistake is choosing the contributor with the most charges as "most significant" because it looks more complex. The opposite is true: fewer charges, more bonds, and negative charge on electronegative atoms win.
Students confuse conjugate acid/base with resonance. If the molecular formula differs by one H (and one charge), it is not a resonance form.
Drawing all closed-shell resonance contributors for a molecule like diazomethanone is worth 6 points. Missing even one valid contributor costs marks.
The "circle the best description" classification question (resonance form, conjugate acid, conjugate base, structural isomer) appears multiple times on this exam and is a reliable exam format.
Alizarin-type questions connect resonance to acidity: if you can draw the resonance contributor that delocalises charge to a specific oxygen, you can predict which OH is more acidic.
"All atoms closed shell, formal charges +1, 0, or -1" is a constraint stated in nearly every resonance question. Violating it (e.g., drawing a formal charge of +2) means the structure is invalid and earns zero points.
True or false: resonance contributors can have atoms in different positions. (False. Only electrons move.)
Fill in the blank: the most significant resonance contributor has the most ___ and the fewest ___. (Covalent bonds; formal charges.)
True or false: if two structures have the same molecular formula but one has an extra H on nitrogen, they are resonance forms. (False. They are conjugate acid/base related.)
Fill in the blank: in the best resonance contributor, negative formal charges sit on the most ___ atom. (Electronegative.)
True or false: a structural isomer of a molecule has the same connectivity of atoms. (False. Structural isomers have different connectivity.)
Q: Diazomethanone (O-C-N-N) is neutral but best represented by contributors with formal charges. Why can you not draw a contributor with all atoms uncharged?
A: The connectivity O-C-N-N does not allow every atom to achieve an octet with only neutral formal charges. The terminal nitrogen needs at least a lone pair or a negative charge, and the internal atoms need positive charges to compensate while maintaining octets throughout. Every valid closed-shell arrangement results in at least two formal charges.
Q: A structure related to Molecule 1 has the same molecular formula and connectivity, but electrons are in different positions. What is the relationship?
A: It is a resonance form of Molecule 1. Same formula, same connectivity, different electron placement.
Q: A structure has the same molecular formula as Molecule 1 but has one additional H on the nitrogen and one fewer lone pair on oxygen. What is the relationship?
A: It is a conjugate acid of Molecule 1. Adding a proton (and therefore a positive charge) to a basic site produces the conjugate acid.
Q: For the alizarin-TiCl4 complex, one resonance contributor has a positive charge on one oxygen. Draw the contributor with the positive charge on a different oxygen. What does this tell you about the relative acidity of the two OH groups?
A: The oxygen that can bear a positive charge through resonance is stabilised by delocalisation and is therefore less willing to lose its proton (less acidic). The other OH, which cannot stabilise positive charge through the ring, is more acidic and is deprotonated first.
Q: Draw a structural isomer of CN2O that contains a ring, has all closed-shell uncharged atoms, and no sp-hybridised atoms.
A: A three-membered ring is needed (with only C, N, N, and O to work with). One possibility: a ring of N-C-O with the second N bonded to the ring carbon and bearing enough H atoms to satisfy all octets with no formal charges and no triple bonds (which would require sp hybridisation).
Resonance connects directly to acid-base chemistry: a conjugate base is more stable (and therefore the acid is stronger) when the negative charge can be delocalised across more atoms. It connects to hybridisation because resonance requires p orbitals to overlap, which means the atoms involved must be sp2 or sp. It also connects to reaction mechanisms: curved-arrow notation in mechanisms is the same skill as drawing resonance contributors, just applied to bond-breaking and bond-forming steps.
Resonance structures, resonance contributors, Lewis structures, formal charge, most significant contributor, major contributor, closed shell, octet rule, structural isomers, constitutional isomers, conjugate acid, conjugate base, delocalisation, electron delocalisation, pi system, diazomethanone, CN2O isomers, bufotenin, alizarin, TiCl4 complex, organic chemistry II, CHEM 210, CHEM 215, University of Michigan