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Organic Chemistry · Polar Covalent Bonds; Acids and Bases

Drawing Resonance Forms

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Science note: All structures and formal-charge assignments follow standard valence-bond conventions for second-row elements; no experimental data were fabricated.
Want it in plain words first? Jump to Eli explains — the same idea, no jargon.
On this page 9 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Check yourself
  8. Study tools
  9. Sources & references

In 30 seconds

Drawing resonance forms is a mechanical skill built from the rules of Topic 5: identify every and , find the electron-poor neighbor, push electrons with curved arrows, and redraw. Three patterns cover most molecules: a lone pair adjacent to a π bond, a π bond adjacent to an , and a π bond between atoms of different electronegativity. This topic gives a step-by-step procedure, works each pattern with real molecules, and catalogs the errors students make most often, so you can produce and check resonance forms quickly on exams and in reaction mechanisms.

Why this matters

Every reaction mechanism in organic chemistry — addition, elimination, substitution, carbonyl chemistry, enzyme catalysis — is written with the same curved-arrow language used to draw resonance. Mastering drawing now means learning to read and write the script of all later chapters. In practice, chemists use resonance drawings to see which atoms carry partial charges, to predict where electrophiles will attack aromatic rings, and to explain why certain reaction intermediates are stabilized. On exams, the ability to draw the missing resonance form correctly is routinely worth several points per problem.

The college version

Core Concepts

The curved-arrow convention

A represents the movement of one electron pair. The tail begins at the electron source — a lone pair or a π bond — and the head points at the electron destination, an adjacent atom where a new bond or lone pair forms. When a π bond moves, the arrow's tail sits on the bond and its head lands on the far atom, which gains the electron pair as a lone pair. When a lone pair moves, the head lands on the adjacent atom to form a new π bond. Drawing two or three arrows in sequence transforms one valid structure into another in a single step.

Pattern 1: A lone pair adjacent to a π bond

The lone pair forms a new π bond while the old π bond becomes a lone pair, shifting the multiple bond one position along the chain. Seen in enolates, enamines, phenol, and amides. Example core: C=C-C- where the anion's lone pair sits next to the double bond.

Pattern 2: A π bond adjacent to an electron-poor atom

The π bond moves onto the atom bearing a positive charge or an empty p orbital, shifting the charge to the far end of the system. Seen in allylic carbocations and in protonated carbonyls. Example core: C=C-C+.

Pattern 3: A π bond between atoms of different electronegativity

The π electrons move onto the more electronegative atom, placing a negative charge on it and a positive charge on its neighbor. Seen in carbonyls, nitro groups, and nitriles. Example core: C=O. Most real molecules combine patterns, and the drawing procedure below handles the combinations.

Step-by-step procedure

  1. Count total valence electrons once and keep them constant across all forms.
  2. Circle every π bond and every lone pair that can move (lone pairs on atoms directly attached to a π system).
  3. Find the electron-poor atom adjacent to a π bond or lone pair: a positive charge, an incomplete octet, or a highly electronegative atom.
  4. Draw the curved arrow from source to destination; add more arrows if the electron flow is a chain.
  5. Redraw the skeleton; place the moved electrons as new bonds or lone pairs.
  6. Recompute formal charges and check octets. Nothing else changes.

Common errors

The three classic mistakes: (1) pointing an arrow at an atom that already has an octet — the electrons have nowhere to go; (2) moving a hydrogen or a σ bond, which changes the molecule into a different species; (3) forgetting to update formal charges after moving electrons. Check each drawing against the rules before moving on.

Common Confusions

Do Not ConfuseWithDifference
Arrow tail on an atomArrow tail on a bondA tail on a lone pair points at the atom; a tail on a π bond sits on the bond itself.
Moving a lone pair into a bondMoving a σ bondOnly π electrons and lone pairs move; σ bonds are fixed.
Enolate carbon formEnolate oxygen formThey are valid resonance forms of the same ion; the oxygen form is more stable and contributes more.
Adding an arrowCompleting a drawingYou must also redraw the structure and recompute formal charges after each arrow.
Resonance formsIntermediates in a mechanismResonance forms are static pictures of one species; intermediates are distinct species along a reaction path.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Drawing resonance forms is like playing musical chairs with electrons. The electrons sit in chairs that are either a bond between two atoms or a spot on one atom. When the music plays, an electron pair slides one chair over to an empty or friendly spot, and you redraw where everyone ended up. The atoms stay in their seats — only the electrons move — and at the end everyone must still fit the octet rule.

Worked examples

Draw all resonance forms of the allyl cation, CH2=CH-CH2+. The central carbon bears the positive charge; the left C=C π bond is adjacent to it. Place the arrow's tail on the left C=C π bond and its head on the central carbon. The electron pair moves onto the central carbon, forming a new C=C bond between the central and right carbons, while the left carbon loses its π bond and acquires the positive charge. The result is CH2+-CH=CH2, the second resonance form. The two forms are equivalent, so the +1 charge is split equally between the two terminal carbons — each carries about +0.5 in the hybrid. The lesson: in an allylic cation, the charge is never "stuck" on one carbon; it is spread across both ends of the allyl unit.

Deprotonation of acetone, CH3COCH3, at the α-carbon (the carbon next to the carbonyl) gives the enolate CH3COCH2-: a lone pair on the α-carbon sits adjacent to the C=O π bond — Pattern 1. Draw the arrow from the α-carbon lone pair to the carbonyl carbon, forming a new C=C bond; then draw a second arrow from the C=O π bond to the oxygen (Pattern 3), giving the oxygen a lone pair and a negative charge. The resulting form is CH3C(O-)=CH2. Because oxygen is more electronegative than carbon, the oxygen-centered form is more stable and contributes more to the hybrid. Both forms are real and useful: the carbon-centered form (the enolate's "carbon end") explains why enolates attack electrophiles with carbon in aldol reactions, while the oxygen-centered form explains O-alkylation products. This single drawing exercise predicts the dual reactivity of one of the most important intermediates in organic synthesis.

For acetamide, CH3CONH2, draw the resonance form that explains amide planarity. Start with the nitrogen lone pair. Arrow 1: from the nitrogen lone pair to the carbonyl carbon, forming a C=N bond. Arrow 2: from the C=O π bond to the oxygen, placing a negative charge on oxygen. The product has C(O-)=N+(H)2: nitrogen now carries a formal positive charge and oxygen a negative charge, with all octets satisfied. Check the formal charges: nitrogen started at 0 with three bonds and a lone pair, and now has four bonds, so it is +1; oxygen started at 0 with a double bond, and now holds a lone pair plus a single bond, so it is −1. The drawing is valid and explains the observed short, rigid, planar C–N bond of every amide and peptide — the structural basis of protein folding.

Key takeaways

  • A curved arrow's tail marks the electron source (lone pair or π bond); its head marks the destination atom.
  • Only π electrons and lone pairs move; atoms and σ bonds never move.
  • Pattern 1: lone pair next to a π bond (enolates, amides) shifts the multiple bond by one position.
  • Pattern 2: π bond next to a positive charge (allylic cations) moves the charge to the far end.
  • Pattern 3: π bond between unlike atoms (C=O, N=O) puts the negative charge on the more electronegative atom.
  • After every drawing: recompute formal charges and verify octets.
  • Never aim an arrow at an atom that already has an octet; never move a hydrogen.
  • Multiple arrows in one step are allowed and usually needed (e.g., amide, enolate).

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

  1. What does the tail of a curved arrow mark, and what does the head mark?

    Show answer

    The tail marks the electron source (a lone pair or π bond); the head marks the destination atom where the pair arrives.

  2. Which two electron sources are allowed for a resonance arrow?

    Show answer

    Lone pairs and π bonds.

  3. Describe in words the curved arrow that converts one allyl cation form into the other.

    Show answer

    Move the C=C π bond onto the central carbon; the left carbon gains the + charge, giving CH2+–CH=CH2.

  4. In the acetone enolate, why does the oxygen-centered form contribute more than the carbon-centered form?

    Show answer

    Oxygen is more electronegative than carbon, so the negative charge is more stable on oxygen.

  5. Which rule prevents you from pointing an arrow at a carbon that already has an octet?

    Show answer

    The octet rule: second-row elements cannot hold more than eight valence electrons.

  6. Name the three classic errors when drawing resonance forms.

    Show answer

    Aiming an arrow at an atom that already has an octet, moving a hydrogen or σ bond, and forgetting to recompute formal charges.

Keep learning

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

curved arrow
Notation for moving one electron pair from a source (tail) to a destination (head).
π bond
A bond formed by side-to-side overlap of p orbitals; its electrons sit above and below the bond axis.
lone pair
A pair of valence electrons localized on one atom.
electron-poor atom
An atom with a positive charge, incomplete octet, or high electronegativity.
allylic system
A unit with a π bond adjacent to a CH2 group or a lone-pair-bearing atom.
formal charge
The charge an atom would have if bonding electrons were split equally between bonded atoms.

Sources & references

  1. openstax.org — Organic Chemistry

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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