Drawing Valid Resonance Contributors

Introduction

Resonance drawing is a constrained electron-accounting exercise. Valid contributors retain the same atoms in the same positions, the same total number of electrons and the same net charge. A curved arrow changes the assignment of an electron pair, not the identity of the compound. These checks catch most incorrect resonance forms.

Core explanation

Start an arrow at an electron source: a lone pair, a pi bond or occasionally a sigma bond in a valid delocalisation model. End the arrow at an atom or bond position that can accept that pair without violating the relevant valence rules. In acetate, an oxygen lone pair on the singly bonded O can form a C=O pi bond; at the same time, the original C=O pi pair moves onto the other oxygen. Two coordinated arrows keep carbon from temporarily having five bond-order units in the finished contributor. The resulting drawing has the same net −1 charge and swapped formal-charge positions.

For the allyl anion CH₂=CH–CH₂⁻, the terminal negatively charged carbon has a lone pair adjacent to a pi bond. One arrow moves that lone pair into a pi bond with the centre carbon, and another shifts the original pi electrons onto the opposite terminal carbon. The alternative contributor moves the formal − charge to the other end. The three-carbon skeleton and attached hydrogens remain fixed. The actual anion has delocalised electron density; neither drawing is a photo of charge parked permanently on one terminal carbon.

For an allyl cation CH₂=CH–CH₂⁺, a pi pair can shift toward the empty p orbital at the positively charged end. The new double bond appears on the other side and formal + charge appears at the first terminal carbon. Again the central three-carbon sigma framework is unchanged. The presence of an adjacent empty p orbital makes this possible; a cation separated from a pi bond by an sp³ carbon does not have the same direct resonance path in the simple picture.

Several invalid moves recur. Moving H⁺ from one atom to another produces a different protonation structure or tautomer, not a resonance contributor. Breaking a C–C sigma framework bond changes connectivity and generally describes a reaction, not routine resonance. Adding an electron to make a negative contributor changes net charge. Drawing a second double bond onto an octet-filled second-period carbon without moving another pair creates an invalid carbon valence. Finally, using a one-headed fishhook for an electron pair mixes radical and electron-pair notation; ordinary resonance pair arrows have full curved heads.

Contributor quality is assessed after validity. Equivalent forms carry equal weight when symmetry relates them. If forms are unequal, complete octets, fewer unnecessary formal charges and appropriate charge placement are useful guides. Do not discard a valid minor contributor simply because its charge separation looks less favourable; it may help predict a reactive site. Conversely, the mere ability to draw a contributor does not prove it contributes equally.

Use the resonance double-headed arrow between contributors, not the equilibrium arrow that implies separate interconverting species. Curved arrows drawn above a contributor explain how to obtain the next drawing; they are not arrows showing molecule travel through time. This symbolic distinction matters in mechanism writing, where a reaction arrow changes nuclei or bonds as a real step.

Step-by-step reasoning

1. Record formula, electron count and net charge before moving pairs. 2. Identify adjacent lone pair, pi bond or vacant p orbital. 3. Draw curved arrows from electron sources to allowed destinations. 4. Check every atom's valence and the finished formal-charge total. 5. Confirm atom positions and sigma skeleton are identical in every contributor.

Visual explanation

Draw acetate with two arrows: O⁻ lone pair → C–O bond, and original C=O pi bond → opposite O. Cross out a false drawing that adds the new double bond but leaves the original one, because carbon would then exceed ordinary octet valence.

Real-world analogy

Rebalancing numbers between columns of one ledger keeps the total fixed. Resonance shifts where electron pairs are represented while conserving the electron total and identity of the molecule.

Real-world example

Mechanism problems often use resonance to identify where an allylic intermediate may be attacked. Correct contributors show charge distribution across positions; an invalid arrow can predict a product at a carbon that was never electronically connected to the intermediate.

Why?

Why are two arrows needed for acetate? Forming a new C=O pi bond from one oxygen lone pair must be accompanied by moving the old C=O pi electrons to the other oxygen, preserving carbon's octet and the ion's electron count.

Common misconception

“A shifted proton is a resonance move.” Atom positions must stay fixed. Proton transfer changes sigma connectivity and is a chemical process, often acid–base or tautomerisation chemistry.

Worked example

Construct the second allyl-anion contributor from ⁻CH₂–CH=CH₂. The left carbon lone pair forms a C1=C2 pi bond. The C2=C3 pi pair shifts to carbon 3, making CH₂=CH–CH₂⁻ with charge at the other end. Each contributor has the same C₃H₅⁻ formula, sigma skeleton and total −1 charge. The physical ion is a delocalised hybrid.

Quick check

1. Can a valid resonance contributor for acetate have two simultaneous C=O double bonds at its carboxyl carbon while also retaining the C–C bond? Answer: Not in the ordinary octet-respecting Lewis representation; that would overbond carbon.

Exam focus

Use full-head arrows for electron pairs, start every arrow at an electron source and preserve atoms, formula and total charge. Verify the final contributor rather than trusting arrow shape alone.

Advanced insight

Resonance contributors are not unique; different valid Lewis sets can represent aspects of the same quantum electron density. Their relative usefulness is judged by electronic structure and experimental evidence, not a literal equilibrium among drawings.

Summary

Valid resonance moves relocate electron pairs along conjugated paths while nuclei and overall charge remain unchanged. Acetate and allyl ions illustrate coordinated arrows; proton transfer, broken skeletons and impossible octets are not ordinary resonance.

Practice questions

1. What must stay the same in all resonance contributors of one species? Answer: Atomic positions/connectivity, electron total and net charge. 2. Where must a curved electron-pair arrow begin? Answer: At a lone pair or bond that supplies the pair. 3. Why is moving H between O atoms not resonance? Answer: It moves a nucleus and changes sigma connectivity. 4. What happens to formal negative charge in the allyl anion's alternate contributors? Answer: It is shown at opposite terminal carbons, while the actual ion is delocalised.