Do resonance structures flip back and forth between forms?
Short answer: The real molecule is one hybrid; resonance structures are drawings. All six C-C bonds are equal.
The trap
The wrong idea
“Resonance structures flip back and forth rapidly between forms.”
It sounds reasonable, and a lot of JEE/NEET aspirants carry it into the exam. Here's a one-question check, then the explanation that makes it click.
Quick check
Try it before you peek
Benzene's C-C bond lengths are:
- Alternating single and double
- All equal, in between
- Constantly switching
Show the answer
B. All equal, in between
The real molecule is one hybrid; resonance structures are drawings. All six C-C bonds are equal.
The click
No. Resonance structures (canonical forms) are not real molecules, and the molecule does not switch between them. Each one is a drawing made with single and double bonds, which is the limit of what our bond notation can show. The real molecule is a single structure, the resonance hybrid, in which some electrons are delocalised over several atoms all the time.
Benzene is the classic proof. If it flipped between the two Kekulé structures, or sat in one of them, it would have alternating C–C single bonds (about 154 pm) and C=C double bonds (about 134 pm). Experiments show all six carbon–carbon bonds have the same length, about 139 pm, in between the two. The six π electrons are spread evenly around the ring.
The hybrid is also more stable than any single canonical form would be. For benzene this extra stability, the resonance energy, shows up in its heat of hydrogenation, which is about 150 kJ/mol smaller than expected for a ring with three separate C=C bonds, and in its preference for substitution reactions over addition reactions.
A well-known analogy: a rhinoceros is not a unicorn one moment and a dragon the next; it is one real animal that you might describe as a blend of two imaginary ones. In exams, treat the curved arrows in resonance as moving electrons on paper, never atoms, and never as a process that happens over time.
Got this one? The Knot Scan finds the misconception you don't know you have, in 8 questions.