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Resonance Chemistry General Organic Chemistry

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Here is a concise and easy-to-understand explanation of Resonance in General Organic Chemistry (GOC) – ideal for JEE/NEET/B.Sc. level:

Resonance – General Organic Chemistry (GOC)

Topic: Resonance (Mesomerism)
Level: Class 11 | JEE | NEET | B.Sc. Chemistry

What is Resonance?

Resonance is a way to represent delocalized electrons in molecules where a single Lewis structure is not sufficient to describe the bonding.

It’s the phenomenon in which a molecule can be represented by two or more Lewis structures (called resonating structures) that differ only in the position of electrons, not atoms.

Example: Benzene (C₆H₆)

Benzene has two major resonating structures:

vbnet
Structure 1 Structure 2
_____ _____
/ \ β‡Œ / \
| | | |
\_____/ \_____/

Electrons in the Ο€ bonds shift positions β€” but the positions of carbon and hydrogen atoms remain the same.

Key Rules of Resonance:

  1. Atoms never move, only Ο€-electrons or lone pairs move.

  2. All resonating structures must have the same position of nuclei.

  3. All valid resonance structures must follow the octet rule (if possible).

  4. Resonance structures differ only in electron arrangement, not atom positions.

  5. The actual molecule is a resonance hybrid, which is more stable than any individual structure.

Why is Resonance Important?

  • Explains extra stability in molecules (called resonance energy).

  • Helps predict acidity/basicity, reactivity, and bond length.

  • Useful for understanding conjugated systems, aromaticity, etc.

Resonance Hybrid

The actual molecule is not flipping between structures – it is a single structure with delocalized electrons.

Think of it like a blended version of all valid structures.

Resonance Energy

This is the difference in energy between the most stable resonating structure and the actual molecule.

More resonance energy β†’ More stability

Examples of Resonating Molecules:

Molecule Resonance Involves
Benzene Alternating Ο€-bonds
Ozone (O₃) Delocalized lone pairs and Ο€-bonds
Carboxylate ion Delocalization of negative charge
Phenol Lone pair on O delocalizes with benzene
Nitromethane N=O N⁺-O⁻ resonance

Common Resonance Arrows & Movement:

  • Curved Arrow (β€Ί or β†’) shows electron movement

  • Electrons move from:

    • Lone pair β†’ Ο€-bond

    • Ο€-bond β†’ lone pair

    • Ο€-bond β†’ Ο€-bond

Important Tips:

  • More the number of valid resonance structures β†’ greater the stability.

  • Charged structures contribute less than neutral ones.

  • Avoid structures that break the octet rule unless necessary.

Memory Trick:

Only Electrons Move in Resonance β†’ "OEM"
Only
Electrons
Move

Practice Questions:

  1. Draw resonance structures of:

    • NO₂⁻ ion

    • Phenoxide ion

  2. Which is more stable: acetate ion or ethoxide ion? Why?

  3. Explain resonance in amide group (–CONHβ‚‚).

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Resonance Chemistry General Organic Chemistry

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