THEORY OF RESONANCE – Introduction
Resonance is a concept used when a single structure cannot correctly describe a molecule. If a molecule can be represented by two or more structures that differ only in the arrangement of electrons, the actual molecule is a resonance hybrid. None of the individual structures truly exists; instead, the real molecule is an average of all contributing forms.
The main principles of resonance are:
- Resonance occurs when structures differ only in electron arrangement, not atom positions.
- More stable structures contribute more to the hybrid.
- The resonance hybrid is more stable than any individual structure. The extra stability is called resonance energy.
Allyl Radical as a Resonance Hybrid
The allyl radical can be drawn using two equivalent structures. Since both structures have equal stability, they contribute equally to the hybrid. The actual molecule does not switch between these structures; instead, the electrons are shared or delocalised over both ends.
This delocalisation gives two identical carbon–carbon bonds that are halfway between a single and double bond — sometimes called 1.5 bonds.
Stability of the Allyl Radical
Because of electron delocalisation, the allyl radical is more stable than expected. This increased stability is the resonance energy. Experiments comparing bond-breaking energies show that the allyl radical has around 10 kcal/mol of resonance stabilisation.
Orbital Picture of the Allyl Radical
Each carbon in the allyl system uses sp2 orbitals, leaving one p-orbital on each carbon. These p-orbitals overlap continuously above and below the carbon chain, forming a π-electron cloud spread across three carbons. This overlap explains delocalisation and resonance stability clearly.
Hyperconjugation
Hyperconjugation is another type of electron-delocalisation. It occurs when σ-bonds (C–H bonds) overlap with an empty or partially filled p-orbital. This effect stabilises radicals and carbocations.
For example:
- Ethyl radicals are more stable than methyl radicals.
- Isopropyl radicals are more stable than ethyl radicals.
- Tert-butyl radicals are even more stable.
The trend is due to the increasing number of hyperconjugation structures.
Allyl Cation as a Resonance Hybrid
The allyl cation also has two equivalent resonance structures. Because of delocalisation, the positive charge is spread over the two terminal carbons. This makes the allyl cation much more stable than a typical primary carbocation.
Evidence includes spectroscopy: instead of showing separate signals for single and double bonds, it shows one intermediate carbon–carbon stretching band.
Allylic Rearrangement
Because the positive charge or radical is delocalised over two carbons, reactions can occur at either end of the allylic system. This explains why substitution or addition reactions at allylic positions often give mixtures of products.
Formation of Conjugated Dienes
Conjugated dienes are more stable than isolated dienes because of electron delocalisation. Therefore, elimination reactions often form conjugated dienes preferentially.
Electrophilic Addition to Conjugated Dienes
When electrophiles react with conjugated dienes, two types of products can form:
- 1,2-addition – reagent attaches to adjacent carbons
- 1,4-addition – reagent attaches at the ends of the conjugated system
This happens because the intermediate carbocation formed is an allylic cation, which allows attack at two positions.
1,2 vs 1,4 Addition: Kinetic vs Thermodynamic Control
Temperature affects which product dominates:
- Low temperature: 1,2-product forms faster → kinetic control
- High temperature: 1,4-product is more stable → thermodynamic control
Thus, fast formation gives the 1,2-product, but higher stability gives the 1,4-product.
Summary
- Resonance describes electron delocalisation.
- Delocalisation stabilises molecules, radicals, and cations.
- Allyl systems (radical and cation) are strongly resonance-stabilised.
- Hyperconjugation also provides stability through σ-bond interactions.
- Conjugated dienes undergo both 1,2- and 1,4-addition depending on conditions.
Detailed Notes:
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