Each double bond is separated from the next by exactly one single bond, so the p orbitals overlap end to end.
1. Conjugation Is Alternating Double–Single Bonds With Overlapping p Orbitals
In 1,3-butadiene all four sp2 carbons carry parallel p orbitals that overlap continuously — even across the central single bond — so the four electrons spread over all four carbons.
2. Conjugated, Isolated, and Cumulated Systems Are Three Different Things
Only the conjugated arrangement lets the π system spread out: an isolated diene has an sp3 CH2 breaking the chain, and a cumulated allene has perpendicular, non-overlapping p orbitals.
3. Conjugated Dienes Are More Stable Than Isolated Ones
1,3-Butadiene releases less heat on hydrogenation than an isolated diene would predict; that shortfall (~15 kJ/mol) is the conjugation stabilization energy delocalization gives the ground state.
4. Allylic Cations, Radicals, and Lone Pairs Are Conjugated Too
A single p orbital beside a π bond delocalizes charge or an electron over two carbons, making allylic and benzylic cations, radicals, and anions unusually stable and easy to form.
5. Conjugation Extends Into Carbonyls, Rings, and Substituents
The rule isn't limited to C=C: a carbonyl conjugated with an alkene (acrolein) turns the β-carbon electrophilic, and rings (styrene) or branches (isoprene) conjugate the same way.
6. A Diene Must Be s-cis to React — Geometry Controls Conjugation
Both s-trans and s-cis conformers are conjugated, but only the U-shaped s-cis brings the ends close enough for a Diels–Alder, so ring-locked s-cis dienes are especially reactive.
7. More Conjugation Means a Smaller HOMO–LUMO Gap (and Color)
Longer conjugation shrinks the HOMO–LUMO gap, shifting absorption to longer wavelengths — the basis of UV–Vis and why long chains like β-carotene are colored; the extreme is aromaticity.
8. Summary
Overlapping p orbitals delocalize electrons · ground-state stability (lower heats of hydrogenation) · resonance-stabilized allylic intermediates · smaller HOMO–LUMO gap and color · leads into resonance, the Diels–Alder reaction, and aromaticity.
Quiz yourself
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1,3-Pentadiene. Its double bonds are separated by a single bond, so all the p orbitals overlap in one chain. In 1,4-pentadiene an sp3 CH2 sits between the double bonds and breaks the overlap — it is isolated (non-conjugated).
Because it starts out more stable. Delocalization over all four carbons lowers the ground-state energy of the conjugated diene, so there is less energy to release when the π bonds are removed. That shortfall is the conjugation (resonance) stabilization energy.
The empty p orbital on the cationic carbon overlaps the adjacent π bond, so the positive charge is delocalized over two carbons (two equivalent resonance structures). Spreading the charge lowers the energy — the same conjugation that stabilizes dienes stabilizes allylic and benzylic intermediates.
Only the s-cis conformation points the two reactive ends of the diene toward the same side, close enough to form two new bonds to the dienophile at once. An s-trans (or s-trans-locked) diene holds the ends too far apart, so it cannot cyclize.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, curved arrows, wedge/dash bonds and a clickable periodic table built in. No account needed.