Conjugated Double Bonds
Definition and meaning of Conjugated Double Bonds in chemistry.
Conjugated double bonds are two or more carbon-carbon double bonds separated by exactly one single bond, arranged so their p orbitals align and overlap into a continuous pi system.
In more detail
Because the alternating single bonds are shorter and stronger than typical single bonds and the pi electrons spread over several atoms rather than staying confined to one bond, the resulting delocalization lowers the molecule's energy relative to an isolated (non-conjugated) arrangement of the same double bonds. This conjugation stabilization can be measured experimentally through lower-than-expected heats of hydrogenation. Extending a conjugated system also narrows the gap between its highest occupied and lowest unoccupied molecular orbitals, shifting light absorption to longer wavelengths, which is why long conjugated chains such as carotenoids and other pigments appear colored.
Key facts
| Example compound | 1,3-butadiene, C4H6 |
|---|---|
| Field | Organic Chemistry |
| Bond pattern | alternating C=C-C=C-C |
| Stabilization energy | ~15 kJ/mol for butadiene |
1,3-Butadiene (CH2=CH-CH=CH2) has its two double bonds separated by a single C-C bond, giving a planar molecule with a delocalized four-carbon pi system and a measured conjugation (resonance) stabilization of about 15 kJ/mol (3.5 kcal/mol) compared to two isolated double bonds.
Frequently asked questions
How does a conjugated diene differ from a cumulated diene?
In a conjugated diene, double bonds alternate with a single bond (C=C-C=C), letting adjacent p orbitals overlap and delocalize. In a cumulated diene (an allene, C=C=C), two double bonds share one carbon and their p orbitals are perpendicular to each other, so no delocalization occurs.
Why are conjugated molecules often colored?
Extending conjugation narrows the HOMO-LUMO energy gap, shifting the wavelength of light a molecule absorbs from the UV region into the visible region, which produces color in compounds like beta-carotene and many synthetic dyes.