conjugated

Meaning: A conjugated system has p orbitals on three or more adjacent atoms overlapping in a continuous chain. The usual pattern is alternating single and double bonds, as in 1,3-butadiene, but a double bond next to a lone pair or empty p orbital (as in an amide, enol, or allylic carbocation) counts. The atoms involved are usually sp² (sometimes sp) hybridized, each contributing a p orbital, so the π electrons are delocalized and can be drawn as resonance structures. Delocalization lowers energy, so a conjugated diene is more stable than an isolated diene and releases less heat on hydrogenation. Longer conjugation shrinks the HOMO–LUMO gap.

On the MCAT: Questions rank stability, predict UV-Vis absorption (longer conjugated systems typically absorb at longer wavelength, and β-carotene absorbs visible light, so it looks colored), or explain why an α,β-unsaturated carbonyl can undergo 1,4-addition. Peptide bonds and aromatic rings are conjugated too.

Common confusion: Conjugated is not conjugate, as in a conjugate acid-base pair. Double bonds that are merely nearby are not conjugated: one sp³ carbon between them, as in 1,4-pentadiene, breaks the overlap. A conjugated protein in biochemistry is different: a protein carrying a non-amino-acid group such as heme.