conjugated π-bonds

Meaning: In a conjugated system, p orbitals on adjacent sp² (or sp) atoms overlap continuously, as in 1,3-butadiene (C=C–C=C). The π electrons are delocalized over all the atoms involved, which is a form of resonance. A lone pair or empty p orbital next to a π bond can join in, as in an amide or an allylic cation. Delocalization lowers energy, so a conjugated diene is more stable than an isolated one and releases less heat per double bond on hydrogenation. The central single bond is a little shorter than an ordinary C–C single bond.

On the MCAT: Expect comparisons of stability, such as 1,3-pentadiene versus 1,4-pentadiene. Longer conjugation narrows the HOMO–LUMO gap, so λmax shifts to longer wavelength. Extended systems like β-carotene absorb visible light and look colored. Conjugated dienes also appear in the Diels-Alder reaction (the diene must adopt the s-cis conformation) and in HBr addition, where the allylic cation gives a 1,2 product (usually favored kinetically at low temperature) and a 1,4 product (usually favored thermodynamically at higher temperature).

Common confusion: Having two double bonds does not make a molecule conjugated. In 1,4-pentadiene an sp³ carbon separates the alkenes, so they are isolated. Check that every atom in the stretch is sp² (or sp) with an aligned p orbital.