Chemistry

Delocalization of $\pi$ electrons requires conjugation—a continuous alternation of single and double bonds that allows $p$-orbitals to overlap. In penta-1,4-diene, the two double bonds are separated by two single bonds, breaking the alternating pattern and preventing orbital overlap, so the $\pi$ electrons remain localized to their individual bonds. In penta-1,3-diene, the double bonds are separated by only one single bond, creating the conjugated system where adjacent $p$-orbitals can overlap and $\pi$ electrons delocalize across all four carbons.

Delocalization of $\pi$ electrons requires conjugation—a continuous alternation of single and double bonds that allows $p$-orbitals to overlap. In penta-1,4-diene, the two double bonds are separated by two single bonds, breaking the alternating pattern and preventing orbital overlap, so the $\pi$ electrons remain localized to their individual bonds. In penta-1,3-diene, the double bonds are separated by only one single bond, creating the conjugated system where adjacent $p$-orbitals can overlap and $\pi$ electrons delocalize across all four carbons.