16.1 : 共役ジエンの構造

Dienes are organic compounds containing two carbon–carbon double bonds. They are classified as cumulated, conjugated, or isolated.

Cumulated dienes have two adjacent double bonds. In comparison, the double bonds in conjugated dienes are separated by one single bond and in isolated dienes by two or more single bonds.

Each carbon atom in conjugated dienes is sp²-hybridized with an unhybridized p orbital perpendicular to the plane of the molecule.

The adjacent p orbitals overlap to form a continuous π system, where the π electrons are delocalized across the entire molecule.

This electron delocalization imparts a partial double bond character to the carbon–carbon single bond, making it shorter than a regular carbon–carbon single bond.

Additionally, the carbons in conjugated systems being sp² hybridized, have a higher s character than sp³-hybridized carbons. Since electrons in an sp² orbital are closer to the nucleus, the overlap between two such orbitals leads to shorter bonds.