doublebond

A double bond is a covalent bond formed when two atoms share two pairs of electrons. In most cases, the first pair forms a sigma bond by end-to-end overlap, and the second pair forms a pi bond by sideways overlap of adjacent p orbitals. The bond order is two, and the pi bond restricts rotation about the bond axis, giving planarity around the involved atoms and enabling cis/trans (E/Z) isomerism in suitably substituted molecules.

Double bonds are shorter and typically stronger than single bonds, but longer and weaker than triple bonds. A representative example is a carbon–carbon double bond in alkenes (C=C), with bond lengths around 1.33 Å and bond energies near 600 kJ/mol. The pi component is more reactive than the sigma component, making double bonds common sites of chemical addition reactions. In organic chemistry, reagents add across the C=C bond to form saturated products.

Electronic structure and geometry: Double bonds arise from sp2-hybridized orbitals forming the sigma framework, while unhybridized p orbitals overlap to form the pi bond. The pi bond’s electron density is concentrated above and below the bond axis, contributing to reactivity and color in some compounds. When conjugated with adjacent pi systems, electrons can be delocalized, stabilizing certain structures through resonance.

Occurrence and significance: Double bonds occur in various functional groups, including alkenes (C=C), carbonyls (C=O), and heteroatom multiple bonds such as N=N. They are central to organic synthesis, biochemistry (unsaturated fats, pigments), and materials science, influencing reactivity, geometry, and physical properties.