Double bond

In chemistry, a double bond is a special connection between two atoms. In this bond, four bonding electrons are shared, instead of just two like in a single bond. This bond is often seen between two carbon atoms, like in alkenes. Double bonds can also form between different elements, such as between carbon and oxygen in a carbonyl group, or between nitrogen atoms in azo compounds.

Double bonds were first used in chemical drawings by the Russian chemist Alexander Butlerov. In drawings, a double bond is shown as two lines (=) between the atoms. These bonds are stronger and shorter than single bonds. Because they have extra electrons, they can react easily with certain other chemicals, like in reactions with halogens.

Some common examples of compounds with double bonds include ethylene, which has a carbon–carbon double bond, acetone with a carbon–oxygen double bond, dimethyl sulfoxide featuring a sulfur–oxygen double bond, and diazene that contains a nitrogen–nitrogen double bond.

Double bonds in alkenes

Double bonds are special connections between atoms where four electrons are shared, instead of just two. They are most common between two carbon atoms, like in a group of compounds called alkenes.

In a molecule called ethylene, each carbon atom has special shapes called orbitals. These orbitals come together to form two types of bonds: a sigma bond and a pi bond. Because of this arrangement, the atoms cannot move around the bond easily, which gives these molecules unique shapes. Double bonds are also shorter and stronger than single bonds between atoms.

Variations

In molecules where double bonds and single bonds change places, parts of atoms called p-orbitals can share space over several atoms. This creates something called a conjugated system. You can see this in dienes and enones. In ring-shaped molecules, this sharing can make the molecule aromatic. In cumulenes, two double bonds sit right next to each other.

Double bonds are often seen between certain elements, like carbon, nitrogen, and oxygen. They are less common with elements in higher rows. Even metals can form multiple bonds with ligands.

Group 14 alkene homologues

Double bonded compounds, called alkene homologues, exist for heavier elements in group 14. Unlike regular alkenes, these compounds do not lie flat. Instead, they twist or bend.

For example, distannene has a tin-tin bond that is short and bends. Each tin atom has a pyramid shape. It breaks apart easily in solution. In disilenes, each silicon atom lies flat, but the whole molecule twists. In plumbenes, the lead-lead bond can be longer than many single bonds. These bonds can be weaker than single bonds. A special model can help predict how these bonds work.

Types of double bonds between atoms