Double bond

In chemistry, a double bond is a special connection between two atoms where four bonding electrons are shared, instead of just two like in a single bond. This kind of bond is most often seen between two carbon atoms, like in alkenes. But 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, and 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 rotate 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 switch back and forth, the parts of atoms called p-orbitals can share space over several atoms in a row. 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 without sharing space.

Double bonds are often seen between certain elements, like carbon, nitrogen, and oxygen, which are in the second row of the periodic table. 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 all the heavier elements in group 14. Unlike regular alkenes, these compounds do not lie flat. Instead, they twist or bend, especially for the heavier elements.

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

Types of double bonds between atoms