Double bond
A double bond in chemistry is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist between two different elements: for example, in a carbonyl group between a carbon atom and an oxygen atom. Other common double bonds are found in azo compounds, imines and sulfoxides. In a skeletal formula, a double bond is drawn as two parallel lines between the two connected atoms; typographically, the equals sign is used for this. Double bonds were first introduced in chemical notation by Russian chemist Alexander Butlerov.
Double bonds involving carbon are stronger than single bonds and are also shorter. The bond order is two. Double bonds are also electron-rich, which makes them potentially more reactive in the presence of a strong electron acceptor.
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Double bonds in alkenes
The type of bonding can be explained in terms of orbital hybridisation. In ethylene each carbon atom has three sp2 orbitals and one p-orbital. The three sp2 orbitals lie in a plane with ~120° angles. The p-orbital is perpendicular to this plane. When the carbon atoms approach each other, two of the sp2 orbitals overlap to form a sigma bond. At the same time, the two p-orbitals approach and together they form a pi bond. For maximum overlap, the p-orbitals have to remain parallel, and, therefore, rotation around the central bond is not possible. This property gives rise to cis-trans isomerism. Double bonds are shorter than single bonds because p-orbital overlap is maximized.| 2 sp2 orbitals approach to form a sp2-sp2 sigma bond | Two p-orbitals overlap to form a pi-bond in a plane parallel to the sigma plane | pi bond in ethylene |
With 133 pm, the ethylene C=C bond length is shorter than the C−C length in ethane with 154 pm. The double bond is also stronger, 636 kJ mol−1 versus 368 kJ mol−1 but not twice as much as the pi-bond is weaker than the sigma bond due to less effective pi-overlap.
In an alternative representation, the double bond results from two overlapping sp3 orbitals as in a bent bond.
Types of double bonds between atoms
Variations
In molecules with alternating double bonds and single bonds, p-orbital overlap can exist over multiple atoms in a chain, giving rise to a conjugated system. Conjugation can be found in systems such as dienes and enones. In cyclic molecules, conjugation can lead to aromaticity. In cumulenes, two double bonds are adjacent.Double bonds are common for period 2 elements carbon, nitrogen, and oxygen, and less common with elements of higher periods. Metals, too, can engage in multiple bonding in a metal ligand multiple bond.