Double bond
Two bond components connect the same carbon pair.
A σ bond lies along the internuclear axis and a π bond arises from sideways orbital overlap.
Molecular compound · alkene
Ethene
C₂H₄
Ethene is C₂H₄, the simplest alkene. Its two carbon atoms are connected by a double bond consisting of one σ bond plus one π bond.
Name note: ethene is the systematic name; ethylene is the widely used common name for the same compound.
Essentials
The C=C double bond defines ethene as an alkene.
H₂C=CH₂ is the simplest molecule that introduces a carbon–carbon π bond.
Detail
The π bond places electron density above and below the molecular plane.
Both carbon centers are locally trigonal planar. Maintaining π overlap restricts free rotation around C=C.
Geometry
Rotation is not free like a C–C single bond.
Twisting the carbons would weaken or break the orbital overlap responsible for the π bond.
Understand
Addition reactions consume the π bond while retaining the σ framework.
Hydrogenation adds one H to each carbon and converts ethene into ethane. The overall change replaces a C=C double bond with a C–C single bond plus two new C–H bonds.
Why alkenes react differently
The π electrons are more exposed than the σ bond.
This makes the double bond a common site for addition chemistry.
Reference identifiers
Names and identifiers that pin down the chemical identity.
| Formula | C₂H₄ / CH₂=CH₂ |
|---|---|
| CAS Registry Number | 74-85-1 |
| PubChem CID | 6325 |
Advanced
Push past the first model without losing the simple picture.
Why is ethene planar?
Each carbon has a trigonal-planar sigma-bond framework while parallel p orbitals overlap to form the π bond.
Can ethene rotate around C=C?
Not freely. Rotation destroys the p-orbital overlap required for the π bond.
Is ethene the same as ethylene?
Yes. Ethylene is the retained/common name; ethene is systematic.
Constituent elements