Alkene Bonding
A concise revision guide to alkene structure, carbon-carbon double bonds, sigma bonds, pi bonds and why alkenes are more reactive than alkanes.
What Makes an Alkene Unsaturated?
Alkenes are hydrocarbons described as unsaturated. This means they do not contain the maximum possible number of hydrogen atoms.
The key structural feature of an alkene is at least one carbon-carbon double bond, written as C=C. For alkenes with one double bond and no rings, the general formula is CnH2n.
Small alkenes include ethene, propene and butene. In longer alkenes, the position of the double bond can change, producing positional isomers such as but-1-ene and but-2-ene.
Key idea: An alkene must contain a C=C double bond, and this double bond is responsible for the characteristic chemistry of alkenes.
Alkenes and positional isomers
Shows unsaturation, the C=C bond, CnH2n, and how butene can exist as but-1-ene or but-2-ene.
The Carbon-Carbon Double Bond
A carbon-carbon double bond is made from two different covalent bonds: one sigma, σ, bond and one pi, π, bond.
The sigma bond forms first and lies directly between the two carbon nuclei. The pi bond forms from sideways overlap of p orbitals, giving electron density above and below the plane of the molecule.
The pi bond is more exposed than the sigma bond. This creates a region of high electron density that can attract electron-deficient species called electrophiles.
Exam focus: When explaining alkene reactivity, link the exposed pi bond to high electron density and attraction to electrophiles.
The C=C double bond contains one sigma bond and one pi bond, with the pi bond forming above and below the molecular plane.
Formation of the Sigma Bond
When two carbon atoms form a double bond, each carbon uses one sp2 hybrid orbital to overlap directly with the other carbon atom.
This direct head-on overlap produces a strong carbon-carbon sigma bond. The electron density is concentrated between the two nuclei, so the sigma bond lies along the straight line joining the nuclei.
A single sigma bond allows free rotation around the bond axis. However, in a C=C double bond, this rotation is restricted because the pi bond must also be maintained.
Remember: Sigma bonds are formed by direct overlap along the internuclear axis.
Direct head-on overlap places electron density between the nuclei, forming the first bond in the carbon-carbon double bond.
σ Bonding Orbital
A visual model of a sigma (σ) bonding orbital formed by the head-on overlap of two atomic orbitals. Electron density is concentrated along and between the two nuclei.
Formation of the Pi Bond
After the sigma bond forms, each carbon retains one unhybridised p orbital perpendicular to the plane of the molecule.
The pi, π, bond forms when these p orbitals overlap sideways. This produces regions of electron density above and below the line joining the two carbon nuclei.
Because sideways overlap is less direct than sigma overlap, the pi bond is weaker than the sigma bond. The pi bond also prevents free rotation around the C=C bond.
Pi bond: A covalent bond formed by sideways overlap of p orbitals, with electron density above and below the internuclear axis.
Formation of the π bond
Sideways p-orbital overlap produces electron density above and below the C-C bond axis, restricting rotation and increasing reactivity.
π Bonding Orbital
A visual model of a pi (π) bonding orbital formed by the sideways overlap of two p orbitals. Electron density is concentrated in two lobes above and below the internuclear axis. The red plane marks the nodal plane where electron density is zero.
Comparing Sigma and Pi Bonds
Exam questions often test whether you can distinguish sigma bonding from pi bonding and connect this bonding to alkene reactivity.
| Feature | Sigma bond | Pi bond |
|---|---|---|
| How it forms | Direct head-on orbital overlap | Sideways overlap of p orbitals |
| Electron density | Between the two nuclei | Above and below the molecular plane |
| Effect in alkenes | Forms the first bond between the carbon atoms | Creates an exposed electron-rich region that reacts with electrophiles |
Check Your Understanding
Use this short activity to check the key terms before moving on to electrophilic addition.
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Some ionic radii are shown.
| Ion | Ionic radius / nm |
|---|---|
| Na+ | 0.102 |
| K+ | 0.138 |
| F− | 0.133 |
| Cl− | 0.180 |
Which compound has the strongest ionic bonding?
Explain why the metallic bonding in magnesium is much stronger than that in sodium.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.