Structure, Bonding & Reactivity (Oxford AQA International A Level Chemistry)
Revision Note
Written by: Alexandra Brennan
Reviewed by: Stewart Hird
Bonding in Alkenes
Alkenes are unsaturated hydrocarbons
They are unsaturated because they contain a double covalent bond
They are hydrocarbons as they consist of hydrogen and carbon atoms only
The general formula of alkanes is CnHn+2
Types of bonds in alkenes
Each carbon atom has four electrons in its outer shell (electronic configuration: 1s22s22p2)
Carbon atoms share these four electrons in four covalent bonds with other atoms to achieve a full outer shell configuration
These electrons are found in orbitals within the respective atoms
When forming a covalent bond, the orbitals overlap in such a way to form two types of bonds:
Sigma bonds (σ)
Pi bonds (π)
When carbon atoms use only three of their electron pairs to form a σ bond, each carbon atom will have a p orbital which contains one spare electron
When the p orbitals of two carbon atoms overlap with each other, a π bond is formed (the π bond contains two electrons)
The two orbitals that form the π bond lie above and below the plane of the two carbon atoms to maximise bond overlap
Ethene
Each carbon atom uses three of its four electrons to form σ bonds
Two σ bonds are formed with the hydrogen atoms
One σ bond is formed with the other carbon atom
The fourth electron from each carbon atom occupies a p orbital which overlaps sideways with another p orbital on the other carbon atom to form a π bond
This means that the C-C is a double bond: one σ and one π bond
The double bond is an area of high electron density
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