Bonding 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
Each carbon atom in ethene forms two sigma bonds with hydrogen atoms and one σ bond with another carbon atom. The fourth electron is used to form a π bond between the two carbon atoms
Exam Tip
A double bond is a combination of a σ and π bond and a triple bond is a combination of one σ and two π bonds.
The strength of the bonds increases as follows: single < double < triple bond
This is due to the increased electron density around the carbon-carbon bond, making the bond stronger and more difficult to break.
You must be able to draw and explain these diagrams for the exam.
Shape of Alkenes
- The carbon of an alkene double bond has three σ bonding pairs of electrons
- Two pairs forming σ bonds with other atoms
- One pair forming σ bonds with the other carbon atom of the double bond
- The three bonding pair of electrons are in the plane of the molecule and repel each other
- This means that the molecule adopts a planar arrangement with bond angles of 120o
- The fourth π bonding pair forms the double bond in combination with the carbon-carbon σ bond
Ethene
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- 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 overlap of the two p orbitals results in the formation of a π bond in ethene in which the bonding pair of electrons repel each other to force the molecule into a planar configuration with bond angles of 120 o
