Structure, Bonding & Reactivity (Oxford AQA International A Level (IAL) Chemistry): Revision Note

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 alkenes is C_nH_2n

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