Alkenes - Introduction (Edexcel International A Level Chemistry)

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Alkenes - Introduction

  • All alkenes contain a double carbon bond, which is shown as two lines between two of the carbon atoms i.e. C=C
  • All alkenes contain a double carbon bond, which is the functional group and is what allows alkenes to react in ways that alkanes cannot
  • Alkenes have the general molecular formula CnH2n
  • They are said to be unsaturated hydrocarbons
    • They contain carbon-carbon double bonds
    • They are made up of hydrogen and carbon atoms only
  • Alkenes are named using the nomenclature rule alk + ene
  • In molecules with a straight chain of 4 or more carbon atoms, the position of the C=C double bond must be specified
  • The carbon atoms on the straight chain must be numbered, starting with the end closest to the double bond
  • The lowest-numbered carbon atom participating in the double bond is indicated just before the -ene:

The First Five Members of the Alkene Family

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

σ bonds

  • Sigma (σ) bonds are formed from the end to end overlap of atomic orbitals
  • s orbitals overlap this way as well as p orbitals

Chemical Bonding Bond Overlap in Sigma Orbitals, downloadable AS & A Level Chemistry revision notes

Sigma orbitals can be formed from the end to end overlap of s orbitals 

  • The electron density in a σ bond is symmetrical about a line joining the nuclei of the atoms forming the bond
  • The pair of electrons is found between the nuclei of the two atoms
  • The electrostatic attraction between the electrons and nuclei bonds the atoms to each other

Hydrogen

  • The hydrogen atom has only one s orbital
  • The s orbitals of the two hydrogen atoms will overlap to form a σ bond

Chemical Bonding Orbital Overlap in Hydrogen, downloadable AS & A Level Chemistry revision notes

π bonds

  • Pi (π) bonds are formed from the sideways overlap of adjacent p orbitals
  • The two lobes that make up the π bond lie above and below the plane of the σ bond
  • This maximises overlap of the p orbitals
  • A single π bond is drawn as two-electron clouds, one arising from each lobe of the p orbitals
  • The two clouds of electrons in a π bond represent one bond containing two electrons

Chemical Bonding Bond Overlap in Pi Orbitals, downloadable AS & A Level Chemistry revision notes

π orbitals can be formed from the sideways overlap of p orbitals

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

Chemical Bonding Electron Density in Ethene, downloadable AS & A Level Chemistry revision notes

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

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Sonny

Author: Sonny

Expertise: Chemistry

Sonny graduated from Imperial College London with a first-class degree in Biomedical Engineering. Turning from engineering to education, he has now been a science tutor working in the UK for several years. Sonny enjoys sharing his passion for science and producing engaging educational materials that help students reach their goals.