4.1.4 Reaction Mechanisms

Homolytic & Heterolytic fission

• Homolytic fission is breaking a covalent bond in such a way that each atom takes an electron from the bond to form two radicals
  • A radical is a reactive chemical species with an unpaired electron
• Heterolytic fission is breaking a covalent bond in such a way that the more electronegative atom takes both the electrons from the bond to form a negative ion and leaving behind a positive ion 

The diagram shows heterolytic fission in which the most electronegative atom takes both electrons in the covalent bond and homolytic fission in which each atom takes one electron from the covalent bond

• Reaction equations tell you about the amount of reactants and products, including their stoichiometry, in a reaction
• Reaction mechanisms tell you about how the reaction actually takes place

• In a reaction mechanism, curly arrows show the movement of electrons
  • Curly arrows can be single headed, sometimes called fish-hook arrows or half curly arrows, to show the movement of one electron which can occur when:
    • A covalent bond undergoes homolytic fission to form two radicals
    • Two radicals terminate forming a covalent bond in the process
    • A radical and a covalent compound propagate as part of a reaction, as shown:

• • Curly arrows can be double headed to show the movement of a pair of electrons, which can occur when:
    • A covalent bond undergoes fission to form a positive and a negative ion
    • A lone pair of electrons attacks a positive or δ+ centre and forms a new covalent bond, as shown:

• Curly arrows are a feature of three main types of reaction:
  1. Addition reactions - where two reactants combine to form one product, e.g. ethene and bromine undergoing addition to form 1,2-dibromoethane
  2. Substitution reactions - where an atom or group of atoms in a compounds is replaced by another atom or group of atoms, e.g. bromoethane reacting with the hydroxide ion to form ethanol and the bromide ion
  3. Elimination reactions - where a small molecule is removed from a larger molecule, e.g. ethanol reacting with an acid catalyst to form ethene alongside a small molecule of water which is eliminated