2.7.2 Reaction Mechanisms: Introduction

• 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
    • The positive ion is electron deficient and is an electrophile
    • The negative ion is electron rich and is a nucleophile
    • 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

• Bond polarity can be used to suggest reaction mechanisms

C₂H₆ + Cl₂ → C₂H₅Cl + HCl

• • A hydrogen atom in ethane is substituted by a chlorine atom
  • All of the bonds in the reaction of ethane with chlorine are either non-polar or only slightly polar
  • This suggests that the type of bond breaking will be homolytic fission
  • The reaction is likely to involve free radicals
  • Therefore, the reaction mechanism is likely to be free radical substitution

C₂H₄ + HBr → C₂H₅Br

• • The hydrogen bromide is added to the ethene molecule
  • The hydrogen bromide molecule contains a polar bond 
  • This suggests that the type of bond breaking will be heterolytic fission 
  • The reaction could involve electrophiles or nucleophiles
  • The ethene molecule contains a carbon-carbon double bond which will attract an electrophile
  • Therefore, the reaction mechanism is most likely to be electrophilic addition