Reaction Mechanisms: Introduction (Edexcel International AS Chemistry): Revision Note

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:

During this propagation step, the unpaired electron from chlorine and one of the electrons from the C-H bond react together to form a new covalent bond and the other electron from the C-H bond moves to form a methyl radical

• 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:

During this step of an addition reaction, the lone pair from the bromide ion reacts with the positive carbocation to form a new covalent bond

• 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 & Mechanisms

• 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