Fundamentals of Reaction Mechanisms (AQA A Level Chemistry)

Reaction Mechanisms: Terminology

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

• 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

Nucleophiles & electrophiles

• A nucleophile is an electron-rich species that can donate a pair of electrons

  • ‘Nucleophile’ means ‘nucleus/positive charge loving’ as nucleophiles are attracted to positively charged species

  • Nucleophilic refers to reactions that involve a nucleophile

• An electrophile is an electron-deficient species that can accept a pair of electrons

  • ‘Electrophile’ means ‘electron/negative charge loving’ as electrophiles are attracted to negatively charged species

  • Electrophilic refers to reactions that involve an electrophile

A nucleophile ‘loves’ a positive charge and an electrophile ‘loves’ a negative charge

Types of reactions

• An addition reaction is an organic reaction in which two (or more) molecules combine to give a single product with no other products

• A substitution reaction is a reaction that involves replacing an atom or group of atoms by another

• An elimination reaction is a reaction in which a small molecule (such as H₂O or HCl) is removed from an organic molecule

• A hydrolysis reaction is a reaction in which a compound is broken down by water (it can also refer to the breakdown of a substance by dilute acids or alkali)

• A condensation reaction is a reaction in which two organic molecules join together and in the process eliminate small molecules (such as H₂O or HCl)

Oxidation & reduction

• An oxidation reaction is a reaction in which oxygen is added, electrons are removed or the oxidation number of a substance is increased

  • In organic chemistry it often refers to the addition of oxygen or removal of hydrogen atoms to a substance

  • In equations for organic redox reactions, the symbol [O] can be used to represent one atom of oxygen from an oxidising agent

• A reduction reaction is a reaction in which oxygen is removed, electrons are added or the oxidation number of a substance is decreased

  • In organic chemistry it often refers to the removal of oxygen or addition of hydrogen atoms to a substance

  • In equations for organic redox reactions, the symbol [H] can be used to represent one atom of hydrogen from a reducing agent

Different types of reactions in organic chemistry

Fundamentals of Mechanisms

• In organic reaction mechanisms, curly arrows represent the movement of electron pairs

• The arrow beings at a bond or a lone pair of electrons and points to the species that accepts the lone pair of electrons

Curly arrows show electron pairs moving from the source (e.g. a nucleophile) to its destination (e.g. an electrophile)