Nucleophilic Substitution (AQA A Level Chemistry)

Mechanism: Nucleophilic Substitution

• 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

• There are various different species which can behave as nucleophiles, and some make better nucleophiles than others

A hydroxide ion is a better nucleophile as it has a full formal negative charge whereas the oxygen atom in water only carries a partial negative charge

• A nucleophilic substitution reaction is one in which a nucleophile attacks a carbon atom which carries a partial positive charge

• An atom that has a partial negative charge is replaced by the nucleophile

• Halogenoalkanes will undergo nucleophilic substitution reactions due to the polar C-X bond (where X is a halogen)

Due to large differences in electronegativity between the carbon and halogen atom, the C-X bond is polar

General Mechanism for Nucleophilic Substitution

• For example, in the following reaction a halogenoalkane reacts with aqueous alkali to form an alcohol

The halogen is replaced by a nucleophile, OH^–

• The mechanism for the reaction is as follows

Nucleophilic substitution reaction of bromoethane and aqueous alkali (e.g. NaOH)