The Nucleophilic Substitution Mechanism (Edexcel International A Level Chemistry)

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Nucleophilic Substitution - Mechanism

  • 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)

Halogen Compounds Polarity of the C-X bond, downloadable AS & A Level Chemistry revision notes

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

Mechanism with aqueous potassium hydroxide

  • In the following reaction a halogenoalkane reacts with aqueous alkali to form an alcohol

Halogen Compounds Electrophilic Substitution by NaOH, downloadable AS & A Level Chemistry revision notes

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)

Mechanism with ammonia

  • When ammonia reacts with a haloalkane a nucleophilic substitution reaction takes place forming a primary amine
    • For example chloromethane reacts with ammonia in two steps to make methylamine and ammonium chloride

CH3Cl + NH3 → [CH3NH3]+Cl-

[CH3NH3]+Cl- + NH3 → CH3NH2 + NH4+Cl-

  • Excess ammonia is used to prevent further substitution and favour the formation of a primary amine 

Mechanism of Nu substitution making amines, downloadable AS & A Level Chemistry revision notes

The mechanism of nucleophilic substitution between ammonia and a halogenoalkane

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Richard

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Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.