Edexcel A Level Chemistry:复习笔记3.4.3 Nucleophilic Substitution Mechanism

Nucleophilic Substitution: Mechanisms

 

  • 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)

3.3-Halogen-Compounds-Polarity-of-the-C-X-bond

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

3.3-Halogen-Compounds-Electrophilic-Substitution-by-NaOH

The halogen is replaced by a nucleophile, OH–

  • The mechanism for the reaction is as follows

Nucleophilic-substitution-hydroxide-ions-

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

7.5.3-Mechanism-of-Nu-substitution-making-amines

The mechanism of nucleophilic substitution between ammonia and a halogenoalkane

 

 

转载自savemyexams

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