Edexcel A Level Chemistry:复习笔记3.4.2 Nucleophilic Substitution

Nucleophilic Substitution: Reactions

 

  • 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

Formation of alcohols

  • The nucleophile in this reaction is the hydroxide ion, OH-
  • An aqueous solution of sodium hydroxide (NaOH) or potassium hydroxide (KOH) with ethanol is used to form an alcohol
  • This reaction is very slow at room temperature, so the reaction mixture is warmed
  • This is an example of a hydrolysis reaction and the product is an alcohol
    • The rate of this reaction depends on the type of halogen in the halogenoalkane
    • The stronger the C-X bond, the slower the rate of the reaction
    • In terms of bond enthalpy, C-F > C-Cl > C-Br > C-I
    • Fluoroalkanes do not react at all, but iodoalkanes have a very fast rate of reaction

     

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

The halogen is replaced by the nucleophile, OH-

 

  • This reaction could also be done with water as the nucleophile, but it is very slow
    • The hydroxide ion is a better nucleophile than water as it carries a full negative charge
    • In water, the oxygen atom only carries a partial charge

     

3.3-Halogen-Compounds-Nucleophilicity

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; this causes the nucleophilic substitution reaction with water to be much slower than the aqueous alkali

 

Reaction with water 

  • The water molecule is a weak nucleophile, but it will eventually substitute for the halogen
  • This occurs much more slowly compared to when warm aqueous sodium hydroxide is used
  • An alcohol is produced
    • RX + H2O → ROH + H+ + X-
    • CH3CH2Br + H2O → CH3CH2OH + H+ + Br-
  • If silver nitrate solution in ethanol is added to the solution, the silver ions will react with the halide ions as soon as they form, giving a silver halide precipitate
    • Ag+ (aq) + X- (aq) → AgX (s)

Formation of nitriles

  • The nucleophile in this reaction is the cyanide ion, CN-
  • An ethanolic solution of potassium cyanide (KCN in ethanol) is heated under reflux with the halogenoalkane
  • The product is a nitrile
    • E.g. bromoethane is heated under reflux with ethanolic potassium cyanide to form propanenitrile

     

3.3-Halogen-Compounds-Electrophilic-Substitution-by-KCN

The halogen is replaced by a cyanide group, CN -

 

  • The nucleophilic substitution of halogenoalkanes with KCN adds an extends the carbon chain by adding an extra carbon atom
  • This reaction can therefore be used by chemists to make a compound with one more carbon atom than the best available organic starting material

Formation of primary amines by reaction with ammonia

  • The nucleophile in this reaction is the ammonia molecule, NH3
  • An ethanolic solution of excess ammonia (NH3 in ethanol) is heated under pressure with a primary halogenoalkane
    • An excess of ammonia is used because the product is more reactive than ammonia so further substitution reactions could occur
  • The product is a primary amine
    • E.g. bromoethane reacts with excess ethanolic ammonia when heated under pressure to form ethylamine

     

3.3-Halogen-Compounds-Electrophilic-Substitution-by-NH3

 

The halogen is replaced by an amine group, NH2

Formation of alkenes 

  • The halogenoalkanes are heated with ethanolic sodium hydroxide causing the C-X bond to break heterolytically, forming an X- ion and leaving an alkene as an organic product
    • E.g. bromoethane is heated with ethanolic sodium hydroxide to form ethene

3.2-Hydrocarbons-Elimination-Reaction

Production of an alkene from a halogenoalkane by reacting it with ethanolic sodium hydroxide and heating it

 

 

3.3-Halogen-Compounds-Elimination

Hydrogen bromide is eliminated to form ethene

 

 

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