Reactions of Halogenoalkanes (Edexcel International A Level Chemistry)

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Halogenoalkanes - Reactions

  • 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

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

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

Halogen Compounds Nucleophilicity, downloadable AS & A Level Chemistry revision notes

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

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

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

Halogen Compounds Electrophilic Substitution by NH3, downloadable AS & A Level Chemistry revision notes

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

Hydrocarbons Elimination Reaction, downloadable AS & A Level Chemistry revision notes

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

 Halogen Compounds Elimination, downloadable AS & A Level Chemistry revision notes

Hydrogen bromide is eliminated to form ethene

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Richard

Author: Richard

Expertise: Chemistry

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.