CIE AS Chemistry (9701) 2019-2021

Revision Notes

3.3.1 Halogenoalkanes

Substitution & Elimination Reactions of Halogenoalkanes

  • Halogenoalkanes are much more reactive than alkanes due to the presence of the electronegative halogens
    • The halogen-carbon bond is polar causing the carbon to carry a partial positive and the halogen 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
  • In an elimination reaction of halogenoalkanes, a halogen halide is eliminated to form an alkene

Hydrolysis

  • Halogenoalkanes can be broken down under reflux by water to form an alcohol
    • The breakdown of a substance by water is also called hydrolysis
  • The reaction is classified as a nucleophilic substitution reaction as a water molecule in the silver nitrate solution acts as a nucleophile and replaces the halogen
    • Eg. bromoethane reacts with aqueous silver nitrate solution to form ethanol

 

Halogen Compounds Hydrolysis of Halogenoalkanes, downloadable AS & A Level Chemistry revision notes

The halogen is replaced by an hydroxyl group, OH

  • This reaction is similar to the nucleophilic substitution reaction of halogenoalkanes with aqueous alkali, however the hydrolysis with water is much slower than with the OH ion
    • The hydroxide ion is a better nucleophile than water as it carries a full formal negative charge
    • In water, the oxygen atom only carries a partial negative 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 with aqueous alkali

 

  • The halogenoalkanes have different rates of hydrolysis, so this reaction can be used as a test to identify halogens in a halogenoalkane by measuring how long it takes for the test tubes containing the halogenoalkane and aqueous silver nitrate solutions to become opaque

Formation of nitriles

  • The nucleophile in this reaction is the cyanide, CN ion
  • Ethanolic solution of potassium cyanide (KCN in ethanol) is heated under reflux with the halogenoalkane
  • The product is a nitrile
    • Eg. bromoethane reacts with ethanolic potassium cyanide when heated under reflux 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 extra carbon atom to the carbon chain
  • 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, NH3 molecule
  • Ethanolic solution of excess ammonia (NH3 in ethanol) is heated under pressure with the halogenoalkane
  • The product is a primary amine
    • Eg. 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,  NH3

Elimination reaction

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

 

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

Hydrogen bromide is eliminated to form ethene

Exam Tip

The reaction conditions in a reaction are extremely important.

If NaOH(ethanol) is used, an elimination reaction takes place to form an alkene from a halogenoalkane.

If NaOH(aq) is used, a nucleophilic substitution reaction takes place to form an alcohol from a halogenoalkane.

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