AQA AS Chemistry

Revision Notes

3.3.3 Reactions of Halogenoalkanes

The Key Reactions of the Halogenoalkanes

Types of Reactions that halogenoalkanes undergo:

  • Halogenoalkanes are much more reactive than alkanes due to the presence of the electronegative halogens
    • The carbon-halogen bond is polar causing the carbon to carry a partial positive and the halogen a partial negative charge

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

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

  • Because of this, halogenoalkanes will undergo two key types of reaction
  • Nucleophilic substitution reactions
    • A halogen is substituted for another atom or group of atoms
    • The products formed when halogenoalkanes undergo this type of reaction are alcohols, amines and nitriles
  • Elimination reactions
    • A hydrogen halide is eliminated during the reaction
    • The key product formed from this type of reaction is an alkene

Formation of alcohols

  • The nucleophile in this reaction is the hydroxide, OH ion
  • An aqueous solution of sodium hydroxide (NaOH) or potassium hydroxide (KOH) with ethanol is used
  • 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

Measuring the rate of hydrolysis

  • Acidified silver nitrate can be used to measure the rate of hydrolysis of halogenoalkanes
  • Set up three test tubes in a 50 oC water bath, with a mixture of ethanol and acidified silver nitrate
  • Add a few drops of a chloroalkane, bromoalkane and an iodoalkane to each test tube and start a stop watch
  • Time how long it takes for the precipitates to form
  • The precipitate will form as the reaction progresses and the halide ions are formed
  • A white precipitate will form from the chloroalkane, a cream precipitate will form from the bromoalkane and a yellow precipitate will form from the iodoalkane
    • The yellow precipitate will form the fastest
    • This is because the C-I bond has the lowest bond enthalpy, so it is the easiest to break and will cause the I ions to form the fastest
    • The white precipitate will form the slowest
    • This is because the C-Cl bond has the highest bond enthalpy, so it is the hardest to break and will cause the Cl ions to form the slowest

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
  • An 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,  NH2

Nucleophilic Substitution

  • The nucleophilic substitution mechanisms for the above reactions are as follows:

Nucleophilic Substitution with OH

Nucleophilic Substitution with NH3

Nucleophilic Substitution with CN

Elimination

  • In an elimination reaction, an organic molecule loses a small molecule
    • In the case of halogenoalkanes this small molecule is a hydrogen halide (eg. HCl)
  • 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
    • For example, bromoethane reacts with ethanolic sodium hydroxide when heated to form etheneHydrocarbons 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

Elimination Reactions

  • The elimination mechanism for the above reaction is as follows:

Elimination with OH

 

Which product will form?

  • Note that the reaction conditions should be stated correctly as different reaction conditions will result in different types of organic reactions
    • NaOH (hot, in ethanol): an elimination reaction occurs to form an alkene
    • NaOH (warm, aqueous): a nucleophilic substitution reaction occurs, and an alcohol is formed
  • The classification of halogenoalkane is also important in determining the type of reaction which takes place
    • Primary halogenoalkanes tend to react by substitution
    • Tertiary halogenoalkanes tend to react by elimination
    • Secondary halogenoalkanes will do both

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