CIE AS Chemistry (9701) exams from 2022

Revision Notes

3.3.2 Substitution Reactions of Halogenoalkanes

Nucleophilic Substitution 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

 

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

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

Reaction with NaOH

  • The reaction of a halogenoalkane with aqueous alkali results in the formation of an alcohol
  • The halogen is replaced by the OH
  • The aqueous hydroxide (OH ion) behaves as a nucleophile by donating a pair of electrons to the carbon atom bonded to the halogen
  • Hence, this reaction is a nucleophilic substitution
    • For example, bromoethane reacts with aqueous alkali when heated to form ethanol

 

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

The halogen is replaced by a nucleophile, OH

Reaction with KCN

  • 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
    • For example, 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

Reaction with NH3

  • 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
    • For example, 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

 

  • It is very important that the ammonia is in excess as the product of the nucleophilic substitution reaction, the ethylamine, can act as a nucleophile and attack another bromoethane to form the secondary amine, diethylamine

Reaction with aqueous silver nitrate

  • Halogenoalkanes can be broken down under reflux by water to form alcohols
    • The breakdown of a substance by water is also called hydrolysis
  • This reaction is classified as a nucleophilic substitution reaction with water molecules in aqueous silver nitrate solution acting as nucleophiles, replacing the halogen in the halogenoalkane
    • For example, 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 a hydroxyl group, OH

  • This reaction is similar to the nucleophilic substitution reaction of halogenoalkanes with aqueous alkali, however, hydrolysis with water is much slower than with the OHion in alkalis
    • 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
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