Reactions of Halogenoalkanes (AQA A Level Chemistry)

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

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

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

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

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

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 ethene

    

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

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