AQA A Level Chemistry复习笔记3.3.2 Reactions of 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

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

Nucleophilic Substitution with OH-

Nucleophilic Substitution with NH3

Nucleophilic Substitution with CN-

• 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

• 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

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