Nucleophilic Substitution (Oxford AQA International A Level Chemistry)

Nucleophilic Substitution Mechanism

• A nucleophile is an electron-rich species that can donate a pair of electrons

  • ‘Nucleophile’ means ‘nucleus/positive charge loving’ as nucleophiles are attracted to positively charged species

  • H₂O, OH^-, NH₃ and CN^- are examples of nucleophiles as they have a full negative charge or a lone pair of electrons

• 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

  Halogenoalkanes will undergo nucleophilic substitution reactions due to the polar C-X bond (where X is a halogen)

General Mechanism for Nucleophilic Substitution

• There are usually two curly arrows to draw in this mechanism

  • Curly arrow from lone pair of electrons on nucleophile to partially positive carbon atom in C-X bond

  • Curly arrow from the C-X bond to the X atom

Formation of nitriles

• The nucleophile in this reaction is the CN^- ion

  • The nucleophilic substitution of halogenoalkanes with KCN is used to extend the carbon chain

• An aqueous ethanolic solution of potassium cyanide is warmed with the halogenoalkane

• The product is a nitrile

  • E.g. bromoethane reacts with ethanolic potassium cyanide when heated under reflux to form propanenitrile

CH₃CH₂Br + CN^- CH₃CH₂CN + Br^-

Formation of primary amines by reaction with ammonia

• The nucleophile in this reaction is NH₃

• An ethanolic solution of excess ammonia (NH₃ in ethanol) is heated under pressure with the halogenoalkane

• The product is a primary amine

  • E.g. bromoethane reacts with excess ethanolic ammonia when heated under pressure to form ethylamine and ammonium bromide

CH₃CH₂Br + 2NH₃ CH₃CH₂NH₂ + NH₄Br

Formation of alcohols by hydrolysis

• The nucleophile is the OH^- ion

• Aqueous sodium (or potassium) hydroxide is the reagent

• Ethanol is used as a solvent (to increase the solubility of the halogenoalkane) and the reaction is warmed

  • The reaction is very slow at room temperature

CH₃CH₂Br + OH^- CH₃CH₂OH + Br^-

Bond Enthalpy of C-X bonds

• The halogenoalkanes have different rates of substitution reactions

• Since substitution reactions involve breaking the carbon-halogen bond the bond energies can be used to explain their different reactivities

Halogenoalkane Bond Energy Table

• The table above shows that the C-I bond requires the least energy to break, and is therefore the weakest carbon-halogen bond

• During substitution reactions the C-I bond will therefore heterolytically break as follows:

  • R₃C-I + OH^-     →    R₃C-OH + I^-

• The C-F bond, on the other hand, requires the most energy to break and is, therefore, the strongest carbon-halogen bond

• Fluoroalkanes will therefore be less likely to undergo substitution reactions