Nucleophilic Substitution (Edexcel A Level Chemistry): Revision Note

Nucleophilic Substitution: Reactions

• 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

  • Nucleophilic refers to reactions that involve a nucleophile

Formation of alcohols

• The nucleophile in this reaction is the hydroxide ion, OH^- 

• An aqueous solution of sodium hydroxide (NaOH) or potassium hydroxide (KOH) is used to form an alcohol

CH₃CH₂Br + OH^- → CH₃CH₂OH + Br^-

• 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

• 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

Reaction with water 

• The water molecule is a weak nucleophile, but it will eventually substitute for the halogen 

• This occurs much more slowly compared to when warm aqueous sodium hydroxide is used

• An alcohol is produced

RX + H₂O → ROH + H⁺ + X^-

CH₃CH₂Br + H₂O → CH₃CH₂OH + H⁺ + Br^-

• If silver nitrate solution in ethanol is added to the solution, the silver ions will react with the halide ions as soon as they form, giving a silver halide precipitate

Ag⁺ (aq) + X^- (aq) → AgX (s) 

Formation of nitriles

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

• An ethanolic solution of potassium cyanide (KCN in ethanol) is heated under reflux with the halogenoalkane

• The product is a nitrile

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

CH₃CH₂Br + CN^- → CH₃CH₂CN + Br^-

• The nucleophilic substitution of halogenoalkanes with KCN extends the carbon chain by adding an extra carbon atom 

• 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 molecule, NH₃ 

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

  • An excess of ammonia is used because the product is more reactive than ammonia so further substitution reactions could occur

• The product is a primary amine

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

CH₃CH₂Br + NH₃ → CH₃CH₂NH₂ + HBr

Formation of alkenes 

• The halogenoalkanes are heated under reflux with ethanolic sodium hydroxide causing the C-X bond to break heterolytically, forming an X^- ion and leaving an alkene as an organic product

Halogenoalkane + NaOH (ethanol) $\stackrel{\mathrm{heat}}{\to }$ alkene + water + NaX

• For example, bromoethane is heated under reflux with ethanolic sodium hydroxide to form ethene

  • Hydrogen bromide is eliminated to form ethene

Bromoethane + NaOH (ethanol) $\stackrel{\mathrm{heat}}{\to }$ CH₂=CH₂ + H₂O + NaBr

CH₃CH₂Br + NaOH (ethanol) $\stackrel{\mathrm{heat}}{\to }$ CH₂=CH₂ + H₂O + NaBr