Nitriles (OCR A Level Chemistry)

Carbon-carbon bond forming 

• Carbon–carbon bond forming reactions are organic reactions in which a new carbon–carbon bond is formed
• They are important in the production of many man-made chemicals such as pharmaceuticals and plastics
• Carbon-carbon bonds can be formed via the following reactions
  • Formation of nitriles to extend the carbon chain
  • Frieidel-Crafts alkylation and acylation

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

Nucleophilic substitution of a haloalkane with CN^-

Addition of HCN to carbonyl compounds

• The nucleophilic addition of hydrogen cyanide to carbonyl compounds is a two-step process, as shown below
• In step 1, the cyanide ion attacks the carbonyl carbon to form a negatively charged intermediate
• In step 2, the negatively charged oxygen atom in the reactive intermediate quickly reacts with aqueous H⁺ (either from HCN, water or dilute acid) to form 2-hydroxynitrile compounds,
  • e.g. 2-hydroxypropanenitrile

Hydrolysis of nitriles

• Nitriles are hydrolysed by either dilute acid or dilute alkali followed by acidification
  • Hydrolysis by dilute acid results in the formation of a carboxylic acid and ammonium salt
  • Hydrolysis by dilute alkali results in the formation of a sodium carboxylate salt and ammonia; Acidification is required to change the carboxylate ion into a carboxylic acid

• The -CN group at the end of the hydrocarbon chain is converted to a -COOH group

Hydrolysis of nitriles by either dilute acid (1) or dilute alkali and acidification (2) will form a carboxylic acid

Reduction of Nitriles

• Nitriles contain a -CN functional group which can be reduced to an -NH₂ group
• The nitrile vapour and hydrogen gas are passed over a nickel catalyst or LiAlH₄ in dry ether can be used to form a primary amine

Nitriles can be reduced with LiAlH₄ or H₂ and Ni catalyst