Nitriles contain the C≡N group. They can be reduced to primary amines, which gives a second route to primary aliphatic amines.Reducing agents:
• LiAlH4 (lithium tetrahydridoaluminate) in dry ether, a good laboratory method;
• hydrogen with a nickel catalyst (catalytic hydrogenation), used in industry because it is cheaper.Using [H] to represent the reducing agent, four hydrogen atoms are added across the C≡N triple bond:RCN + 4[H] → RCH2NH2 For catalytic hydrogenation this is written with H2: CH3CN + 2H2 → CH3CH2NH2, so ethanenitrile is reduced to ethylamine.Making the nitrile. Nitriles are made from halogenoalkanes by nucleophilic substitution with potassium cyanide in ethanol (heated under reflux). This adds one carbon atom to the chain.Worked example: convert bromoethane into propylamine in two steps.
Step 1: CH3CH2Br + KCN → CH3CH2CN + KBr, giving propanenitrile.
Step 2: CH3CH2CN + 4[H] → CH3CH2CH2NH2, giving propylamine, with one more carbon than bromoethane.Comparing routes. Reduction of nitriles gives only the primary amine, with no further substitution, so there is no mixture to separate. The disadvantages are that it needs two steps from a halogenoalkane, KCN is highly toxic, and LiAlH4 is expensive.
• LiAlH4 (lithium tetrahydridoaluminate) in dry ether, a good laboratory method;
• hydrogen with a nickel catalyst (catalytic hydrogenation), used in industry because it is cheaper.Using [H] to represent the reducing agent, four hydrogen atoms are added across the C≡N triple bond:
Step 1: CH3CH2Br + KCN → CH3CH2CN + KBr, giving propanenitrile.
Step 2: CH3CH2CN + 4[H] → CH3CH2CH2NH2, giving propylamine, with one more carbon than bromoethane.Comparing routes. Reduction of nitriles gives only the primary amine, with no further substitution, so there is no mixture to separate. The disadvantages are that it needs two steps from a halogenoalkane, KCN is highly toxic, and LiAlH4 is expensive.