Reduction of Nitriles

Nitriles are organic compounds where a cyanide group (-CN) is attached to the alkyl group. They are colourless and have a particular smell. They can be present in solid or liquid phase. Nitriles can be manufactured via various processes one of which being the heating of carboxylic acids with ammonia and a catalyst. Nitriles undergo reduction in presence of lithium aluminium hydride in presence of some catalyst to form either primary or tertiary amines. Nitriles are used for various other things like production of nitrile gloves, it can also be used as an antidiabetic drug. They are also useful in automotive systems, aircraft systems, etc.

Nitriles and their properties

• The nitriles are colourless solids or liquids and have a characteristic odour.
• Their boiling point ranges from 82-118℃.
• They show strong dipole movements.
• They also have van der waals force of dispersion between the molecules of nitriles.
• Nitriles are highly electronegative and show high polarity.
• Their solubility in water is high but the solubility decreases with increase in the chain length.
• In nitriles the C-N bond shows linear geometry with sp hybridization of the carbon atom.
• The C-N bond is 1.16 Å and it is a triple bond.
• Nitriles take part in various chemical reactions like hydrolysis, alkylation and reduction.
• Nitrile hydrolysis in presence of acid or base leads to formation of carboxamides and carboxylic acids.
• Alkylation of nitriles forms nitrile anions.
• Nitrile reduction forms primary and secondary amines.

Nitrile Reduction

Nitrile group is reduced to aldehyde or amine with the help of a catalyst by 3 different mechanisms.

• Hydrogenation- Hydrogenation of nitriles leads to formation of primary amines and is the most economical method for production of primary amines. Group 10 elements nickel, palladium etc acts as a catalyst for such reactions. In commercial manufacturing, this process is used for production of hexamethylenediamine and adiponitrile, which is a nylon 66 precursor. Under specific reaction conditions imines which are formed as an intermediate can be attacked by the products of amine reaction to form tertiary and primary amine. Generally these reactions have an intermediate enamine. The catalyst choice plays a very important role in formation of primary amine.
• Reduction to aldehydes- in this process reducing agents are used like lithium aluminium hydride, diborane etc. Nitriles can be reduced to aldehyde via Stephen aldehyde synthesis method. Tin(II) chloride and HCl is used for this reaction. Aldehyde can be formed using a hydrogen donor followed by in-situ hydrolysis of an imine. Formic acid or metal hydrides are very useful reagents.
• Diisopropylaminoborane with lithium borohydride is used to reduce aliphatic and aromatic nitriles.
• Nitriles can also be reduced by electrochemical methods. 
• Aliphatic and Aromatic nitriles can be reduced to alcohol with reductive deamination. The reaction takes place in the presence of paraformaldehyde, aqueous solvent and a catalyst.

Reduction of nitrile to primary amine

• It is a nucleophilic addition reaction where hydride ions initially undergo nucleophilic addition to form an imine salt.
• The imine salt then again undergoes nucleophilic addition with the help of AlH3 forming a highly reactive amine derivative.
• This reactive amine reacts with water to form primary amine.
• LiAlH2 can also be used in this reaction for reducing the nitrile.

Reduction of nitriles to aldehyde

• DIBAL is a mild reducing agent which can reduce nitriles to aldehyde.
• First step is the hydride addition to the triple bond between C-N and leads to formation of iminium anion.
• The iminium anion is then hydrolyzed to form aldehyde.
• Aromatic nitriles are reduced to aldehydes with the help of calcium hypophosphite and nickel(II) complex base and water ethanol mixture.

Conclusion

Nitriles are organic compounds, where a cyanide group CN is attached to the alkyl group. Nitriles can be manufactured via various processes one of which being the heating of carboxylic acids with ammonia and a catalyst. Nitriles undergo reduction with the help of various catalysts to form primary and secondary amines, aldehydes and alcohol. The nitriles are colourless solids or liquids and have a characteristic odour. Their boiling point ranges from 82-118℃. Nitriles are highly electronegative and show high polarity. Their solubility in water is high but the solubility decreases with increase in the chain length. In nitriles the N-C-C bond shows linear geometry with sp hybridization of the carbon atom. The C-N bond is 1.16 Å and it is a triple bond. Nitriles take part in various chemical reactions like hydrolysis, alkylation and reduction. Nitrile hydrolysis in presence of acid or base leads to formation of carboxamides and carboxylic acids. Nitriles are used for various other things like production of nitrile gloves, it can also be used as an antidiabetic drug, in automotive systems, aircraft systems, etc.