Nitriles and hydroxynitriles — common mistakes

For exam purposes, when asked for the reagents for the reduction of a nitrile, stating 'LiAlH₄ in dry ether' is sufficient. The '[H]' in the equation represents the hydrogen atoms supplied by the reducing agent. Remember this reaction also extends the carbon chain and produces a primary amine.

When naming hydroxynitriles, always count the carbon from the nitrile group as part of the main chain. For example, reacting propanal (3 carbons) with HCN adds a carbon, so the main chain has 4 carbons. The product is 2-hydroxybutanenitrile, not 2-hydroxypropanenitrile.

Using an ethanolic solvent is crucial for two reasons. Firstly, water is a nucleophile itself and would compete with the cyanide ion, leading to the formation of an alcohol (hydrolysis). Secondly, the cyanide ion can also act as a base. In the presence of water, it can cause an elimination reaction, producing an alkene instead of the desired nitrile. Ethanol is a less polar solvent and a weaker nucleophile, favouring the substitution pathway.

A racemic mixture (or racemate) is an equimolar (50:50) mixture of two enantiomers (non-superimposable mirror-image molecules). It is formed in this reaction because the starting aldehyde or unsymmetrical ketone has a planar carbonyl group. The incoming cyanide nucleophile has an equal probability of attacking from above the plane or below the plane, leading to the formation of both enantiomers in equal amounts. As a result, the product mixture is not optically active.

Yes, hydrogen cyanide is an extremely toxic and volatile liquid (boiling point 26°C) that is lethal if inhaled or ingested. For safety reasons, it is almost never used directly in a laboratory setting. Instead, it is generated 'in situ' (within the reaction flask) by reacting a stable salt like potassium cyanide (KCN) or sodium cyanide (NaCN) with a dilute acid.