The aldehyde and nitrile play distinct roles in the Strecker synthesis.

In the Strecker synthesis, an aldehyde or ketone is combined with ammonia and potassium cyanide to yield an α-amino nitrile. The aldehyde acts as the primary starting material for this reaction, supplying the necessary carbonyl group. This carbonyl group undergoes a nucleophilic addition with ammonia, resulting in the formation of an imine intermediate. Subsequently, this intermediate reacts with the cyanide ion to produce the α-amino nitrile.

Conversely, the nitrile can also serve as a starting material in the Strecker synthesis. In this scenario, the nitrile reacts with ammonia and either an aldehyde or ketone to generate the α-amino nitrile. Here, the nitrile provides the cyano group, which undergoes nucleophilic addition with ammonia to form an imine intermediate. This intermediate then reacts with the aldehyde or ketone to yield the α-amino nitrile.

In summary, the primary distinction between the aldehyde and nitrile in the Strecker synthesis lies in their roles within the reaction. The aldehyde is the starting material that provides the carbonyl group, while the nitrile serves as an alternative starting material that contributes the cyano group.