Strecker synthesis

Meaning: The Strecker synthesis builds an α-amino acid in two stages. Ammonia adds to the aldehyde carbonyl and loses water to give an imine (protonated, an iminium ion). Cyanide then attacks the electrophilic C=N carbon, forming an α-aminonitrile. Aqueous acid hydrolyzes the nitrile (−C≡N) to a carboxylic acid. The aldehyde carbon becomes the α-carbon, the aldehyde R group becomes the side chain, and the cyanide carbon becomes the COOH carbon.

Equation: RCHO + NH₃ + HCN → RCH(NH₂)C≡N + H₂O, then RCH(NH₂)C≡N + 2H₂O + 2H⁺ → RCH(NH₃⁺)COOH + NH₄⁺. R is the side chain (an alkyl or aryl group); no SI units apply because this is a reaction scheme.

On the MCAT: You may be given a target amino acid and asked for the starting aldehyde (formaldehyde gives glycine, acetaldehyde gives alanine), or asked to order the steps or name the intermediate. You may also be asked about optical activity. Expect comparison with the Gabriel synthesis or with α-bromination followed by amination, which also give amino acids.

Common confusion: Students expect a single enantiomer, as in biological amino acids. Cyanide attacks the planar imine carbon from either face, and no chiral reagent is present, so the product is a racemic mixture with no net optical rotation. Glycine is achiral, so it is the exception. Also, the nitrogen comes from ammonia, not from the cyanide.