Strecker synthesis: practice and mechanism
With NH₃ the aldehyde becomes an imine, cyanide adds, and hydrolysis turns C≡N into COOH. In the end the former carbonyl carbon carries NH₂ and COOH: an α-amino acid.
Example
The aldehyde and ammonia form an imine, cyanide adds to it: an α-aminonitrile forms. Acidic hydrolysis turns the nitrile into a carboxylic acid. Result: an α-amino acid (racemic) whose side chain comes from the aldehyde.
Mechanism step by step
- 1
Ammonia adds to the aldehyde; after loss of water an imine (or iminium ion) forms.
- 2
Cyanide attacks the imine carbon: an α-aminonitrile forms.
- 3
Acidic hydrolysis converts C≡N via the amide into COOH: the α-amino acid forms (racemic).
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- That is the α-hydroxy acid from the cyanohydrin (without ammonia). With NH₃ the imine forms first, so in the end there is an NH₂ group instead of OH.
- The α-aminonitrile is only the intermediate after step 1. Only the acidic hydrolysis in step 2 turns C≡N into the carboxylic acid.
Often confused with: Cyanohydrin formation · Reductive amination · Imine formation
Frequently asked questions
Which aldehyde gives which amino acid?
The group on the aldehyde becomes the side chain: acetaldehyde → alanine, 2-methylpropanal → valine, phenylacetaldehyde → phenylalanine. Formaldehyde gives glycine.
Why is a racemate formed?
The imine is planar and cyanide attacks equally from both faces. Pure L-amino acids need chiral catalysts or a resolution.