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There are two methods used to synthesize α-amino acids: Strecker synthesis and Gabriel synthesis. This post covers Strecker synthesis.
The Strecker synthesis method uses aldehydes as a starting reactant. The structure of the aldehyde determines the side chain of the synthesized amino acid. The three key steps of the Strecker synthesis are (1) imine formation, (2) α-aminonitrile formation, and (3) hydrolysis. Figure 1 provides an overview of this process.
Step 1 of Strecker synthesis begins when weakly acidic ammonium protonates the aldehyde’s carbonyl .Delving into the details of this mechanism reveals to us that step 1 proceeds through protonation of the oxygen of the aldehyde’s carbonyl oxygen. This full positive charge attached to oxygen makes the carbonyl carbon an excellent electrophilic center that the remaining, nucleophilic ammonia will attack. This attack and the subsequent dehydration of the original reactant ultimately leads to imine formation.
By observing the structure of imines, you will find that they are structurally similar to carbonyls. In the same way, the imine carbon is also electrophilic because nitrogen is more electronegative than carbon. Therefore, the introduction of a molecule like potassium cyanide (KCN), results in a similar electrophile-nucleophile reaction. In this step of Strecker synthesis, cyanide (CN–) acts as a nucleophile and attacks the imine carbon, resulting in the formation of an α-aminonitrile.
Remember that hydrolysis is simply breaking apart a compound with water. In the instance of Strecker synthesis, we have an acidic aqueous solution that hydrolyzes our nitrile group, forming a carboxylic acid and, therefore, our amino acid product.
That covers the procedure and mechanism of Strecker amino acid synthesis. Our other MCAT posts cover the Gabriel amino acid synthesis as well as the basic structure and stereochemistry of amino acids.