Strecker Synthesis of Amino Acids

MCAT Biochemistry Chapter 1 - Section 3.1 - Amino Acids - Strecker Synthesis of Amino Acids

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Sample MCAT Question - Strecker Synthesis of Amino Acids

What is the correct sequence of steps for Strecker synthesis?

I. Hydrolysis

II. Imine Formation I

II. ɑ-aminonitrile Formation

a) I, II, III

b) II, I, III

c) II, III, I

d) III, II, I

Click to Reveal the Answer

C is correct. II, III, I.

The first step of the Strecker synthesis is imine formation, where the aldehyde is converted to an imine by reacting it with a weak acid (NH₄Cl). The second step is ɑ-aminonitrile formation, where a strong nucleophile (CN^–) attacks the imine to form an ɑ-aminonitrile. The final step is hydrolysis, which protonates the amino group and hydrolyzes the nitrile group into a carboxyl group.

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Strecker Synthesis of Amino Acids

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.

Strecker Synthesis Step 1: Imine Formation

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.

Strecker Synthesis Step 2: α-Aminonitrile 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.

Strecker Synthesis Step 3: Hydrolysis

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.