Strecker Synthesis of Amino Acids – MCAT Biochemistry | MedSchoolCoach

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

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.

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. If instead you’re looking for information on Gabriel synthesis, please see this post.

 

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.

Overview of Strecker Synthesis - MCAT Biochemistry
Figure 1. Overview of 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.

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.

Steps 1 and 2 of Strecker Synthesis - MCAT Biochemistry
Figure 2. Steps 1 and 2 of 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.

Third Step of Strecker Synthesis - MCAT Biochemistry
Figure 3. Third Step of Strecker Synthesis

That covers the procedure and mechanism of Strecker amino acid synthesis. To learn about the other method of amino acid synthesis, Gabriel synthesis, please visit this post. Alternatively, if you’d like to learn more about amino acids, visit this post for information about their structure and stereochemistry or this post for information about how they are classified.

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