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Chapter 32: Problem 732

It is thought that the first amino acids were synthesized from formaldehyde, hydrogen cyanide, ammonia and water in the primitive atmosphere. A possible synthesis involves a series of nucleophilic attacks and proton transfers. Propose a mechanism for the synthesis of glycine using the above mentioned compounds.

Short Answer

Expert verified
The synthesis of glycine can be proposed using formaldehyde, hydrogen cyanide, ammonia, and water through a series of nucleophilic attacks and proton transfers. Initially, hydrogen cyanide reacts with formaldehyde to form a cyanohydrin. A proton transfer occurs, followed by nucleophilic addition of ammonia, forming an amino nitrile intermediate. Finally, hydrolysis of the nitrile group results in the formation of glycine.

Step by step solution

01

Identify the nucleophiles and electrophiles in the reactants

To begin the synthesis, we need to identify the nucleophilic and electrophilic centers of the reactants. Formaldehyde (H2CO) contains a polar carbonyl group with a partially positive carbon atom, making it electrophilic. Hydrogen cyanide (HCN) has a polar C≡N triple bond with a negative charge on the carbon atom, making it nucleophilic. Ammonia (NH3) has a lone pair on the nitrogen atom, making it nucleophilic as well. Water (H2O), which will be the solvent for the reaction, acts as a source of protons (H+) and a proton acceptor (OH-) for proton transfers.
02

Nucleophilic addition of HCN to H2CO

The first step in the synthesis is the nucleophilic addition of hydrogen cyanide (HCN) to formaldehyde (H2CO). The nucleophilic carbon atom of HCN attacks the electrophilic carbon atom of the carbonyl group in H2CO, forming a new C-C bond. The negative charge on the oxygen is now neutralized by a proton transfer from HCN to the oxygen atom, resulting in the formation of an imine and releasing a hydroxide ion (OH-). The intermediate product will be a cyanohydrin with the formula OHC-CH2-CN.
03

Proton transfer and nucleophilic addition of ammonia

The next step involves a proton transfer from the hydroxyl group of cyanohydrin to the water molecule, resulting in the formation of an oxonium ion and a hydroxide ion (OH-). The oxonium ion can now react with ammonia (NH3) through a nucleophilic attack. The nitrogen of ammonia attacks the electrophilic carbon of the oxonium ion, forming a nitrogen-carbon bond and breaking the carbon-oxygen bond.
04

Proton transfer to form the amino nitrile intermediate

The oxygen atom with the negative charge acquires a proton from water, generating a hydroxide ion again and stabilizing the intermediate. At this stage, we have an amino nitrile intermediate with the formula NH2-CH2-CN.
05

Hydrolysis of the nitrile group to form glycine

The final step in the synthesis of glycine involves the hydrolysis of the nitrile group in the amino nitrile intermediate. The hydroxide ion attacks the carbon of the nitrile group, resulting in the formation of a new carbon-oxygen bond. This is followed by a proton transfer from water to the nitrogen atom, yielding cyanic acid, which further hydrolyses to produce CO2 and another hydroxide ion. The hydroxide ion then removes a proton from the ammonium ion, resulting in the formation of glycine NH2-CH2-COOH. The proposed mechanism used formaldehyde, hydrogen cyanide, ammonia, and water to synthesize glycine through several nucleophilic attacks and proton transfers.

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Most popular questions from this chapter

Suggest a way to separate a mixture of amino acids into three fractions: monoamino monocarboxylic acids, monoamino dicarboxylic acids (the acidic amino acids), and diamino monocarboxylic acids (the basic amino acids).

Many years before the Hofmann degradation of optically active a-phenylpropionamide was studied, the following observations were made: when the cyclopentane derivative, \(\mathrm{I}\), in which the \(-\mathrm{COOH}\) and \(-\mathrm{CONH}_{2}\) groups are cis to each other, was treated with hypobromite, compound II was obtained; compound II could be converted by heat into the amide III (called a lactam). What do these results show about the mechanism of the rearrangement? (Use models.)

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