Reactions of Amino Acids: Hydrogenolysis - Online Tutor, Practice Problems & Exam Prep

Hydrogenolysis is a reaction involving the cleavage of bonds in amino acids, specifically through catalytic hydrogenation. Benzyl esters yield an amino acid and toluene by breaking the carbon-oxygen bond, facilitated by metal catalysts like palladium. In the case of n-benzyl oxycarbonyl derivatives, the reaction produces toluene and an unstable carbamic acid, which quickly decarboxylates to form an amino acid. This process highlights the importance of understanding bond cleavage and the formation of amino acids in organic synthesis.

Reactions of Amino Acids: Hydrogenolysis Concept 1

Everyone, so here we're going to take a look at hydrogenolysis as one of the reactions of amino acids. We're going to say benzyl esters and N-benzyl oxycarbonyl derivatives can be broken down by hydrogenolysis. Here we have the cleavage of a bond through catalytic hydrogenation. So we know about catalytic hydrogenation.

Here we're going to see the same reagents being used, but it's going to apply differently to give us new types of products. So we're going to say here that hydrogenolysis of benzyl esters, we're going to say the CO bond is cleaved to produce an amino acid and toluene. So, if we take a look here, we have our benzoyl ester and what's going to happen here is we're going to cleave this bond right here and since this is catalytic hydrogenation, that means we have H₂ over a metal catalyst, we have, as options, palladium or nickel or platinum. We tend to see H₂ and palladium as the matchup most of the time, but, again, the other two metal catalysts can also be used. So we cut through that carbon-oxygen bond.

We add an H here to this oxygen to make an amino acid. And remember, this is a CH₂. It also gains an H as well making it a CH₃. So what you have here is benzene with a methyl group or what we call toluene. So, again, we know about catalytic hydrogenation.

We've seen in the past that it adds hydrogens by syn addition to pi bonds. But here we don't have pi bonds, but it's still doing the same type of idea. It's adding hydrogens. Here though, it's adding the hydrogen to oxygen and to the carbon. Each one would have to sever the link they have to one another though for each of them to gain a hydrogen apiece.

Right? So that's basically what's happening. We're creating an amino acid and toluene as products from this process.

Reactions of Amino Acids: Hydrogenolysis Example 1

Here we're told to write a reaction of hydrogenolysis of Phenylalanine Benzoyl Ester. Right? So in this process, remember, we're using catalytic hydrogenation, which is just H₂ with our metal catalyst, in this case, palladium. We're going to cleave the CO bond here. And remember, what happens when we cut that bond, the oxygen gains an H to create an amino acid and the CH₂ gains an H, becoming a CH₃ group.

So what we're going to have here is we still have everything else here the same. Here's the carboxylic acid we just created, plus this CH₂ gains an H to become a CH₃, which is still connected to benzene. So we just created methylbenzene or toluene. So again, in this process, we're changing this beginning reactant into an amino acid plus toluene.

Reactions of Amino Acids: Hydrogenolysis Example 2

In this example, we're told to provide all products including the intermediates of our hydrogenolysis of N-Benzoyl oxycarbonyl aspartame. Alright. So what's going to happen first is we're going to do H2 over our metal catalyst. So we're doing catalytic hydrogenation. Remember that's going to target the carbon-oxygen bond here, cleaving it.

This CH2 gains an H, this oxygen gains an H. So what we're going to get initially is toluene plus a carboxylic acid. So, we're going to have our carboxylic acid still connected to NH, connected to the alpha carbon, carboxylate group, and then this R group here. Now remember, this carbamic acid form is unstable and will undergo decarboxylation. So we're going to lose CO2.

We're going to lose this CO2 right here. So at the end, what we're going to have is this carbamic acid which is unstable, changing into an amino acid. So we have NH2 over here, still connected to the alpha carbon. We're going to have a carboxylic acid group here, and on the alpha carbon, we still have this R group. So this would be all of our products and our intermediates from this specific example question.

Here’s what students ask on this topic:

What is hydrogenolysis in the context of amino acids?

Hydrogenolysis in the context of amino acids refers to the cleavage of bonds through catalytic hydrogenation. This process involves breaking specific bonds, such as the carbon-oxygen bond in benzyl esters, using hydrogen gas (H₂) and a metal catalyst like palladium, nickel, or platinum. The reaction typically produces an amino acid and toluene. In the case of n-benzyl oxycarbonyl derivatives, hydrogenolysis results in toluene and an unstable carbamic acid, which quickly decarboxylates to form an amino acid. This reaction is crucial for understanding bond cleavage and amino acid formation in organic synthesis.

How does hydrogenolysis of benzyl esters produce amino acids?

Hydrogenolysis of benzyl esters involves the cleavage of the carbon-oxygen bond through catalytic hydrogenation. Using hydrogen gas (H₂) and a metal catalyst like palladium, the C-O bond in the benzyl ester is broken. The oxygen gains a hydrogen atom to form a carboxylic acid group, while the carbon gains a hydrogen atom to form toluene (C₆H₅CH₃). The overall reaction produces an amino acid and toluene as the final products. This process highlights the role of catalytic hydrogenation in breaking specific bonds to yield desired products in organic chemistry.

What are the products of hydrogenolysis of n-benzyl oxycarbonyl derivatives?

The hydrogenolysis of n-benzyl oxycarbonyl derivatives (abbreviated as N-Cbz) produces toluene and an unstable carbamic acid. The reaction involves the cleavage of the carbon-oxygen bond using hydrogen gas (H₂) and a metal catalyst like palladium. The carbamic acid formed is unstable and undergoes immediate decarboxylation, losing CO₂ to form an amino acid. The overall products of this reaction are toluene (C₆H₅CH₃) and an amino acid, demonstrating the importance of understanding bond cleavage and decarboxylation in organic synthesis.

What role do metal catalysts play in the hydrogenolysis of amino acids?

Metal catalysts such as palladium, nickel, or platinum play a crucial role in the hydrogenolysis of amino acids by facilitating the cleavage of specific bonds through catalytic hydrogenation. These catalysts provide a surface for the hydrogen gas (H₂) to adsorb and dissociate into atomic hydrogen, which then interacts with the substrate (e.g., benzyl esters or n-benzyl oxycarbonyl derivatives). The metal catalysts help break the carbon-oxygen bonds, leading to the formation of amino acids and other products like toluene. The choice of catalyst can influence the efficiency and selectivity of the reaction.

Why is carbamic acid unstable in the hydrogenolysis of n-benzyl oxycarbonyl derivatives?

Carbamic acid is unstable in the hydrogenolysis of n-benzyl oxycarbonyl derivatives because it readily undergoes decarboxylation. Upon formation, carbamic acid (R-NH-COOH) loses CO₂ almost immediately, resulting in the formation of an amino acid. This instability is due to the inherent nature of carbamic acid to decompose, driven by the release of CO₂, which is a thermodynamically favorable process. The rapid decarboxylation ensures that the final product of the reaction is an amino acid, highlighting the transient existence of carbamic acid in this context.