Problem 74 - Chemistry and Chemical Reactivity

Question

Benzoic acid occurs in many berries. When humans eat berries, benzoic acid is converted to hippuric acid in the body by reaction with the amino acid glycine, \(\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{CO}_{2} \mathrm{H} .\) Draw the structure of hippuric acid, recognizing that it is an amide formed by reaction of the carboxylic acid group of benzoic acid and the amino group of glycine. Why is hippuric acid referred to as an acid?

Step-by-Step Solution

Answer

Hippuric acid is an amide with a carboxylic acid group from glycine, making it an acid.

Step 1: Analyzing Benzoic Acid and Glycine

Benzoic acid has a benzene ring attached to a carboxylic acid group, giving it the formula: \( \text{C}_6\text{H}_5\text{COOH} \). Glycine, the simplest amino acid, has the structure: \( \text{H}_2\text{NCH}_2\text{CO}_2\text{H} \). The reaction occurs between the carboxylic acid group of benzoic acid and the amino group of glycine.

Step 2: Formation of Amide Bond

The carboxylic acid group from benzoic acid reacts with the amino group of glycine to form an amide bond. This involves the removal of a water molecule, combining the structures: \([\text{C}_6\text{H}_5\text{CO}]-[\text{NHCH}_2\text{CO}_2\text{H}]\), which is the backbone of hippuric acid.

Step 3: Completing the Structure of Hippuric Acid

Connect the benzene ring from benzoic acid with the amide link to glycine's structure. Therefore, hippuric acid is:\[ \text{C}_6\text{H}_5\text{CONHCH}_2\text{CO}_2\text{H} \], showing the benzene ring connected to a carbonyl group which is linked to the amino group, with the glycine structure completing it.

Step 4: Understanding Why It's an Acid

Hippuric acid is referred to as an acid because it contains a carboxylic acid group (\( \text{CO}_2\text{H} \)) at the end of the glycine segment. This group can donate a proton (\( \text{H}^+ \)), classifying it as an acid by definition.

Key Concepts

Hippuric Acid

Hippuric acid is an interesting compound that forms in the human body when we consume benzoic acid, found in various berries, and it reacts with glycine, a simple amino acid. This transformation highlights a fascinating biochemical process where the components interact to produce a new molecule.
Hippuric acid combines the aromatic benzene ring of benzoic acid and the structural simplicity of glycine. This process effectively connects the aromatic world of benzoic acid with the basic building blocks of proteins and enzymes present in glycine.

• Benzoic acid has the formula: \( \text{C}_6\text{H}_5\text{COOH} \)
• Glycine is structured as: \( \text{H}_2\text{NCH}_2\text{CO}_2\text{H} \)

This combination results in hippuric acid, which can be structurally represented as \( \text{C}_6\text{H}_5\text{CONHCH}_2\text{CO}_2\text{H} \). This molecule is significant as it plays a role in detoxification pathways within the human body, helping to process and eliminate some organic compounds that might otherwise be harmful.

Amide Bond

An amide bond is a key feature in many biological molecules, and it's the link that forms between two functional groups: a carboxylic acid and an amino group. In the case of hippuric acid, this bond is crucial in creating the connection between benzoic acid and glycine.
The formation of an amide bond occurs through a dehydration synthesis reaction. Essentially, a water molecule (\( \text{H}_2\text{O}\)) is removed to join the carboxylic acid group (\( \text{COOH} \)) from benzoic acid with the amino group (\( \text{NH}_2 \)) of glycine. This type of bond provides stability and specific properties to the resultant compound.

• Amide bonds are stronger than single covalent bonds due to their partial double-bond character.
• They are planarity-inducing, which restricts the possible conformations of molecules.

This specific linkage in hippuric acid not only stabilizes the compound but also gives it certain physical and chemical characteristics that would not be present were these two entities unlinked.

Carboxylic Acid Group

The carboxylic acid group is a fundamental organic functional group characterized by its \( \text{COOH} \) formula. This group is often found in a variety of organic compounds, including benzoic acid and hippuric acid. It is crucial in many chemical reactions due to its ability to donate a hydrogen ion (proton, \( \text{H}^+ \)).
This proton donator property classifies any compound containing it, such as hippuric acid, as an acid. The carboxyl group's presence at the end of the glycine segment in hippuric acid is what allows it to retain its acidic property.

• The reactivity of this group stems from the resonance stability of the carboxylate ion formed after proton donation.
• It's responsible for the solubility in water and reactivity with bases.

In hippuric acid, the carboxylic acid group not only defines its chemical reactivity but also plays a significant role in determining its biological functionality, making it an important entity in biochemical pathways and detoxification processes in the body.