Question

Draw the structure of the carboxyl group, \(-\mathrm{COOH}\). Show how a molecule containing the carboxyl group behaves as an acid when dissolved in water.

Short answer

The carboxyl group, -COOH, consists of a carbon atom double-bonded to an oxygen atom and single-bonded to a hydroxyl group (-OH) with the structure: O=C-O-H. When a molecule containing a carboxyl group is dissolved in water, it behaves as an acid by releasing a hydrogen ion (H+) from the hydroxyl group: R-COOH + H2O -> R-COO⁻ + H3O⁺. This occurs because the acidic hydrogen atom in the carboxyl group can easily be released as a hydrogen ion, allowing the molecule to donate a proton and exhibit acidic behavior.

Step-by-step solution

Drawing the carboxyl group

A carboxyl group has the molecular formula -COOH and consists of a carbon atom double-bonded to an oxygen atom and single-bonded to a hydroxyl group (-OH). Here is the structure of the carboxyl group: O=C-O-H

Dissolving a molecule containing a carboxyl group in water

When a molecule containing a carboxyl group is dissolved in water, it can behave as an acid by releasing a hydrogen ion (H+) from the hydroxyl group. To visualize this, let's take a molecule containing a carboxyl group (R-COOH) and observe its interaction with water: R-COOH + H2O -> R-COO⁻ + H3O⁺ Here, R represents the rest of the molecule containing the carboxyl group.

Explaining the acidic behavior of the carboxyl group

The carboxyl group (COOH) has an acidic hydrogen atom bound to the oxygen of the hydroxyl group (-OH). This hydrogen atom can easily be released as a hydrogen ion (H+), which reacts with a water molecule to form a hydronium ion (H3O⁺). As a result, the carboxyl group donates a proton (H+), causing the solution to become acidic: R-COOH -> R-COO⁻ + H+ In this reaction, the carboxyl group behaves as a Bronsted-Lowry acid because it donates a proton to the water molecule. The water molecule, in turn, behaves as a Bronsted-Lowry base, as it accepts a proton from the carboxyl group. In conclusion, the carboxyl group (COOH) has an acidic hydrogen atom in its structure that can easily be released as a hydrogen ion (H+) when dissolved in water. This allows the molecule containing the carboxyl group to behave as an acid.

Key concepts

Acidic Behavior

Acidic behavior in chemistry refers to a substance's ability to donate protons (H+) when dissolved in a solution. The carboxyl group, which has the structure -COOH, is a prime example of a functional group that exhibits this behavior.
The carbon atom in a carboxyl group is bound to a double-bonded oxygen and a hydroxyl group (-OH). The hydrogen in the hydroxyl part is what makes the carboxyl group acidic. When a carboxyl group is in water, it tends to release the hydrogen ion (H+) from the hydroxyl group. This release leads to the formation of a carboxylate ion (R-COO⁻) and a hydronium ion (H3O⁺).
The easy release of H+ ions is what makes a solution acidic. In the case of carboxyl groups, this often means that any molecule containing them can potentially lower the pH of a solution, depending on the concentration of the carboxyl groups present.

Bronsted-Lowry Acid

The concept of acids and bases can be understood using the Bronsted-Lowry theory. According to this model, an acid is a substance that donates protons, while a base is one that accepts protons. The carboxyl group fits perfectly into this definition.
In the reaction where a molecule containing a carboxyl group \( ext{(R-COOH)} \) interacts with water, the carboxyl group donates a proton (H+) to the water molecule. This proton donation confirms that the carboxyl group is acting as a Bronsted-Lowry acid.
When the carboxyl group donates its proton, it leaves behind a negatively charged carboxylate ion \( ext{(R-COO}^- ext{)} \). This reaction highlights the versatility of the Bronsted-Lowry definition, which embraces a wide range of acidic species, including those like the carboxyl group that we commonly encounter in organic chemistry.

Molecular Structure

The molecular structure of a carboxyl group is crucial for understanding its properties and reactions. The carboxyl group has the formula -COOH and features a carbon atom double-bonded to an oxygen and single bonded to a hydroxyl group (-OH).
The specific arrangement of atoms in the carboxyl group makes it polar, with the oxygen atoms having partial negative charges due to their electronegativity. This polarity aids in the release of the hydrogen ion when the molecule is dissolved in water.
In a broader molecular context, the carboxyl group contributes to the overall acidity and reactivity of the organic molecules they are part of. Often found in organic acids such as acetic acid and citric acid, the carboxyl group is a common and important functional group in biochemistry and organic chemistry.

Hydronium Ion

The hydronium ion \( ext{(H}_3 ext{O}^+ ext{)} \) is an essential player in the behavior of acids in aqueous solutions. When a carboxyl group donates a hydrogen ion (H+) to the water, a hydronium ion is formed.
The formation of the hydronium ion occurs as follows: the donated proton from the carboxyl group attaches to a water molecule \( ext{(H}_2 ext{O)} \), resulting in the creation of a hydronium ion.

• This reaction increases the concentration of H+ in the solution.
• As the H+ concentration rises, the pH of the solution decreases, indicating higher acidity.

The hydronium ion plays a significant role in the acid-base balance of solutions. It acts as an indicator of acidity and helps determine the extent of an acid's strength. The presence of hydronium ions is thus a crucial element in assessing the behavior of substances that act as acids, such as those containing the carboxyl group.