Question

Although carboxylic acids and alcohols both contain an $-\mathrm{OH}$ group, one is acidic in water and the other is not. Explain the difference.

Short answer

The difference in acidity between carboxylic acids and alcohols stems from their structural differences. Carboxylic acids contain a carboxyl group (R-COOH), which undergoes a reaction in water to form hydronium ions (H3O+) and carboxylate ions (RCOO-), making the solution acidic. In contrast, alcohols have a hydroxyl group (R-OH) connected to a carbon atom and do not dissociate in water or lose a proton, thus not making the solution acidic. The presence of the carbonyl group (C=O) in carboxylic acids allows for resonance stabilization of the carboxylate ion, making the hydroxyl group more acidic than the one in alcohols.

Step-by-step solution

Identify the structure of carboxylic acids and alcohols.

Carboxylic acids contain a carboxyl group, which consists of one carbonyl group (C=O) connected to a hydroxyl group (-OH). Their general formula is R-COOH, where R represents an alkyl group. Alcohols, on the other hand, have a hydroxyl group (-OH) connected directly to a carbon atom in the molecule. The general formula of an alcohol is R-OH, where R represents an alkyl group.

Understand the behavior of carboxylic acids in water

When carboxylic acids dissolve in water, they undergo a reaction to form a hydronium ion (H3O+) and a carboxylate ion (RCOO-). This reaction is an acid-base reaction, where the carboxylic acid donates a proton (H+) to a water molecule. The equilibrium lies to the left, so only a small amount of the carboxylic acid will ionize in water. Nonetheless, the presence of hydronium ions makes the solution acidic. The reaction is as follows: $RCOOH+H_{2}O\rightleftharpoons RCO{O}^{-}+H_3{O}^{+}$

Understand the behavior of alcohols in water

When alcohols dissolve in water, they do not dissociate or give away a proton. Because the hydroxyl group is attached directly to a carbon atom, which is less electronegative, it is more difficult for the alcohol to lose a proton in an acid-base reaction. As a result, alcohols do not increase the concentration of hydronium ions (H3O+) in the solution, and thus, the solution is not acidic.

Explain the difference in acidity

The difference in acidity between carboxylic acids and alcohols stems from their structural differences. In carboxylic acids, the presence of the carbonyl group (C=O) makes the adjacent hydroxyl group more acidic due to resonance stabilization of the resulting carboxylate ion. In alcohols, no such resonance stabilization occurs. As a result, the hydroxyl group in carboxylic acids is more acidic than the one in alcohols, leading to the observed difference in acidity in water.

Key concepts

Acid-Base Reactions

Acid-base reactions are a fundamental concept in chemistry, where acids donate protons (H⁺ ions) and bases accept them. Carboxylic acids like acetic acid are known for their acidic behavior in water. When dissolved, they engage in acid-base reactions by donating a proton to water, forming hydronium ions.
This process is represented by the chemical equilibrium:

• Carboxylic Acid: $$\text{RCOOH}+{\text{H}}_2\text{O}\leftrightarrows {\text{RCOO}}^{-}+{\text{H}}_3{\text{O}}^{+}$$

The reaction above highlights the essence of the acid's behavior: the carboxylic acid donates a proton, forming a negatively charged carboxylate ion.
Although the equilibrium lies mostly to the left, where the acid remains intact, enough hydronium ions form to make the solution acidic. This reaction sets carboxylic acids apart from alcohols, which do not typically participate in similar reactions under standard conditions, making them non-acidic in nature.

Hydronium Ion

The hydronium ion is a significant player in the acidity of a solution. When carboxylic acids dissolve in water, they donate protons, resulting in the formation of hydronium ions ${\text{H}}_3{\text{O}}^{+}$.
These ions are crucial because they directly determine the acidity of a solution. Higher concentrations of hydronium ions equate to lower pH levels, indicating more acidic conditions.

• Formation: Originates when a hydrogen ion ${\text{H}}^{+}$ interacts with a water molecule.
• Significance: The increase or decrease in hydronium ion concentration significantly influences the acidic or basic properties of a solution.
• Example: Presence in the equation $\text{RCOOH}+{\text{H}}_2\text{O}\rightleftharpoons {\text{RCOO}}^{-}+{\text{H}}_3{\text{O}}^{+}$

Understanding the role of hydronium ions is essential for grasping how certain substances affect the pH of their environment, which is a fundamental observation when comparing carboxylic acids to other organic compounds like alcohols.

Resonance Stabilization

Resonance stabilization explains why carboxylic acids are more acidic than alcohols. In a carboxylic acid, when a proton is donated, a carboxylate ion forms. This ion benefits from resonance stabilization, where the negative charge is delocalized over the oxygen atoms.

• Resonance: A mechanism allowing molecules like carboxylate ions to have multiple structures, each contributing to the overall stability.
• Stabilization Effects: Because the charge is spread out, the ion is more stable than the individual, non-resonant structure.

This structural property decreases the energy of the resulting ion, making the loss of a proton more favorable—thus enhancing the acidity of carboxylic acids. In contrast, alcohols lack this feature, as their negative charge remains localized on the oxygen atom when they hypothetically lose a proton, making them less stable and less acidic. Hence, resonance stabilization plays a vital role in distinguishing the acidity properties of carboxylic acids from alcohols.