Carboxylic Acid Reactions: Study with Video Lessons, Practice Problems & Examples

Carboxylic acids, such as ethanoic acid, are weak acids that react with bases in acid-base reactions. When a base removes an H⁺ ion from a carboxylic acid, it forms a carboxylate anion, changing the suffix from "oic acid" to "oate" (e.g., ethanoic acid becomes ethanoate). This transformation highlights the relationship between acids and their conjugate bases, essential for understanding acid dissociation constants (K_a) and the behavior of weak acids in various chemical reactions.

Acid-Base Reactions Concept 1

In this video, we're going to take a look at carboxylic acid reactions. Now here we do this by examining acid-base reactions. We're going to say, under this type of reaction, a base reacts with a carboxylic acid, which represents a weak acid. If we take a look here, we have a carboxylic acid in the form of ethanoic acid. Here we're going to use some type of base, which is going to remove an H+ from the carboxylic acid, and in doing this creates its conjugate base form, which is a carboxylate anion. Now, here the carboxylate anion, it's ending differently. In its carboxylic acid form it's oic acid. In its carboxylate anion form, you change the oic acid to oate. So ethanoic acid becomes ethanoate. Right? So just remember, when we're talking about a carboxylic acid reaction, we're basically saying the carboxylic acid is an acid, we can tell from its name, and it's reacting with some type of base which removes an H+ from it, thereby creating a carboxylate anion product.

Carboxylic Acid Reactions Example 1

Predict the products formed from the following reaction between propanoic acid and ammonia. Alright. So here, this is our acid, ammonia, which is a neutral nitrogen compound we know is our weak base. Now, since both of these species are weak, we know that we have reversible arrows to show that an equilibrium has been established. If you don't remember this, make sure you go back and take a look at my videos, where we talk about reactions and equilibrium between acids and bases.

So we know following the Bronsted and Lowry definition of acids and bases, that the carboxylic acid gives away an H⁺ to the ammonia, it becomes a carboxylate anion in the process. The ammonia gains an H⁺ to become the ammonium ion, so it becomes NH₄⁺. So although we're talking about an organic molecule in terms of carboxylic acid, this is kind of familiar because it follows our Bronsted-Lowry definitions of acids and bases. The acid donates an H⁺, the base accepts the H⁺. And in that way, we make our two ions as products.

Name the carboxylate anion formed in the following reaction.

• A. 2-methylpentanoate
• B. 4-methylhexanoate
• C. 4-methylpentanoate
• D. 2-methylhexanoate

Here’s what students ask on this topic:

What is the general reaction mechanism for carboxylic acids with bases?

Carboxylic acids react with bases in an acid-base reaction where the base removes a proton (H⁺) from the carboxylic acid. This results in the formation of a carboxylate anion and water. The general reaction can be represented as:

${R-\mathrm{COOH}}_{}+{\mathrm{OH}}_{}-\to {R-\mathrm{COO}}_{}-+{H2O}_{}$

Here, R-COOH represents the carboxylic acid, and OH^- is the base. The product is a carboxylate anion (R-COO^-) and water (H₂O).

How does the suffix of a carboxylic acid change when it forms a carboxylate anion?

When a carboxylic acid forms a carboxylate anion, the suffix of its name changes from "-oic acid" to "-oate." For example, ethanoic acid (CH₃COOH) becomes ethanoate (CH₃COO^-) when it loses a proton (H⁺). This change in suffix indicates the transformation from the acid form to its conjugate base form.

What is the significance of the carboxylate anion in acid-base reactions?

The carboxylate anion is significant in acid-base reactions because it represents the conjugate base of a carboxylic acid. Understanding the formation of carboxylate anions helps in studying the acid dissociation constant (K_a) and the behavior of weak acids. The carboxylate anion is more stable than the carboxylic acid due to resonance stabilization, which is crucial for predicting the reactivity and strength of carboxylic acids in various chemical reactions.

Why are carboxylic acids considered weak acids?

Carboxylic acids are considered weak acids because they do not completely dissociate in water. Instead, they partially ionize, releasing a small proportion of H⁺ ions. The equilibrium between the carboxylic acid and its conjugate base (carboxylate anion) lies significantly towards the undissociated form. This partial dissociation is quantified by the acid dissociation constant (K_a), which is relatively low for weak acids like carboxylic acids.

What role does resonance play in the stability of carboxylate anions?

Resonance plays a crucial role in the stability of carboxylate anions. When a carboxylic acid loses a proton (H⁺), the resulting carboxylate anion has two resonance structures. These structures delocalize the negative charge over the two oxygen atoms, reducing the overall energy and increasing stability. This resonance stabilization is a key factor in the behavior and reactivity of carboxylate anions in chemical reactions.