Acid-Base Properties of Carboxylic Acids - SS3 Chemistry Lesson Note

Acid-base properties are central to the behaviour of carboxylic acids. Carboxylic acids are weak acids, meaning they partially dissociate in water, releasing a hydrogen ion (H+) to form carboxylate ions (RCOO-). The acid-base properties of carboxylic acids have important implications in various chemical and biological processes. Let's explore the acid-base properties of carboxylic acids in detail:

The ionisation of Carboxylic Acids:

Carboxylic acids contain the carboxyl group (-COOH), which is composed of a carbonyl group (C=O) and a hydroxyl group (OH) attached to the same carbon atom. In aqueous solutions, carboxylic acids undergo ionisation to release a hydrogen ion (H+) and form the carboxylate ion (RCOO-):

RCOOH (Carboxylic Acid) ⇌ RCOO- (Carboxylate Ion) + H+ (Hydrogen Ion)

The ionisation of carboxylic acids is a reversible reaction, with the extent of ionisation depending on the concentration of the acid and the pH of the solution.

Weak Acid Behavior:

Carboxylic acids are weak acids compared to strong mineral acids like hydrochloric acid (HCl) or sulfuric acid (H2SO4). This means that in dilute solutions, only a small fraction of carboxylic acid molecules dissociate into carboxylate ions and hydrogen ions. The equilibrium constant for the ionisation of carboxylic acids is denoted as Ka, and it represents the acid dissociation constant.

Conjugate Base - Carboxylate Ion:

The carboxylate ion (RCOO-) formed after the loss of a hydrogen ion is the conjugate base of the carboxylic acid. It is more stable than carboxylic acid due to resonance stabilisation. In the carboxylate ion, the negative charge is delocalized over both oxygen atoms (carbonyl and hydroxyl), leading to greater stability.

Effect of Substituents on Acid Strength:

The acidity of carboxylic acids can be influenced by the nature of the substituents attached to the carboxyl group. Electron-withdrawing groups, such as halogens or nitro groups, increase the acidity of carboxylic acids by stabilising the carboxylate ion through inductive and resonance effects. Conversely, electron-donating groups, like alkyl groups, decrease the acidity of carboxylic acids by destabilising the carboxylate ion.

Amphiprotic Nature:

Carboxylic acids can act as both acids and bases. While they are weak acids, they can also behave as weak bases by accepting a proton. When a carboxylic acid accepts a proton, it forms a carboxylate ion. This property makes carboxylic acids amphiprotic, meaning they can donate or accept protons depending on the conditions.

Salt Formation:

Carboxylic acids readily react with strong bases to form carboxylate salts. For example, the reaction of acetic acid (CH3COOH) with sodium hydroxide (NaOH) yields sodium acetate (CH3COONa):

CH3COOH + NaOH → CH3COONa + H2O

These carboxylate salts are often used in various applications, including as food additives and buffering agents.

The acid-base properties of carboxylic acids are fundamental to their reactivity and behaviour in various chemical reactions and biological processes. Understanding these properties is essential in organic chemistry, biochemistry, and medicinal chemistry, as carboxylic acids are crucial components of many biomolecules and pharmaceuticals.