Organic Chemistry: Aldehydes, Ketones, and Carboxylic Acids - SS3 Chemistry Past Questions and Answers - page 4

When a carboxylic acid reacts with an amine, an amidation reaction occurs, forming a carboxamide and water. The reaction involves the replacement of the -OH group of the carboxylic acid with the amino group (-NH2) of the amine.

Note: Option "b" (Alcohol and carboxylic acid) represents an esterification reaction, not the formation of a carboxylic acid. Option "d" (Aldehyde and alcohol) represents an aldol condensation reaction, not the formation of a carboxylic acid.

Acid-Base Properties of Carboxylic Acids:

Carboxylic acids exhibit unique acid-base properties due to the presence of the carboxyl functional group (COOH). In aqueous solutions, they can act as both acids and bases, making them amphoteric. The carboxylic acid functional group consists of a carbonyl group (C=O) and a hydroxyl group (-OH). The carbonyl group is electron-withdrawing, leading to an increase in the acidity of the hydroxyl group's hydrogen (H). This hydrogen is more acidic than typical alcohol (-OH) due to resonance effects.

1.    Acidity of Carboxylic Acids:

Carboxylic acids donate a proton (H+) to a base to form a carboxylate ion (RCOO-). The ionisation of carboxylic acid can be represented as follows:

RCOOH ⇌ RCOO- + H+

The equilibrium lies to the left in dilute solutions due to the weak acid nature of carboxylic acids. However, they are stronger acids than other organic acids, such as alcohols and phenols, because of the resonance stabilisation of the carboxylate ion.

Factors Influencing Acidity:

The acidity of carboxylic acids is influenced by the following factors:

a.    Substituent Effects: Electron-withdrawing groups attached to the carboxyl group increase acidity by stabilising the carboxylate ion. Electron-donating groups decrease acidity.

b.    Inductive Effects: Electronegative atoms adjacent to the carboxyl group can withdraw electron density, enhancing acidity.

c.     Resonance Stabilisation: Resonance structures involving delocalization of the negative charge over the two oxygen atoms stabilise the carboxylate ion, increasing acidity.

2. Basicity of Carboxylic Acids:

Carboxylic acids can also act as weak bases by accepting a proton from a strong acid. The ionisation of the carboxylic acid as a base can be represented as follows:

RCOOH + HCl ⇌ RCOOH2+ + Cl-

In this case, the carboxylic acid acts as a proton acceptor and forms the carboxylic acid cation (RCOOH 2) in the presence of a stronger acid (HCl).

Examples of Acid-Base Behavior:

1.    Acetic Acid (CH3COOH): In an aqueous solution, acetic acid donates a proton to water, forming the acetate ion (CH3COO-) and a hydronium ion (H3O+):

CH3COOH + H2O ⇌ CH3COO- + H3O+

2.    Formic Acid (HCOOH): Formic acid is a stronger acid than acetic acid due to the electron-withdrawing effect of the hydrogen attached to the carbonyl group. It ionises more extensively in water:

HCOOH ⇌ HCOO- + H+

In conclusion, carboxylic acids exhibit distinct acid-base properties, acting as weak acids by donating a proton and as weak bases by accepting a proton. The electron-withdrawing nature of the carbonyl group and resonance stabilisation of the carboxylate ion contribute to their relatively higher acidity compared to other organic acids. Understanding the acid-base properties of carboxylic acids is essential for organic chemists, as these reactions play a vital role in various chemical processes and biological systems.