Organic Chemistry III: Aldehydes, Ketones, and Carboxylic Acids - SS2 Chemistry Past Questions and Answers - page 4

Carboxylic acids are organic compounds that contain a carboxyl group (-COOH) attached to a carbon atom. They exhibit distinct properties and characteristics compared to other organic compounds. Some key features of carboxylic acids include:

1.    Acidity: Carboxylic acids are acidic compounds due to the presence of the carboxyl group, which can donate a proton (H+) in solution. They are stronger acids than alcohols and aldehydes but weaker than mineral acids. The acidity of carboxylic acids allows them to react with bases to form salts known as carboxylates.

2.    Hydrogen Bonding: Carboxylic acids can form strong intermolecular hydrogen bonds due to the presence of the hydroxyl (-OH) group and the carbonyl (C=O) group. These hydrogen bonds contribute to the higher boiling points and solubility in water compared to hydrocarbons and other organic compounds.

3.    Odour: Carboxylic acids often have strong, pungent odours. For example, acetic acid is responsible for the distinctive smell of vinegar, while butyric acid gives off a rancid odour resembling spoiled butter.

4.    Reactivity: Carboxylic acids can undergo various reactions, including acid-base reactions, esterification, oxidation, and decarboxylation. These reactions allow for the synthesis of derivatives and the transformation of carboxylic acids into other functional groups.

Examples illustrating the properties of carboxylic acids include the acidity of acetic acid (CH3COOH), its ability to form intermolecular hydrogen bonds, and the odour of formic acid (HCOOH).

Carboxylic acids can undergo various reactions to form derivatives. Three common derivatives of carboxylic acids are esters, amides, and acid chlorides. These derivatives possess unique properties and have diverse applications. Let's explore their synthesis and properties:

1.    Esters:

●     Synthesis: Esters are formed through esterification reactions, which involve the reaction between a carboxylic acid and an alcohol in the presence of an acid catalyst or heat. This reaction leads to the formation of an ester and water. For example, the reaction between acetic acid and ethanol produces ethyl acetate.

●     Properties: Esters are characterised by their pleasant, fruity odours and are often used as fragrance and flavour compounds. They have relatively low boiling points, are generally insoluble in water, and are commonly used as solvents, plasticizers, and intermediates in organic synthesis.

2.    Amides:

●     Synthesis: Amides are formed by the reaction of a carboxylic acid with an amine. This reaction is known as amidation and is typically carried out in the presence of an acid catalyst. The reaction between acetic acid and ammonia, for instance, yields acetamide.

●     Properties: Amides are less reactive than carboxylic acids and exhibit higher boiling points and greater stability due to the presence of strong intermolecular hydrogen bonding. They are found in various biologically important compounds, such as proteins and nucleic acids, and are used as synthetic intermediates, solvents, and drugs.

3.    Acid Chlorides:

●     Synthesis: Acid chlorides are synthesised through the reaction of a carboxylic acid with thionyl chloride (SOCl2) or phosphorus trichloride (PCl3). This reaction leads to the replacement of the hydroxyl group with a chlorine atom. For example, the reaction between acetic acid and thionyl chloride produces acetyl chloride.

●     Properties: Acid chlorides are highly reactive compounds and can be used as acylating agents in various chemical reactions, such as the synthesis of amides, esters, and anhydrides. They are corrosive and can undergo vigorous reactions with water, alcohols, and amines.

These derivatives of carboxylic acids exhibit different properties and reactivity compared to the parent carboxylic acids. Their synthesis and unique properties make them valuable in various applications, including the production of fragrances, pharmaceuticals, polymers, and other industrial chemicals.