Organic Chemistry: Aldehydes, Ketones, and Carboxylic Acids - SS3 Chemistry Past Questions and Answers - page 3

Carboxylic acids are organic compounds containing the carboxyl functional group (COOH). They exhibit unique properties, and reactions, and have various applications. Carboxylic acid derivatives are compounds derived from carboxylic acids by replacing the -OH group with other functional groups. The major types of derivatives are esters, amides, and anhydrides.

Properties:

1.    Acidity: Carboxylic acids are weak acids, capable of donating a proton (H+) to a base to form a carboxylate ion (RCOO-).

2.    Hydrogen Bonding: Carboxylic acids and their derivatives can form hydrogen bonds due to the presence of the -OH or nitrogen-containing functional groups.

3.    Solubility: Smaller carboxylic acids are soluble in water due to hydrogen bonding, while solubility decreases with increasing carbon chain length.

Reactions:

1.    Esterification: Carboxylic acids react with alcohols in the presence of acid catalysts to form esters and water.

Example: Ethanoic acid + Methanol → Methyl ethanoate + Water

2.    Amidation: Carboxylic acids react with ammonia or amines to form amides and water.

Example: Ethanoic acid + Ammonia → Acetamide + Water

3.    Dehydration of Carboxylic Acids: Carboxylic acids can undergo dehydration to form anhydrides.

Example: Ethanoic acid + Ethanoic acid → Ethanoic anhydride + Water

Applications:

1.    Pharmaceuticals: Carboxylic acids and their derivatives are important in the synthesis of pharmaceutical drugs, including aspirin (acetylsalicylic acid), a commonly used analgesic.

2.    Flavour and Fragrance: Esters derived from carboxylic acids contribute to the flavours and fragrances of various fruits and flowers.

3.    Polymers: Carboxylic acid derivatives, such as polyesters and polyamides, are used in the production of synthetic fibres and plastics.

4.    Biological Functions: Carboxylic acids are essential in various biological processes. For example, fatty acids are carboxylic acids that are important components of lipids and cell membranes.

5.    Organic Synthesis: Carboxylic acid derivatives serve as versatile intermediates in the synthesis of a wide range of organic compounds.

Real-World Examples:

1.    Aspirin (Acetylsalicylic Acid): Aspirin is a widely used medication that acts as an analgesic, antipyretic, and anti-inflammatory agent. It is derived from salicylic acid through esterification.

2.    Ethyl Acetate: Ethyl acetate is an ester used as a solvent in paints, varnishes, and glues due to its pleasant fruity smell and low toxicity.

3.    Nylon: Nylon is a polyamide synthesised from the condensation of a dicarboxylic acid and a diamine. It is commonly used in textiles and engineering plastics.

In conclusion, carboxylic acids and their derivatives are essential compounds in various industries and biological systems. Their unique properties, reactivity, and versatility make them valuable building blocks in organic synthesis, pharmaceuticals, flavours, and fragrances, and polymer chemistry. The formation and applications of carboxylic acid derivatives play a significant role in improving and diversifying our daily lives and industrial processes

The general formula for a carboxylic acid is R-COOH, where R represents an organic group.

Smaller carboxylic acids are soluble in water due to hydrogen bonding, but solubility decreases with increasing carbon chain length.

The reaction between an alcohol and a carboxylic acid in the presence of an acid catalyst forms an ester.

Reactions and Synthesis of Carboxylic Acids:

1.    Oxidation of Primary Alcohols:

Primary alcohols can be oxidised to carboxylic acids using oxidising agents such as potassium dichromate (K2Cr2O7) or potassium permanganate (KMnO4). The oxidation reaction involves the conversion of the alcohol's -OH group into a carboxyl group (COOH).

Example: Ethanol → Ethanoic Acid

CH3CH2OH + [O] → CH3COOH

Significance: This method is widely used in the laboratory and industrial synthesis of carboxylic acids from readily available primary alcohols.

2.    Oxidation of Aldehydes:

Aldehydes can be further oxidised to carboxylic acids using the same oxidising agents. The aldehyde group (-CHO) is converted into a carboxyl group (-COOH) during the oxidation process.

Example: Ethanal → Ethanoic Acid

CH3CHO + [O] → CH3COOH

Significance: This oxidation reaction allows the transformation of aldehydes, which are common intermediates in organic synthesis, into carboxylic acids, expanding the range of available compounds.

3.    Hydrolysis of Nitriles:

Nitriles can be hydrolyzed to carboxylic acids using either acidic or basic conditions. The nitrile group (-CN) is converted into a carboxyl group (-COOH) by adding water.

Example: Ethyl Cyanide → Ethanoic Acid

CH3CH2CN + 2H2O → CH3COOH + NH3

Significance: This method is important for the synthesis of carboxylic acids from nitriles, which can be obtained from a variety of starting materials, providing versatility in organic synthesis.

4.    Carboxylation of Grignard Reagents:

Carboxylic acids can be synthesised by reacting Grignard reagents (RMgX) with carbon dioxide (CO2) to form carboxylate ions. Acidification of the reaction mixture yields the corresponding carboxylic acids.

Example: Ethylmagnesium Bromide + Carbon Dioxide → Ethanoic Acid

CH3CH2MgBr + CO2 → CH3COOH

Significance: This method enables the direct incorporation of carbon dioxide into organic molecules, allowing the efficient synthesis of carboxylic acids with different alkyl substituents.

Real-World Applications:

1.    Pharmaceutical Synthesis: The synthesis of carboxylic acids is crucial in pharmaceutical manufacturing as many drugs contain carboxyl groups in their structures.

2.    Food Industry: Carboxylic acids play a vital role in the food industry as they are responsible for the sour taste of fruits (e.g., citric acid in citrus fruits) and are used as food preservatives (e.g., benzoic acid).

3.    Biological Significance: Carboxylic acids are involved in various biological processes, such as in the citric acid cycle, where they act as intermediates in cellular respiration.

In conclusion, carboxylic acids can be synthesised through various methods, including oxidation of primary alcohols and aldehydes, hydrolysis of nitriles, and carboxylation of Grignard reagents. These reactions have significant applications in organic synthesis, pharmaceuticals, and the food industry. The versatility and importance of carboxylic acids in various fields make their synthesis a key aspect of organic chemistry.

Carboxylic acids are weak acids, meaning they do not ionise completely in water. Instead, they partially ionise to form carboxylate ions (RCOO-) and release hydrogen ions (H+). The equilibrium lies more towards the reactant side, resulting in partial ionisation and weak acid behaviour.

When a carboxylic acid solution is diluted with water, the concentration of the carboxylate ions (RCOO-) increases relative to the concentration of the undissociated acid (RCOOH). As a result, the concentration of hydrogen ions (H+) decreases, leading to an increase in pH. Dilution makes the solution less acidic.

When a carboxylic acid reacts with an alkali metal hydroxide (e.g., sodium hydroxide), a neutralisation reaction occurs, producing water and a carboxylate salt. The carboxylate salt has the general formula RCOONa, where R represents the alkyl group of the carboxylic acid.