Reactions and Synthesis of Carboxylic Acids - SS3 Chemistry Lesson Note

Carboxylic acids are versatile organic compounds that can be synthesised through various methods, and they participate in a wide range of chemical reactions. Understanding these reactions and synthesis methods is crucial for the preparation and utilisation of carboxylic acids in diverse applications. Let's explore the reactions and synthesis of carboxylic acids in detail:

Synthesis of Carboxylic Acids:

a.    Oxidation of Primary Alcohols:

Primary alcohols can be oxidised to carboxylic acids using oxidising agents such as potassium dichromate (K2Cr2O7) or potassium permanganate (KMnO4) in the presence of acidic conditions. The alcohol is first converted to an aldehyde and then further oxidised to carboxylic acid. For example:

R-CH2-OH (Primary Alcohol) → R-CHO (Aldehyde) → R-COOH (Carboxylic Acid)

b.    Oxidation of Aldehydes:

Aldehydes can be further oxidised to carboxylic acids using the same oxidising agents mentioned above. This reaction proceeds directly from the aldehyde to the carboxylic acid. For example:

R-CHO (Aldehyde) → R-COOH (Carboxylic Acid)

c.     Hydrolysis of Nitriles:

Nitriles (RCN) can be hydrolyzed under acidic or basic conditions to form carboxylic acids. In acidic hydrolysis, the nitrile is first converted to an amide, which then undergoes further hydrolysis to yield the carboxylic acid. In basic hydrolysis, the nitrile is directly hydrolyzed to the carboxylate salt, which can be acidified to obtain the carboxylic acid. For example:

RCN (Nitrile) → RCONH2 (Amide) → RCOOH (Carboxylic Acid)

d.    Grignard Reaction with Carbon Dioxide:

Grignard reagents (RMgX) react with carbon dioxide (CO2) to produce carboxylate salts, which can be acidified to form carboxylic acids. This reaction is a versatile method for synthesising various carboxylic acids. For example:

RMgX + CO2 → RCOOMgX (Carboxylate Salt) → RCOOH (Carboxylic Acid)

Reactions of Carboxylic Acids:

a.    Esterification:

Carboxylic acids react with alcohols in the presence of an acid catalyst to form esters. This reaction involves the substitution of the hydroxyl group of the carboxylic acid with the alkyl group of the alcohol. For example:

RCOOH (Carboxylic Acid) + R'OH (Alcohol) → RCOOR' (Ester) + H2O

b.    Amidation:

Carboxylic acids react with amines to form amides. This reaction involves the substitution of the hydroxyl group of the carboxylic acid with the amino group of the amine. For example:

RCOOH (Carboxylic Acid) + R'NH2 (Amine) → RCONHR' (Amide) + H2O

c.     Decarboxylation:

Certain carboxylic acids can undergo decarboxylation, where they lose a carbon dioxide molecule to form an alkyl or aryl group. This reaction is commonly observed in the biosynthesis of fatty acids and the synthesis of aromatic compounds. For example:

RCOOH (Carboxylic Acid) → R-H (Alkyl or Aryl Group) + CO2

These are just a few examples of the many reactions and synthesis methods involving carboxylic acids. Carboxylic acids and their derivatives play crucial roles in organic synthesis, pharmaceuticals, food chemistry, and various other industries. Understanding these reactions allows chemists to design and optimise synthetic routes to obtain specific carboxylic acids and their derivatives for desired applications