Aromatic Acids

Definition of Aromatic Acids:

• Aromatic acids are organic compounds containing a carboxylic acid group (-COOH) attached to an aromatic ring, such as benzene or naphthalene.

• They exhibit properties characteristic of both carboxylic acids and aromatic compounds.

Types and Classification:

Benzoic Acid Derivatives:

• Contain a carboxylic acid group directly attached to a benzene ring.

• Classified based on the presence and position of additional substituents on the benzene ring (ortho-, meta-, or para- substituted).

Naphthoic Acid Derivatives:

• Contain a carboxylic acid group directly attached to a naphthalene ring.

• Classified similarly to benzoic acid derivatives based on substituents.

Phenolic Acids:

• Contain both a carboxylic acid group and a hydroxyl group (-OH) attached to the aromatic ring.

• Classified as hydroxybenzoic acids (e.g., salicylic acid) or hydroxycinnamic acids (e.g., caffeic acid).

Applications:

Aromatic acids have various applications across different industries and fields, including:

1. Pharmaceuticals: Used as active ingredients in drugs; for example, aspirin is derived from salicylic acid.

2. Food Industry: Aromatic acids like benzoic acid are used as preservatives due to their antimicrobial properties.

3. Polymers and Plastics: Terephthalic and isophthalic acids are monomers in polyester production, including PET, used in packaging and textiles.

4. Dyes and Pigments: Serve as precursors in synthesizing dyes and pigments, such as azo dyes.

Examples:

1. Benzoic acid: Used as a food preservative and precursor to plasticizers and dyes.

2. Salicylic acid: Precursor to aspirin and used in skincare for exfoliation.

3. Terephthalic acid: Monomer in polyester production.

4. Anthranilic acid: Used in azo dye synthesis, perfumes, and pharmaceuticals.

General Methods of Preparation

Oxidation of Alkylbenzenes:

• Process: Alkylbenzenes like toluene are oxidized to benzoic acid using strong oxidizing agents like KMnO₄ or H₂CrO₄. The intermediate benzylic alcohol and aldehyde are further oxidized to form benzoic acid.

• Reaction: 

  C6H5CH3 + 2KMnO4 + 3H2O → C6H5COOH + 2MnO2 + 2KOH + H2O

Hydrolysis of Nitriles:

• Process: Aromatic nitriles like benzonitrile are hydrolyzed to carboxylic acids using acids or bases. Water adds to the nitrile, followed by tautomerization and protonation/deprotonation to yield the acid.

• Reaction: 

  C6H5CN + 2H2O + H+ → C6H5COOH + NH4+

Grignard Reagent and CO2:

• Process: Benzoic acid is synthesized by reacting phenylmagnesium bromide (Grignard reagent) with CO₂, followed by acidification to produce the acid.

• Reaction: 

  C6H5MgBr + CO2 → C6H5COOMgBr

  C6H5COOMgBr + H2O + H+ → C6H5COOH + MgBrOH